Dangling else

The dangling else is a problem in <a href="/facts/Computer_programming/QmSsFnu0">programming</a> of <a href="/facts/Parser_generator/Ud51JPrv">parser generators</a> in which an optional else clause in an <a href="/facts/Conditional_(computer_programming)/8gDQTqUD">if–then(–else)</a> statement can make nested conditional statements ambiguous. Formally, the reference <a href="/facts/Context-free_grammar/yu1FFdK4">context-free grammar</a> of the language is <a href="/facts/Ambiguous_grammar/elcVDDL4">ambiguous</a>, meaning there is more than one correct <a href="/facts/Parse_tree/FeBEtXcz">parse tree</a>.
In many <a href="/facts/Programming_language/cWTYbgWa">programming languages</a>, one may write conditionally executed code in two forms: the if-then form, or the if-then-else form. (The else clause is optional.):

if a then s
if b then s1 else s2

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:

if a then if b then s1 else s2

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:

if a then { if b then s1 } else s2
if a then { if b then s1 else s2 }

The dangling-else problem dates back to <a href="/facts/ALGOL_60/QnpxUYQm">ALGOL 60</a>, and subsequent languages have resolved it in various ways. In <a href="/facts/LR_parser/bpMod8K0">LR parsers</a>, the dangling else is the archetypal example of a <a href="/facts/Shift-reduce_conflict/bpMod8K0">shift-reduce conflict</a>.

Dangling else open-in-new

Dangling else