Fortran language features

This is a comprehensive overview of features of the Fortran 95 language, the version supported by almost all existing Fortran compilers. Old features that have been superseded by new ones are not described — few of those historic features are used in modern programs (although most have been retained in the language to maintain backward compatibility). The current standard is known as Fortran 2003, but still, as of 2008, none of the compilers on the market supports the entire set of its enhancements .

Language elements

Note. Fortran is case-insensitive. Convention of writing Fortran keywords in upper case and all other names in lower case is adopted below (except, by way of contrast, in the input/output descriptions (Data transfer and Operations on external files))..


The basic component of the Fortran language is its character set. Its members are:

  • the letters A ... Z and a ... z (which are equivalent outside a character context);
  • the numerals 0 ... 9;
  • the underscore _; and
  • the special characters = : + blank - * / ( ) , . $ ' ! " % & ; < > ?

Tokens that have a syntactic meaning to the compiler are built from those components. There are six classes of tokens:

Label 123
Constant 123.456789_long
Operator .add.
Name solve_equation (up to 31 characters, including _)
Separator / ( ) (/ /) , = => : :: ; %

From the tokens, statements are built. These can be coded using the new free source form which does not require positioning in a rigid column structure:

FUNCTION string_concat(s1, s2)                             ! This is a comment
  TYPE (string), INTENT(IN) :: s1, s2
  TYPE (string) string_concat
  string_concat%string_data = s1%string_data(1:s1%length) // &
     s2%string_data(1:s2%length)                          ! This is a continuation
  string_concat%length = s1%length + s2%length
END FUNCTION string_concat

Note the trailing comments and the trailing continuation mark. There may be 39 continuation lines, and 132 characters per line. Blanks are significant. Where a token or character constant is split across two lines:

               ...        start_of&
              ...   'a very long &
a leading & on the continued line is also required.

Automatic conversion of source form for existing programs can be carried out by convert.f90

Its options are:

  • significant blank handling;
  • indentation;
  • CONTINUE replaced by END DO;
  • name added to subprogram END statement; and
  • INTEGER*2 etc. syntax converted.

Intrinsic data types

Fortran has five intrinsic data types: INTEGER, REAL, COMPLEX, LOGICAL and CHARACTER. Each of those types can be additionally charactericized by a kind. Kind, basically, defines internal representation of the type: for the three numeric types, it defines the precision and range, and for the other two, the specifics of storage representation. Thus, it is an abstract concept which models the limits of data types' representation; it is expressed as a member of a set of whole numbers (e.g. it may be {1, 2, 4, 8} for integers, denoting bytes of storage), but those values are not specified by the Standard and not portable. For every type, there is a default kind, which is used if no kind is explicitly specified. For each intrinsic type, there is a corresponding form of literal constant. Numeric types can only be signed.

Literal constants and kinds

Integer literal constants of the default kind take the form:
1   0   -999   32767   +10

Kind can be defined as a named constant. If the desired range is ±10kind, the portable syntax for defining the appropriate kind, two_bytes is:


that allows subsequent definition of constants of the form:

-1234_two_bytes   +1_two_bytes

Here, two_bytes is the kind type parameter; it can also be an explicit default integer literal constant, like

but such use is non-portable.

The KIND function supplies the value of a kind type parameter:

KIND(1)            KIND(1_two_bytes)

and the RANGE function supplies the actual decimal range (so the user must make the actual mapping to bytes):


Also, in DATA (initialization) statements (see below), binary (B), octal (O) and hexcadecimal (Z) constants may be used (often informally referred to as "BOZ constants"):

B'01010101'   O'01234567'   Z'10fa'

There are at least two real kinds—the default, and one with greater precision (this replaces DOUBLE PRECISION). SELECTED_REAL_KIND functions returns the kind number for desired range and precision; for at least 9 decimal digits of precision and a range of 10-99 to 1099, it can be specified as:

and literals subsequently specified as:
Also, there are the intrinsic functions
KIND(1.7_long)   PRECISION(1.7_long)   RANGE(1.7_long)
that give in turn the kind type value, the actual precision (here at least 9), and the actual range (here at least 99).

COMPLEX data type is built of two integer or real components:

(1, 3.7_long)

The forms of literal constants for CHARACTER data type are:

'A string'   "Another"   'A "quote"'   '

(the last being an empty string). Different kinds are allowed (for example, to distinguish ASCII and UNICODE strings), but not widely supported by compilers. Again, the kind value is given by the KIND function:


There are only two basic values of logical constants: .TRUE. and .FALSE.. Here, there may also be different kinds (to allow for packing into bits or bytes). Logicals don't have their own kind inquiry functions, but use the kinds specified for INTEGERs; default kind of LOGICAL is the same as of INTEGER.

.FALSE.   .true._one_bit

and the KIND function operates as expected:


Number model and intrinsic functions

The numeric types are based on number models with associated inquiry functions (whose values are independent of the values of their arguments; arguments are used only to provide kind). These functions are important for portable numerical software:

DIGITS(X) Number of significant digits
EPSILON(X) Almost negligible compared to one (real)
HUGE(X) Largest number
MAXEXPONENT(X) Maximum model exponent (real)
MINEXPONENT(X) Minimum model exponent (real)
PRECISION(X) Decimal precision (real, complex)
RADIX(X) Base of the model
RANGE(X) Decimal exponent range
TINY(X) Smallest positive number (real)

Scalar variables

Scalar variables corresponding to the five intrinsic types are specified as follows:

REAL(KIND=long) :: a
COMPLEX         :: current
LOGICAL         :: Pravda
CHARACTER(LEN=20) :: word
CHARACTER(LEN=2, KIND=Kanji) :: kanji_word

where the optional KIND parameter specifies a non-default kind, and the :: notation delimits the type and attributes from variable name(s) and their optional initial values, allowing full variable specification and initialization to be typed in one statement (in previous standards, attributes and initializers had to be declared in several statements). While it is not required in above examples (as there are no additional attributes and initialization), most Fortran-90 programmers acquire the habit to use it everywhere.

LEN= specifier is applicable only to CHARACTERs and specifies the string length (replacing the older *len form). The explicit KIND= and LEN= specifiers are optional:

CHARACTER(2, Kanji) :: kanji_word

works just as well.

There are some other interesting character features. Just as a substring as in

CHARACTER(80) :: line
... = line(i:i)                     ! substring
was previously possible, so now is the substring

Also, zero-length strings are allowed:

line(i:i-1)       ! zero-length string

Finally, there is a set of intrinsic character functions, examples being:

REPEAT SCAN(for one of a set)
TRIM VERIFY(for all of a set)

Derived data types

For derived data types, the form of the type must be defined first:

TYPE person
   CHARACTER(10) name
   REAL          age
END TYPE person

and then, variables of that type can be defined:

TYPE(person) you, me

To select components of a derived type, % qualifier is used:


Literal constants of derived type have the form TypeName(1stComponentLiteral, 2ndComponentLiteral, ...):

you = person('Smith', 23.5)
which is known as a structure constructor. Definitions may refer to a previously defined type:

TYPE point
   REAL x, y
END TYPE point
TYPE triangle
   TYPE(point) a, b, c
END TYPE triangle

and for a variable of type triangle, as in

TYPE(triangle) t
each component of type point is accessed as:
t%a   t%b   t%c
which, in turn, have ultimate components of type real:
t%a%x   t%a%y   t%b%x   etc.
(Note that the % qualifier was chosen rather than dot (.) because of potential ambiguity with operator notation, like .OR.).

Implicit and explicit typing

Unless specified otherwise, all variables starting with letters I, J, K, L, M and N are default INTEGERs, and all others are default REAL; other data types must be explicitly declared. This is known as implicit typing and is a heritage of early FORTRAN days. Those defaults can be overridden by IMPLICIT TypeName (CharacterRange) statements, like:
However, it is a good practice to explicitly type all variables, and this can be forced by inserting the statement
at the beginning of each program unit.


Arrays are considered to be variables in their own right. Every array is characterized by its type, rank, and shape (which defines the extents of each dimension). Bounds of each dimension are by default 1 and size, but arbitrary bounds can be explicitly specified. DIMENSION keyword is optional and considered an attribute; if omitted, the array shape must be specified after array-variable name. For example:

REAL:: a(10)
INTEGER, DIMENSION(0:100, -50:50) :: map

declares two arrays, rank-1 and rank-2, whose elements are in column-major order. Elements are, for example,

a(1)  a(i*j)
and are scalars. The subscripts may be any scalar integer expression.

Sections are parts of the array variables, and are arrays themselves:

a(i:j)               ! rank one
map(i:j, k:l:m)      ! rank two
a(map(i, k:l))       ! vector subscript
a(3:2)               ! zero length

Whole arrays and array sections are array-valued objects. Array-valued constants (constructors) are available, enclosed in (/ ... /):

(/ 1, 2, 3, 4 /)
(/ ((/ 1, 2, 3 /), i = 1, 4) /)
(/ (i, i = 1, 9, 2) /)
(/ (0, i = 1, 100) /)
(/ (0.1*i, i = 1, 10) /)
making use of an implied-DO loop notation. Fortran 2003 allows the use of brackets: [1, 2, 3, 4] and [([1,2,3], i=1,4)] instead of the first two examples above, and many compilers support this now. A derived data type may, of course, contain array components:
TYPE triplet
  REAL, DIMENSION(3) :: vertex
END TYPE triplet
TYPE(triplet), DIMENSION(4) :: t
so that
t(2)           is a scalar (a structure)
t(2)%vertex    is an array component of a scalar

Data initialization

Variables can be given initial values as specified in a specification statement:
REAL, DIMENSION(3) :: a = (/0.1, 0.2, 0.3 /)
and a default initial value can be given to the component of a derived data type:
TYPE triplet
   REAL, DIMENSION(3) :: vertex = 0
END TYPE triplet

PARAMETER attribute

A named constant can be specified directly by adding the PARAMETER attribute and the constant values to a type statement:
REAL, DIMENSION(3), PARAMETER :: field = (/ 0., 1., 2. /)
TYPE(triplet), PARAMETER :: t =   &
  triplet(0., (/ 0., 0., 0. /) )

DATA statement

The DATA statement can be used for scalars and also for arrays and variables of derived type. It is also the only way to initialise just parts of such objects, as well as to initialise to binary, octal or hexadecimal values:
TYPE(triplet) :: t1, t2
DATA t1/triplet(0., (/ 0., 1., 2. /) )/, t2%u/0./
DATA array(1:64) / 64*0/
DATA i, j, k/ B'01010101', O'77', Z'ff'/

Initialization expressions

The values used in DATA and PARAMETER statements, or with these attributes, are constant expressions that may include references to: array and structure constructors, elemental intrinsic functions with integer or character arguments and results, and the six transformational functions REPEAT, SELECTED_INT_KIND, TRIM, SELECTED_REAL_KIND, RESHAPE and TRANSFER (see Intrinsic procedures):
                      array(3) = (/ 1, 2, 3 /)

Specification expressions

It is possible to specify details of variables using any non-constant, scalar, integer expression that may also include inquiry function references:
SUBROUTINE s(b, m, c)
 USE mod                                 ! contains a
 REAL, DIMENSION(:, :)             :: b
 REAL, DIMENSION(UBOUND(b, 1) + 5) :: x
 INTEGER                           :: m
 CHARACTER(LEN=*)                  :: c
 CHARACTER(LEN= m + LEN(c))        :: cc

Expressions and assignments

Scalar numeric

The usual arithmetic operators are available — +, -, *, /, ** (given here in increasing order of precedence._

Parentheses are used to indicate the order of evaluation where necessary:

a*b + c     ! * first
a*(b + c)   ! + first

The rules for scalar numeric expressions and assignments accommodate the non-default kinds. Thus, the mixed-mode numeric expression and assignment rules incorporate different kind type parameters in an expected way:

real2 = integer0 + real1

converts integer0 to a real value of the same kind as real1; the result is of same kind, and is converted to the kind of real2 for assignment.

Scalar relational operations

For scalar relational operations of numeric types, there is a set of built-in operators:
<    <=    ==   /=   >   >=
.LT. .LE. .EQ. .NE. .GT. .GE.
(the forms above are new to Fortran-90, and older equivalent forms are given below them). Example expressions:

IF (a < b .AND. i /= j) THEN ! for numeric variables
flag = a == b                ! for logical variable flags

Scalar characters

In the case of scalar characters and given
CHARACTER(8) result

it is legal to write

result(3:5) = result(1:3)    ! overlap allowed
result(3:3) = result(3:2)    ! no assignment of null string

Derived-data types

No built-in operations (except assignment, defined on component-by component basis) exist between derived data types mutually or with intrinsic types. Meaning of existing or user-specified operators can be (re)defined though:
TYPE string
  INTEGER       length
  CHARACTER(80) value
END TYPE string
CHARACTER::    char1, char2, char3
TYPE(string):: str1,  str2,  str3
we can write
str3  = str1//str2       ! must define operation
str3  = str1.concat.str2 ! must define operation
char3 = char2//char3     ! intrinsic operator only
str3  = char1            ! must define assignment

Notice the "overloaded" use of intrinsic symbol // and of named operator, .concat. . A difference is that, for an intrinsic operator token, the usual precedence rules apply, whereas for named operators, precedence is the highest as a unary operator or the lowest as a binary one. In

vector3 = matrix    *    vector1  + vector2
vector3 =(matrix .times. vector1) + vector2
the two expressions are equivalent only if appropriate parentheses are added as shown. In each case there must be defined, in a module, procedures defining the operator and assignment, and corresponding operator-procedure association, as follows:
INTERFACE OPERATOR(//) !Overloads the // operator as invoking string_concat procedure
   MODULE PROCEDURE string_concat

The string concatenation function was shown already in Basics.

MODULE string_type
   TYPE string
      INTEGER length
      CHARACTER(LEN=80)   :: string_data
   END TYPE string
      MODULE PROCEDURE c_to_s_assign, s_to_c_assign
      MODULE PROCEDURE string_concat
   SUBROUTINE c_to_s_assign(s, c)
      TYPE (string), INTENT(OUT)    :: s
      CHARACTER(LEN=*), INTENT(IN)  :: c
      s%string_data = c
      s%length = LEN(c)
   END SUBROUTINE c_to_s_assign
   SUBROUTINE s_to_c_assign(c, s)
      TYPE (string), INTENT(IN)     :: s
      c = s%string_data(1:s%length)
   END SUBROUTINE s_to_c_assign
   FUNCTION string_concat(s1, s2)
   END FUNCTION string_concat
END MODULE string_type

Defined operators such as these are required for the expressions that are allowed also in structure constructors (see Derived-data types):

str1 = string(2, char1//char2)  ! structure constructor


In the case of arrays then, as long as they are of the same shape (conformable), operations and assignments are extended in an obvious way, on an element-by-element basis. For example, given declarations of
REAL, DIMENSION(10, 20) :: a, b, c
REAL, DIMENSION(5)      :: v, w
LOGICAL                    flag(10, 20)
it can be written:
a = b                                       ! whole array assignment
c = a/b                                     ! whole array division and assignment
c = 0.                                      ! whole array assignment of scalar value
w = v + 1.                                  ! whole array addition to scalar value
w = 5/v + a(1:5, 5)                         ! array division, and addition to section
flag = a==b                                 ! whole array relational test and assignment
c(1:8, 5:10) = a(2:9, 5:10) + b(1:8, 15:20) ! array section addition and assignment
v(2:5) = v(1:4)                             ! overlapping section assignment
The order of expression evaluation is not specified in order to allow for optimization on parallel and vector machines. Of course, any operators for arrays of derived type must be defined.

Some real intrinsic functions that are useful for numeric computations are:

CEILING         FLOOR         MODULO (also integer)
These are array valued for array arguments (elemental), like all FORTRAN 77 functions (except LEN):
INT             REAL          CMPLX
AINT            ANINT         NINT
ABS             MOD           SIGN
DIM             MAX           MIN

SQRT            EXP           LOG
LOG10           SIN           COS
TAN             ASIN          ACOS
ATAN            ATAN2
SINH            COSH          TANH

AIMAG           CONJG

LGE             LGT           LLE
LLT             ICHAR         CHAR
(the last seven are for characters).

Control statements

Branching and conditions

The simple GO TO label exists, but is usually avoided — in most cases, a more specific branching construct will accomplish the same logic with more clarity.

The simple conditional test is the IF statement:

     IF (a > b) x = y
A full-blown IF construct is illustrated by:
     IF (i < 0) THEN
        IF (j < 0) THEN
           x = 0.
           z = 0.
        END IF
     ELSE IF (k < 0) THEN
        z = 1.
        x = 1.
     END IF

CASE construct

The CASE construct is a replacement for the computed GOTO, but is better structured and does not require the use of statement labels:

     SELECT CASE (number)       ! number of type integer
     CASE (:-1)                 ! all values below 0
        n_sign = -1
     CASE (0)                   ! only 0
        n_sign = 0
     CASE (1:)                  ! all values above 0
        n_sign = 1
Each CASE selector list may contain a list and/or range of integers, character or logical constants, whose values may not overlap within or between selectors:
     CASE (1, 2, 7, 10:17, 23)
A default is available:
There is only one evaluation, and only one match.

DO construct

A simplified but sufficient form of the DO construct is illustrated by

  outer: DO
  inner:    DO i = j, k, l      ! from j to k in steps of l (l is optional)
               IF (...) CYCLE
               IF (...) EXIT outer
            END DO inner
         END DO outer
where we note that loops may be optionally named so that any EXIT or CYCLE statement may specify which loop is meant.

Many, but not all, simple loops can be replaced by array expressions and assignments, or by new intrinsic functions. For instance

         tot = 0.
         DO i = m, n
            tot = tot + a(i)
         END DO
becomes simply
         tot = SUM(a(m:n) )

Program units and procedures


In order to discuss this topic we need some definitions. In logical terms, an executable program consists of one main program and zero or more subprograms (or procedures) - these do something. Subprograms are either functions or subroutines, which are either external, internal or module subroutines. (External subroutines are what we knew from FORTRAN 77.)

From an organizational point of view, however, a complete program consists of program units. These are either main programs, external subprograms or modules'' and can be separately compiled.

An example of a main (and complete) program is:

  PROGRAM test
     PRINT *, 'Hello world!'
An example of a main program and an external subprogram, forming an executable program, is:
  PROGRAM test
     CALL print_message
  SUBROUTINE print_message
     PRINT *, 'Hello world!'
  END SUBROUTINE print_message
The form of a function is:
  FUNCTION name(arg1, arg2) ! zero or more arguments
     name = ...
The form of reference of a function is:
  x = name(a, b)

Internal procedures

An internal subprogram is one contained in another (at a maximum of one level of nesting) and provides a replacement for the statement function:

    SUBROUTINE outer
       REAL x, y
       SUBROUTINE inner
          REAL y
          y = x + 1.
       END SUBROUTINE inner     ! SUBROUTINE mandatory
We say that outer is the host of inner, and that inner obtains access to entities in outer by host association (e.g. to x), whereas y is a local variable to inner.

The scope of a named entity is a scoping unit, here outer less inner, and inner.

The names of program units and external procedures are global, and the names of implied-DO variables have a scope of the statement that contains them.


Modules are used to package

  • global data (replaces COMMON and BLOCK DATA from Fortran 77);
  • type definitions (themselves a scoping unit);
  • subprograms (which among other things replaces the use of ENTRY from Fortran 77);
  • interface blocks (another scoping unit, see Interface blocks);
  • namelist groups (see any textbook).
An example of a module containing a type definition, interface block and function subprogram is:
    MODULE interval_arithmetic
       TYPE interval
          REAL lower, upper
       END TYPE interval
          MODULE PROCEDURE add_intervals
       FUNCTION add_intervals(a,b)
          TYPE(interval), INTENT(IN) :: a, b
          TYPE(interval) add_intervals
          add_intervals%lower = a%lower + b%lower
          add_intervals%upper = a%upper + b%upper
       END FUNCTION add_intervals             ! FUNCTION mandatory
    END MODULE interval_arithmetic
and the simple statement
USE interval_arithmetic
provides use association to all the module's entities. Module subprograms may, in turn, contain internal subprograms.

Controlling accessibility

The PUBLIC and PRIVATE attributes are used in specifications in modules to limit the scope of entities. The attribute form is
    REAL, PUBLIC     :: x, y, z           ! default
    INTEGER, PRIVATE :: u, v, w
and the statement form is
    PUBLIC  :: x, y, z, OPERATOR(.add.)
    PRIVATE :: u, v, w, ASSIGNMENT(=), OPERATOR(*)
The statement form has to be used to limit access to operators, and can also be used to change the overall default:
    PRIVATE                        ! sets default for module
    PUBLIC  :: only_this
For derived types there are three possibilities: the type and its components are all PUBLIC, the type is PUBLIC and its components PRIVATE (the type only is visible and one can change its details easily), or all of it is PRIVATE (for internal use in the module only):
    MODULE mine
       TYPE, PUBLIC :: list
          REAL x, y
          TYPE(list), POINTER :: next
       END TYPE list
       TYPE(list) :: tree
    END MODULE mine

The USE statement's purpose is to gain access to entities in a module. It has options to resolve name clashes if an imported name is the same as a local one:

    USE mine, local_list => list
or to restrict the used entities to a specified set:
    USE mine, ONLY : list
These may be combined:
    USE mine, ONLY : local_list => list


We may specify the intent of dummy arguments:
    SUBROUTINE shuffle (ncards, cards)
       INTEGER, INTENT(IN)  :: ncards
       INTEGER, INTENT(OUT), DIMENSION(ncards) :: cards
Also, INOUT is possible: here the actual argument must be a variable (unlike the default case where it may be a constant).

Arguments may be optional:

    SUBROUTINE mincon(n, f, x, upper, lower, equalities, inequalities, convex, xstart)
       REAL, OPTIONAL, DIMENSION :: upper, lower
allows us to call mincon by
       CALL mincon (n, f, x, upper)
       IF (PRESENT(lower)) THEN   ! test for presence of actual argument
Arguments may be keyword rather than positional (which come first):
       CALL mincon(n, f, x, equalities=0, xstart=x0)
Optional and keyword arguments are handled by explicit interfaces, that is with internal or module procedures or with interface blocks.

Interface blocks

Any reference to an internal or module subprogram is through an interface that is 'explicit' (that is, the compiler can see all the details). A reference to an external (or dummy) procedure is usually 'implicit' (the compiler assumes the details). However, we can provide an explicit interface in this case too. It is a copy of the header, specifications and END statement of the procedure concerned, either placed in a module or inserted directly:
    REAL FUNCTION minimum(a, b, func)
! returns the minimum value of the function func(x) ! in the interval (a,b)
       REAL, INTENT(in) :: a, b
          REAL FUNCTION func(x)
             REAL, INTENT(IN) :: x
          END FUNCTION func
       REAL f,x
       f = func(x)   ! invocation of the user function.
    END FUNCTION minimum
An explicit interface is obligatory for:
  • optional and keyword arguments;
  • POINTER and TARGET arguments (see Pointers);
  • POINTER function result;
  • new-style array arguments and array functions (Array handling).
It allows full checks at compile time between actual and dummy arguments.

Overloading and generic interfaces

Interface blocks provide the mechanism by which we are able to define generic names for specific procedures:
    INTERFACE gamma                   ! generic name
       FUNCTION sgamma(X)             ! specific name
          REAL (SELECTED_REAL_KIND(6)) sgamma, x
       FUNCTION dgamma(X)             ! specific name
          REAL (SELECTED_REAL_KIND(12)) dgamma, x
where a given set of specific names corresponding to a generic name must all be of functions or all of subroutines. If this interface is within a module, then it is simply
    INTERFACE gamma
       MODULE PROCEDURE sgamma, dgamma
We can use existing names, e.g. SIN, and the compiler sorts out the correct association.

We have already seen the use of interface blocks for defined operators and assignment (see Modules).


Indirect recursion is useful for multi-dimensional integration. For
    volume = integrate(fy, ybounds)
We might have
    RECURSIVE FUNCTION integrate(f, bounds)
       ! Integrate f(x) from bounds(1) to bounds(2)
       REAL integrate
          FUNCTION f(x)
             REAL f, x
          END FUNCTION f
       REAL, DIMENSION(2), INTENT(IN) :: bounds
    END FUNCTION integrate
and to integrate f(x, y) over a rectangle:
    FUNCTION fy(y)
       USE func           ! module func contains function f
       REAL fy, y
       yval = y
       fy = integrate(f, xbounds)
Direct recursion is when a procedure calls itself, as in
    RECURSIVE FUNCTION factorial(n) RESULT(res)
       INTEGER res, n
       IF(n.EQ.1) THEN
          res = 1
          res = n*factorial(n-1)
       END IF
Here, we note the RESULT clause and termination test.

Pure Procedures

This is a feature for parallel computing.

In the FORALL Statement and Construct, any side effects in a function can impede optimization on a parallel processor -- the order of execution of the assignments could affect the results. To control this situation, we add the PURE keyword to the SUBROUTINE or FUNCTION statement -- an assertion that the procedure (expressed simply):

  • alters no global variable,
  • performs no I/O,
  • has no saved variables (variables with the SAVE attribute that retains values between invocations), and
  • does not alter its INTENT(IN) arguments for subroutines, or any for functions.
A compiler can check that this is the case, as in:
  PURE FUNCTION calculate (x)
All the intrinsic functions are pure.

Array handling

Array handling is included in Fortran for two main reasons:

  • the notational convenience it provides, bringing the code closer to the
  •  underlying mathematical form;
  • for the additional optimization opportunities it gives compilers (although
  •  there are plenty of opportunities for degrading optimization too!).
At the same time, major extensions of the functionality in this area have been added. We have already met whole arrays above and here - now we develop the theme.

Zero-sized arrays

A zero-sized array is handled by Fortran as a legitimate object, without special coding by the programmer. Thus, in
    DO i = 1,n
       x(i) = b(i) / a(i, i)
       b(i+1:n) = b(i+1:n) - a(i+1:n, i) * x(i)
    END DO
no special code is required for the final iteration where i = n. We note that a zero-sized array is regarded as being defined; however, an array of shape (0,2) is not conformable with one of shape (0,3), whereas
   x(1:0) = 3
is a valid 'do nothing' statement.

Assumed-shape arrays

These are an extension and replacement for assumed-size arrays. Given an actual argument like:
    REAL, DIMENSION(0:10, 0:20) :: a
    CALL sub(a)
the corresponding dummy argument specification defines only the type and rank of the array, not its size. This information has to be made available by an explicit interface, often using an interface block (see Interface blocks). Thus we write just
    REAL, DIMENSION(:, :) :: da
and this is as if da were dimensioned (11,21). However, we can specify any lower bound and the array maps accordingly. The shape, not bounds, is passed, where the default lower bound is 1 and the default upper bound is the corresponding extent.

Automatic arrays

A partial replacement for the uses to which EQUIVALENCE was put is provided by this facility, useful for local, temporary arrays, as in
    SUBROUTINE swap(a, b)
       REAL, DIMENSION(:)       :: a, b
       REAL, DIMENSION(SIZE(a)) :: work
       work = a
       a = b
       b = work
The actual storage is typically maintained on a stack.


Fortran provides dynamic allocation of storage; it relies on a heap storage mechanism (and replaces another use of EQUIVALENCE). An example, for establishing a work array for a whole program, is
    MODULE work_array
       INTEGER n
       REAL, DIMENSION(:,:,:), ALLOCATABLE :: work
    PROGRAM main
       USE work_array
       READ (input, *) n
       ALLOCATE(work(n, 2*n, 3*n), STAT=status)
       DEALLOCATE (work)
The work array can be propagated through the whole program via a USE statement in each program unit. We may specify an explicit lower bound and allocate several entities in one statement. To free dead storage we write, for instance,
    DEALLOCATE(a, b)
Deallocation of arrays is automatic when they go out of scope.

Elemental operations, assignments and procedures

We have already met whole array assignments and operations:
  REAL, DIMENSION(10) :: a, b
  a = 0.          ! scalar broadcast; elemental assignment
  b = sqrt(a)     ! intrinsic function result as array object
In the second assignment, an intrinsic function returns an array-valued result for an array-valued argument. We can write array-valued functions ourselves (they require an explicit interface):
  PROGRAM test
     REAL, DIMENSION(3) :: a = (/ 1., 2., 3./),       &
                           b = (/ 2., 2., 2. /),  r
     r = f(a, b)
     PRINT *, r
     FUNCTION f(c, d)
     REAL, DIMENSION(:) :: c, d
     REAL, DIMENSION(SIZE(c)) :: f
     f = c*d        ! (or some more useful function of c and d)
Elemental procedures are specified with scalar dummy arguments that may be called with array actual arguments. In the case of a function, the shape of the result is the shape of the array arguments.

Most intrinsic functions are elemental and Fortran 95 extends this feature to non-intrinsic procedures, thus providing the effect of writing, in Fortran 90, 22 different versions, for ranks 0-0, 0-1, 1-0, 1-1, 0-2, 2-0, 2-2, ... 7-7, and is further an aid to optimization on parallel processors. An elemental procedure must be pure.

     REAL, INTENT(INOUT)  :: a, b
     REAL                 :: work
     work = a
     a = b
     b = work
The dummy arguments cannot be used in specification expressions (see above) except as arguments to certain intrinsic functions (BIT_SIZE, KIND, LEN, and the numeric inquiry ones, (see below).


Often, we need to mask an assignment. This we can do using the WHERE, either as a statement:
WHERE (a /= 0.0) a = 1.0/a  ! avoid division by 0
(note: the test is element-by-element, not on whole array), or as a construct:
    WHERE (a /= 0.0)
       a = 1.0/a
       b = a             ! all arrays same shape
    WHERE (a /= 0.0)
       a = 1.0/a
       a = HUGE(a)
  • it is permitted to mask not only the WHERE statement of the WHERE construct, but also any ELSEWHERE statement that it contains;
  • a WHERE construct may contain any number of masked ELSEWHERE statements but at most one ELSEWHERE statement without a mask, and that must be the final one;
  • WHERE constructs may be nested within one another, just FORALL constructs;
  • a WHERE assignment statement is permitted to be a defined assignment, provided that it is elemental;
  • a WHERE construct may be named in the same way as other constructs.

The FORALL Statement and Construct

When a DO construct is executed, each successive iteration is performed in order and one after the other -- an impediment to optimization on a parallel processor.

  FORALL(i = 1:n) a(i, i) = x(i)
where the individual assignments may be carried out in any order, and even simultaneously. The FORALL may be considered to be an array assignment expressed with the help of indices.
  FORALL(i=1:n, j=1:n, y(i,j)/=0.) x(j,i) = 1.0/y(i,j)
with masking condition.

The FORALL construct allows several assignment statements to be executed in order.

  a(2:n-1,2:n-1) = a(2:n-1,1:n-2) + a(2:n-1,3:n) + a(1:n-2,2:n-1) + a(3:n,2:n-1)
  b(2:n-1,2:n-1) = a(2:n-1,2:n-1)
is equivalent to the array assignments
  FORALL(i = 2:n-1, j = 2:n-1)
      a(i,j) = a(i,j-1) + a(i,j+1) + a(i-1,j) + a(i+1,j)
      b(i,j) = a(i,j)
The FORALL version is more readable.

Assignment in a FORALL is like an array assignment: as if all the expressions were evaluated in any order, held in temporary storage, then all the assignments performed in any order. The first statement must fully complete before the second can begin. A FORALL may be nested, and may include a WHERE. Procedures referenced within a FORALL must be pure.

Array elements

For a simple case: given
        REAL, DIMENSION(100, 100) :: a
we can reference a single element as, for instance, a(1, 1). For a derived-data type like
    TYPE triplet
       REAL                  u
       REAL, DIMENSION(3) :: du
    END TYPE triplet
we can declare an array of that type:
    TYPE(triplet), DIMENSION(10, 20) :: tar
and a reference like
                    tar(n, 2)
is an element (a scalar!) of type triplet, but
                    tar(n, 2)%du
is an array of type real, and
                     tar(n, 2)%du(2)
is an element of it. The basic rule to remember is that an array element always has a subscript or subscripts qualifying at least the last name.

Array subobjects (sections)

The general form of subscript for an array section is
      [lower] : [upper] [:stride]

(where [] indicates an optional item) as in

      REAL a(10, 10)
      a(i, 1:n)                ! part of one row
      a(1:m, j)                ! part of one column
      a(i, : )                 ! whole row
      a(i, 1:n:3)              ! every third element of row
      a(i, 10:1:-1)            ! row in reverse order
      a((/ 1, 7, 3, 2 /), 1)  ! vector subscript
      a(1, 2:11:2)             ! 11 is legal as not referenced
      a(:, 1:7)                ! rank two section
Note that a vector subscript with duplicate values cannot appear on the left-hand side of an assignment as it would be ambiguous. Thus,
      b((/ 1, 7, 3, 7 /) ) = (/ 1, 2, 3, 4 /)
is illegal. Also, a section with a vector subscript must not be supplied as an actual argument to an OUT or INOUT dummy argument. Arrays of arrays are not allowed:
      tar%du             ! illegal
We note that a given value in an array can be referenced both as an element and as a section:
      a(1, 1)            !  scalar (rank zero)
      a(1:1, 1)          !  array section (rank one)
depending on the circumstances or requirements. By qualifying objects of derived type, we obtain elements or sections depending on the rule stated earlier:

      tar%u              !  array section (structure component)
      tar(1, 1)%u        !  component of an array element

Arrays intrinsic functions

Vector and matrix multiply
     DOT_PRODUCT        Dot product of 2 rank-one arrays
     MATMUL             Matrix multiplication
Array reduction
     ALL                True if all values are true
     ANY                True if any value is true. Example:
                            IF (ANY(a > b)) THEN
     COUNT              Number of true elements in array
     MAXVAL             Maximum value in an array
     MINVAL             Minimum value in an array
     PRODUCT            Product of array elements
     SUM                Sum of array elements
Array inquiry
     ALLOCATED          Array allocation status
     LBOUND             Lower dimension bounds of an array
     SHAPE              Shape of an array (or scalar)
     SIZE               Total number of elements in an array
     UBOUND             Upper dimension bounds of an array
Array construction
     MERGE              Merge under mask
     PACK               Pack an array into an array of rank
     SPREAD             Replicate array by adding a dimension
     UNPACK             Unpack an array of rank one into an array under mask
Array reshape
     RESHAPE            Reshape an array
Array manipulation
     CSHIFT             Circular shift
     EOSHIFT            End-off shift
     TRANSPOSE          Transpose of an array of rank two
Array location
     MAXLOC             Location of first maximum value in an array
     MINLOC             Location of first minimum value in an array



Pointers are variables with the POINTER attribute; they are not a distinct data type (and so no 'pointer arithmetic' is possible).
         REAL, POINTER :: var
They are conceptually a descriptor listing the attributes of the objects (targets) that the pointer may point to, and the address, if any, of a target. They have no associated storage until it is allocated or otherwise associated (by pointer assignment, see below):
         ALLOCATE (var)
and they are dereferenced automatically, so no special symbol required. In
         var = var + 2.3
the value of the target of var is used and modified. Pointers cannot be transferred via I/O. The statement
         WRITE *, var
writes the value of the target of var and not the pointer descriptor itself.

A pointer can point to other pointers, and hence to their targets, or to a static object that has the TARGET attribute:

         REAL, POINTER :: object
         REAL, TARGET  :: target_obj
         var => object                  ! pointer assignment
         var => target_obj
but they are strongly typed:
         INTEGER, POINTER :: int_var
         var => int_var                 ! illegal - types must match
and, similarly, for arrays the ranks as well as the type must agree.

A pointer can be a component of a derived type:

       TYPE entry                       ! type for sparse matrix
          REAL value
          INTEGER index
          TYPE(entry), POINTER :: next  ! note recursion
       END TYPE entry
and we can define the beginning of a linked chain of such entries:
       TYPE(entry), POINTER :: chain
After suitable allocations and definitions, the first two entries could be addressed as
       chain%value           chain%next%value
       chain%index           chain%next%index
       chain%next            chain%next%next
but we would normally define additional pointers to point at, for instance, the first and current entries in the list.


A pointer's association status is one of
  • undefined (initial state);
  • associated (after allocation or a pointer assignment);
  • disassociated:
  •          DEALLOCATE (p, q)  ! for returning storage
             NULLIFY (p, q)     ! for setting to 'null'
Some care has to be taken not to leave a pointer 'dangling' by use of DEALLOCATE on its target without nullifying any other pointer referring to it.

The intrinsic function ASSOCIATED can test the association status of a defined pointer:

              IF (ASSOCIATED(pointer)) THEN
or between a defined pointer and a defined target (which may, itself, be a pointer):
              IF (ASSOCIATED(pointer, target)) THEN
An alternative way to initialize a pointer, also in a specification statement, is to use the NULL function:
  REAL, POINTER, DIMENSION(:) :: vector => NULL() ! compile time
  vector => NULL()                                ! run time

Pointers in expressions and assignments

For intrinsic types we can 'sweep' pointers over different sets of target data using the same code without any data movement. Given the matrix manipulation y = B C z, we can write the following code (although, in this case, the same result could be achieved more simply by other means):
     REAL, TARGET  :: b(10,10), c(10,10), r(10), s(10), z(10)
     REAL, POINTER :: a(:,:), x(:), y(:)
     INTEGER mult
     DO mult = 1, 2
        IF (mult == 1) THEN
           y => r              ! no data movement
           a => c
           x => z
           y => s              ! no data movement
           a => b
           x => r
        END IF
        y = MATMUL(a, x)       ! common calculation
     END DO
For objects of derived type we have to distinguish between pointer and normal assignment. In
     TYPE(entry), POINTER :: first, current
     first => current
the assignment causes first to point at current, whereas
     first =  current
causes current to overwrite first and is equivalent to
     first%value = current%value
     first%index = current%index
     first%next => current%next

Pointer arguments

If an actual argument is a pointer then, if the dummy argument is also a pointer,
  • it must have same rank,
  • it receives its association status from the actual argument,
  • it returns its final association status to the actual argument (note: the
  •  target may be undefined!),
  • it may not have the INTENT attribute (it would be ambiguous),
  • it requires an interface block.
If the dummy argument is not a pointer, it becomes associated with the target of the actual argument:
    REAL, POINTER :: a (:,:)
    ALLOCATE (a(80, 80))
    CALL sub(a)
    REAL c(:, :)

Pointer functions

Function results may also have the POINTER attribute; this is useful if the result size depends on calculations performed in the function, as in
    USE data_handler
    REAL x(100)
    REAL, POINTER :: y(:)
    y => compact(x)
where the module data_handler contains
    FUNCTION compact(x)
       REAL, POINTER :: compact(:)
       REAL x(:)
 ! A procedure to remove duplicates from the array x
       INTEGER n
       :              ! Find the number of distinct values, n
       :              ! Copy the distinct values into compact
    END FUNCTION compact
The result can be used in an expression (but must be associated with a defined target).

Arrays of pointers

These do not exist as such: given
    TYPE(entry) :: rows(n)
    rows%next              ! illegal
would be such an object, but with an irregular storage pattern. For this reason they are not allowed. However, we can achieve the same effect by defining a derived data type with a pointer as its sole component:
    TYPE row
       REAL, POINTER :: r(:)
and then defining arrays of this data type:
    TYPE(row) :: s(n), t(n)
where the storage for the rows can be allocated by, for instance,
    DO i = 1, n
       ALLOCATE (t(i)%r(1:i)) ! Allocate row i of length i
    END DO
The array assignment
    s = t
is then equivalent to the pointer assignments
    s(i)%r => t(i)%r
for all components.

Pointers as dynamic aliases

Given an array
 REAL, TARGET :: table(100,100)

that is frequently referenced with the fixed subscripts

    table(m:n, p:q)
these references may be replaced by
    REAL, DIMENSION(:, :), POINTER :: window
    window => table(m:n, p:q)
The subscripts of window are 1:n-m+1, 1:q-p+1. Similarly, for
(as defined in already), we can use, say,
          taru => tar%u
to point at all the u components of tar, and subscript it as
          taru(1, 2)
The subscripts are as those of tar itself. (This replaces yet more of EQUIVALENCE.)

In the pointer association

pointer => array_expression
the lower bounds for pointer are determined as if lbound was applied to array_expression. Thus, when a pointer is assigned to a whole array variable, it inherits the lower bounds of the variable, otherwise, the lower bounds default to 1. Fortran 2003 allows specifying arbitrary lower bounds on pointer association, like
window(r:,s:) => table(m:n,p:q)
so that the bounds of window become r:r+n-m,s:s+q-p. Fortran 95 does not have this feature; however, it can be simulated using the following trick (based on the pointer association rules for assumed shape array dummy arguments):
function remap_bounds2(lb1,lb2,array) result(ptr)
  integer,intent(in)                          :: lb1,lb2
  real,dimension(lb1:,lb2:),intent(in),target :: array
  real,dimension(:,:),pointer                 :: ptr
  ptr => array
end function
window => remap_bounds2(r,s,table(m:n,p:q))

The source code of an extended example of the use of pointers to support a data structure is in pointer.f90

Intrinsic procedures

Most of the intrinsic functions have already been mentioned. Here, we deal only with their general classification and with those that have so far been omitted. All intrinsic procedures can be used with keyword arguments:

and many have optional arguments.

The intrinsic procedures are grouped into four categories:

  1. elemental - work on scalars or arrays, e.g. ABS(a);
  3. inquiry - independent of value of argument (which may be undefined), e.g.
  4.  PRECISION(a);
  5. transformational - array argument with array result of different shape,
  6.  e.g. RESHAPE(a, b);
  7. subroutines, e.g. SYSTEM_CLOCK.
The procedures not already introduced are::
Bit inquiry
     BIT_SIZE           Number of bits in the model
Bit manipulation
     BTEST              Bit testing
     IAND               Logical AND
     IBCLR              Clear bit
     IBITS              Bit extraction
     IBSET              Set bit
     IEOR               Exclusive OR
     IOR                Inclusive OR
     ISHFT              Logical shift
     ISHFTC             Circular shift
     NOT                Logical complement
Transfer function, as in
           INTEGER :: i = TRANSFER('abcd', 0)
                          (replaces part of EQUIVALENCE)
     DATE_AND_TIME      Obtain date and/or time
     MVBITS             Copies bits
     RANDOM_NUMBER      Returns pseudorandom numbers
     RANDOM_SEED        Access to seed
     SYSTEM_CLOCK       Access to system clock
     CPU_TIME           Returns processor time in seconds

Data transfer

(This is a subset only of the actual features and, exceptionally, lower case is used in the code examples.)

Formatted input/output

These examples illustrate various forms of I/O lists with some simple formats (see below):
  integer             :: i
  real, dimension(10) :: a
  character(len=20)   :: word
  print "(i10)",     i
  print "(10f10.3)", a
  print "(3f10.3)",  a(1),a(2),a(3)
  print "(a10)",     word(5:14)
  print "(3f10.3)",  a(1)*a(2)+i, sqrt(a(3:4))
Variables, but not expressions, are equally valid in input statements using the read statement:
     read "(i10)", i

If an array appears as an item, it is treated as if the elements were specified in array element order.

Any pointers in an I/O list must be associated with a target, and transfer takes place between the file and the targets.

An item of derived type is treated as if the components were specified in the same order as in the type declaration, so

     read "(8f10.5)", p, t  ! types point and triangle
has the same effect as the statement
  read "(8f10.5)", p%x, p%y, t%a%x, t%a%y, t%b%x, &
                             t%b%y, t%c%x, t%c%y
An object in an I/O list is not permitted to be of a derived type that has a pointer component at any level of component selection. Note that a zero-sized array may occur as an item in an I/O list. Such an item corresponds to no actual data transfer.

The format specification may also be given in the form of a character expression:

  character(len=*), parameter :: form="(f10.3)"
  print form, q
or as an asterisk -- this is a type of I/O known as list-directed I/O (see below), in which the format is defined by the computer system:
     print *, "Square-root of q = ", sqrt(q)
Input/output operations are used to transfer data between the storage of an executing program and an external medium, specified by a unit number. However, two I/O statements, print and a variant of read, do not reference any unit number: this is referred to as terminal I/O. Otherwise the form is:
  read (unit=4,     fmt="(f10.3)") q
  read (unit=nunit, fmt="(f10.3)") q
  read (unit=4*i+j, fmt="(f10.3)") a
where unit= is optional. The value may be any nonnegative integer allowed by the system for this purpose (but 5 and 6 often denote the keyboard and terminal).

An asterisk is a variant -- again from the keyboard:

  read (unit=*, fmt="(f10.3)") q

A read with a unit specifier allows exception handling:

  read (unit=nunit, fmt="(3f10.3)", iostat=ios) a,b,c
  if (ios == 0) then
! Successful read - continue execution.
! Error condition - take appropriate action.
     call error (ios)
  end if

There a second type of formatted output statement, the write statement:

     write (unit=nout, fmt="(10f10.3)", iostat=ios) a

Internal files

These allow format conversion between various representations to be carried out by the program in a storage area defined within the program itself.
  integer, dimension(30)         :: ival
  integer                        :: key
  character(len=30)              :: buffer
  character(len=6), dimension(3), parameter :: form=(/ "(30i1)", "(15i2)","(10i3)" /)
  read (unit=*, fmt="(a30,i1)")      buffer, key
  read (unit=buffer, fmt=form (key)) ival(1:30/key)
If an internal file is a scalar, it has a single record whose length is that of the scalar. If it is an array, its elements, in array element order, are treated as successive records of the file and each has length that of an array element. An example using a write statement is
  integer           :: day
  real              :: cash
  character(len=50) :: line
! write into line
  write (unit=line, fmt="(a, i2, a, f8.2, a)") "Takings for day ", day, " are ", cash, " dollars"
that might write
     Takings for day  3 are  4329.15 dollars

List-directed I/O

An example of a read without a specified format for input is:
  integer               :: i
  real                  :: a
  complex, dimension(2) :: field
  logical               :: flag
  character(len=12)     :: title
  character(len=4)      :: word
  read *, i, a, field, flag, title, word
If this reads the input record
10 6.4 (1.0,0.0) (2.0,0.0) t test/
(in which blanks are used as separators), then i, a, field, flag, and title will acquire the values 10, 6.4, (1.0,0.0) and (2.0,0.0), .true. and test respectively, while word remains unchanged.

Quotation marks or apostrophes are required as delimiters for a string that contains a blank.

Non-advancing I/O

This is a form of reading and writing without always advancing the file position to ahead of the next record. Whereas an advancing I/O statement always repositions the file after the last record accessed, a non-advancing I/O statement performs no such repositioning and may therefore leave the file positioned within a record.
  character(len=3) :: key
  integer      :: u, s, ios
  read(unit=u, fmt="(a3)", advance="no", size=s, iostat=ios) key
  if (ios == 0) then
! key is not in one record
     key(s+1:) = ""
  end if
A non-advancing read might read the first few characters of a record and a normal read the remainder.

In order to write a prompt to a terminal screen and to read from the next character position on the screen without an intervening line-feed, we can write:

  write (unit=*, fmt="(a)", advance="no") "enter next prime number:"
  read  (unit=*, fmt="(i10)") prime_number
Non-advancing I/O is for external files, and is not available for list-directed I/O.

Edit descriptors

It is possible to specify that an edit descriptor be repeated a specified number of times, using a repeat count::
The slash edit descriptor (see below) may have a repeat count, and a repeat count can also apply to a group of edit descriptors, enclosed in parentheses, with nesting:
  print "(2(2i5,2f8.2))", i(1),i(2),a(1),a(2), i(3),i(4),a(3),a(4)
Repeats are possible:
  print "(10i8)", (/ (i(j), j=1,100) /)
will write 100 values eight to a line (apart from the last).

Data edit descriptors

  • Integer: iW iW.M
  • Real: fW.D esW.D esW.DeE
  • Complex: pairs of f or es edit descriptors
  • Logical: lW
  • Character: a aW
  • Derived types: are edited by the appropriate sequence of edit descriptors corresponding to the intrinsic types of the ultimate components of the derived type.
      type, public :: string
         integer   :: length
         character(len=20) :: word
      end type string
      type(string) :: text
      read(unit=*, fmt="(i2, a)") text

Control edit descriptors

Control edit descriptors setting conditions:

The ss (sign suppress) edit descriptor suppresses leading plus signs. To switch on plus sign printing, the sp (sign print) descriptor is used. The s edit descriptor restores the option to the processor.

This descriptor remains in force for the remainder of the format specification, unless another of them is met.

Control edit descriptors for immediate processing:

  • Tabulation: tN trN tlN
         read (unit=*, fmt="(t3,i4, tl4,i1, i2)") i,j,k
  • New records: / N/
         read "(i5,i3,/,i5,i3,i2)", i, j, k, l, m
    Note that
         print "(i5,4/,i5)", i, j
    separates the two values by three blank records.
  • Colon editing: : terminates format control if there are no further items in an I/O list.
      print "(i5, :, /, i5, :, /, i5)", (/(l(i), i=1,n)/)
    stops new records if n equals 1 or 2.

Unformatted I/O

This type of I/O should be used only in cases where the records are generated by a program on one computer, to be read back on the same computer or another computer using the same internal number representations:
  open(unit=4, file='test', form='unformatted')
  read(unit=4) q
  write(unit=nout, iostat=ios) a  ! no fmt=

Direct-access files

This form of I/O is also known as random access or indexed I/O. Here, all the records have the same length, and each record is identified by an index number. It is possible to write, read, or re-write any specified record without regard to position.
  integer, parameter :: nunit=2, length=100
  real, dimension(length)            :: a
  real, dimension(length+1:2*length) :: b
  integer                            :: i, rec_length
  inquire (iolength=rec_length) a
  open (unit=nunit, access="direct", recl=rec_length, status="scratch", action="readwrite")
! Write array b to direct-access file in record 14
  write (unit=nunit, rec=14) b
! ! Read the array back into array a
  read (unit=nunit, rec=14) a
  do i = 1, length/2
     a(i) = i
  end do
! ! Replace modified record
  write (unit=nunit, rec=14) a
The file must be an external file and list-directed formatting and non-advancing I/O are unavailable.

Operations on external files

Once again, this is an overview only.

File positioning statements

  • The backspace statement:
      backspace (unit=u [,iostat=ios])      ! where [] means optional
  • The rewind statement:
      rewind (unit=u [,iostat=ios])
  • The endfile statement:
      endfile (unit=u [,iostat=ios])

The open statement

The statement is used to connect an external file to a unit, create a file that is preconnected, or create a file and connect it to a unit. The syntax is
     open (unit=u, status=st, action=act [,olist])
where olist is a list of optional specifiers. The specifiers may appear in any order.
  open (unit=2, iostat=ios, file="cities", status="new", access="direct",  &
        action="readwrite", recl=100)
Other specifiers are form and position.

The close statement

This is used to disconnect a file from a unit.
  close (unit=u [,iostat=ios] [,status=st])
as in
     close (unit=2, iostat=ios, status="delete")

The inquire statement

At any time during the execution of a program it is possible to inquire about the status and attributes of a file using this statement. Using a variant of this statement, it is similarly possible to determine the status of a unit, for instance whether the unit number exists for that system Another variant permits an inquiry about the length of an output list when used to write an unformatted record.

For inquire by unit:

     inquire (unit=u, ilist)
or for inquire by file:
     inquire (file=fln, ilist)
or for inquire by I/O list:
     inquire (iolength=length) olist
As an example:
  logical            :: ex, op
  character (len=11) :: nam, acc, seq, frm
  integer            :: irec, nr
  inquire (unit=2, exist=ex, opened=op, name=nam, access=acc, sequential=seq, form=frm, &
           recl=irec, nextrec=nr)
ex      .true.
op      .true.
nam      cities
acc      DIRECT
seq      NO
irec     100
nr       1
(assuming no intervening read or write operations).

Other specifiers are iostat, opened, number, named, formatted, position, action, read, write, readwrite.

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