Dangling_else
Dangling else
if a then if b then s1 else s2which can be understood in two ways. Either as
if a then
{
if b then
{
s1
}
else
{
s2
}
}
or as
if a then
{
if b then
{
s1
}
}
else
{
s2
}
This is a problem that often comes up in compiler construction. It can be solved either at the implementation level, by telling the parser the right way to solve the ambiguity, or at the grammar level by using a Parsing expression grammar or equivalent. Python solves this problem by requiring the nested if statements to be indented.
if foo:
if bar:
print 'Inner True'
else:
print 'Inner False'
else:
print 'Outer False'
C and languages with similar syntax use braces to clearly define the body of the statement.
The problem can also be solved by removing the possibility for ambiguity, e.g. by designing the language to have an "end if". Examples of such languages are ALGOL 68, Ada, and REALbasic.
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Last updated on Thursday September 25, 2008 at 09:42:55 PDT (GMT -0700)
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