In most languages, a switch statement is defined across many individual statements. A typical syntax is that the first line contains the actual word "switch" followed by either the name of a variable or some other expression allowed by the language's syntax. This variable or expression is usually referred to as the "control variable" of the switch statement. After this line, following lines define one or more blocks of code that represent possible branches that program execution may take.
Each block begins with a line containing the case keyword followed a value that the control variable may have. If the value of the control variable matches this value, program execution will jump to that block of code. If not, the value specified in the next block (if present) is examined and the process repeats.
An optional special block is also allowed, which does not specify any value and which begins with the default keyword instead of the case keyword. If this block is present and if none of the values listed for any other block matches that of the control variable, program execution will jump to the statement following the default keyword.
The method of terminating a block is also of note. Typically, a break keyword is used to signal the end of the block. When encountered, this keyword causes program execution to continue with the first statement after the series of statements within the switch statement, thus completing execution of the switch statement. If no break keyword is present at the end of the block, in many languages program execution "falls through" to the code associated with the next block in the switch statement, as if its value also matched the value of the control variable. Notable exceptions include C#, in which fallthrough is not permitted unless the block is empty and all blocks must be terminated via a break, or by using another keyword. Similarly, almost all BASIC dialects that feature this type of statement do not allow fallthrough.
Python doesn’t have the “fall through” mechanism; a function per case is needed, and hashing is used to determine which function is called.
Ruby doesn’t have the “fall through” mechanism; it also uses case instead of switch, when instead of case and else instead of default.
Haskell's case construct is different in three ways: unlike C-influenced languages, it has no fall-through behaviour; it is an expression which returns a value; and it can deconstruct values using pattern matching.
case list of
(f:r) -> "Not empty, first item is " ++ show f
 -> "List is empty!"
For example in PHP you can do:
The reason this works is because the variable being switched for is the boolean true. So having
x=='hello' as a condition would either return true or false, so if that is true, it matches the thing which is being checked.
You could also use this for checking multiple variables against one value instead of checking multiple values for one variable:
In Ruby, due to its handling of
=== equality, the statement can be used to test for variable’s class:
Ruby also returns a value that can be assigned to a variable, and doesn’t actually require the
case to have any parameters (acting a bit like an
else if statement):
If the constants form a compact range then a switch statement can be implemented very efficiently as if it were a choice based on whole numbers. This is often done by using a jump table.
In some languages and programming environments, a case or switch statement is considered easier to read and maintain than an equivalent series of if-else statements, because it is more concise. However, when implemented with fall-through, switch statements are a frequent source of bugs among programmers new to the switch statement.
One alternative to a switch statement can be the use of a lookup table which contains as keys the case values and as values the part under the case statement. In some languages, only actual data types are allowed as values in the lookup table. In other languages, it is also possible to assign functions as lookup table values, gaining the same flexibility as a real switch statement (this is one way to implement switch statements in Lua which has no built-in switch ).
In some cases, lookup tables are more efficient than switch statements as many languages can optimize the table lookup whereas switch statements are often not optimized that much.
Another "alternative" to switch statements is the extensive use of polymorphism.
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