Editing Control flow (section)

=== Multiple early exit/exit from nested loops ===
[[Zahn's construct|This construct]] was proposed by [[Zahn's construct|Zahn]] in 1974.<ref>Zahn, C. T. "A control statement for natural top-down structured programming" presented at Symposium on Programming Languages, Paris, 1974.</ref> A modified version is presented here.
    '''exitwhen''' EventA '''or''' EventB '''or''' EventC;
        xxx
    '''exits'''
        EventA: actionA
        EventB: actionB
        EventC: actionC
    '''endexit''';

'''exitwhen''' is used to specify the events which may occur within ''xxx'',
their occurrence is indicated by using the name of the event as a statement. When some event does occur, the relevant action is carried out, and then control passes just after '''{{not a typo|endexit}}'''. This construction provides a very clear separation between determining that some situation applies, and the action to be taken for that situation.

'''exitwhen''' is conceptually similar to [[exception handling]], and exceptions or similar constructs are used for this purpose in many languages.

The following simple example involves searching a two-dimensional table for a particular item.

    '''exitwhen''' found '''or''' missing;
        '''for''' I := 1 '''to''' N '''do'''
            '''for''' J := 1 '''to''' M '''do'''
                '''if''' table[I,J] = target '''then''' found;
        missing;
    '''exits'''
        found:   print ("item is in table");
        missing: print ("item is not in table");
    '''endexit''';