Dangling else

The dangling else is a problem in programming of parser generators in which an optional else clause in an if–then(–else) statement can make nested conditional statements 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, or the if-then-else form. (The else clause is optional.): Ambiguous interpretation becomes possible when there are nested statements; specifically when an if-then-else form replaces the statement s inside the above if-then construct: In this example, s1 gets executed if and only if a is true and b is true. But what about s2? One person might be sure that s2 gets executed whenever a is false (by attaching the else to the first if), while another person might be sure that s2 gets executed only when a is true and b is false (by attaching the else to the second if). In other words, someone could interpret the previous statement as being equivalent to either of the following unambiguous statements: The dangling-else problem dates back to ALGOL 60, and subsequent languages have resolved it in various ways. In LR parsers, the dangling else is the archetypal example of a shift-reduce conflict. Concrete examples follow. In C, the grammar reads, in part: Thus, without further rules, the statement could ambiguously be parsed as if it were either: or: The C standard clarifies that an else block is associated with the nearest if.