Editing Control flow (section)

=== Loop with test in the middle ===
The following was proposed by [[Ole-Johan Dahl|Dahl]] in 1972:<ref>Dahl & Dijkstra & Hoare, "Structured Programming" Academic Press, 1972.</ref>

    '''loop'''                           '''loop'''
        xxx1                           read(char);
    '''while''' test;                    '''while''' '''not''' atEndOfFile;
        xxx2                           write(char);
    '''repeat''';                        '''repeat''';

If ''xxx1'' is omitted, we get a loop with the test at the top (a traditional '''while''' loop). If ''xxx2'' is omitted, we get a loop with the test at the bottom, equivalent to a '''do while''' loop in many languages. If '''while''' is omitted, we get an infinite loop. The construction here can be thought of as a '''do''' loop with the while check in the middle. Hence this single construction can replace several constructions in most programming languages.

Languages lacking this construct generally emulate it using an equivalent infinite-loop-with-break idiom:
 '''while''' (true) {
     xxx1
     '''if''' ('''not''' test)
         '''break'''
     xxx2
 }

A possible variant is to allow more than one '''while''' test; within the loop, but the use of '''exitwhen''' (see next section) appears to cover this case better.

In [[Ada (programming language)|Ada]], the above loop construct ('''loop'''-'''while'''-'''repeat''') can be represented using a standard infinite loop ('''loop''' - '''end loop''') that has an '''exit when''' clause in the middle (not to be confused with the '''exitwhen''' statement in the following section).

<syntaxhighlight lang="ada">
with Ada.Text_IO;
with Ada.Integer_Text_IO;

procedure Print_Squares is 
    X : Integer;
begin
    Read_Data : loop
        Ada.Integer_Text_IO.Get(X);
    exit Read_Data when X = 0;
        Ada.Text IO.Put (X * X);
        Ada.Text IO.New_Line;
    end loop Read_Data;
end Print_Squares;
</syntaxhighlight>

Naming a loop (like ''Read_Data'' in this example) is optional but permits leaving the outer loop of several nested loops.