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The dangling else is a problem in computer programming in which an optional else clause in an if–then(–else) statement results in nested conditionals being ambiguous. Formally, the reference context-free grammar of the language is ambiguous, meaning there is more than one correct parse tree. In many programming languages one may write conditionally executed code in two forms: the if-then form, and the if-then-else form – the else clause is optional: if a then s if a then s1 else s2 This gives rise to an ambiguity in interpretation when there are nested statements, specifically whenever an if-then form appears as s1 in an if-then-else form:if a then if b then s else s2 In this example, s is unambiguously executed when a is true and b is true, but one may interpret s2 as being executed when a is false (thus attaching the else to the first if) or when a is true and b is false (thus attaching the else to the second if). In other words, one may see the previous statement as either of the following expressions:if a then (if b then s) else s2 or if a then (if b then s else s2) The dangling else problem dates to ALGOL 60, and has been resolved in various ways in subsequent languages. In LR parsers, the dangling else is the archetypal example of a shift-reduce conflict. ==Avoiding ambiguity while keeping the syntax== This is a problem that often comes up in compiler construction, especially scannerless parsing. The convention when dealing with the dangling else is to attach the else to the nearby if statement,〔(5.2 Shift/Reduce Conflicts ) from GNU Operating System web site〕 allowing for unambiguous context-free grammars, in particular. Programming languages like Pascal〔ISO 7185:1990 (Pascal) 6.8.3.4: ''An if-statement without an else-part shall not be immediately followed by the token else.''〕 and C〔(ISO 9899 ):1999 (C): 6.8.4.1(3): "An else is associated with the lexically nearest preceding if that is allowed by the syntax.", available at (WG14 N1256 ), p. 134〕 follow this convention, so there is no ambiguity in the semantics of the ''language'', though the use of a parser generator may lead to ambiguous ''grammars''. In these cases alternative grouping is accomplished by explicit blocks, such as begin...end in Pascal〔''Pascal,'' Nell Dale and Chip Weems, "Dangling Else", (p. 160–161 )〕 and Depending on the compiler construction approach, one may take different corrective actions to avoid ambiguity: *If the parser is produced by an SLR, LR(1) or LALR LR parser generator, the programmer will often rely on the generated parser feature of preferring shift over reduce whenever there is a conflict.〔 Alternatively, the grammar can be rewritten to remove the conflict, at the expense of an increase in grammar size (see below). *If the parser is produced by a Pruning and Deep Pruning LR generator, one can issue directives that prune away the ambiguities completely.〔http://www.mightyheave.com/blog/?p=17#more-17〕 *If the parser is hand written, the programmer may use a ''non-ambiguous'' context-free grammar. Alternatively, one may rely on a non-context-free grammar or a parsing expression grammar. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dangling else」の詳細全文を読む スポンサード リンク
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