2.15 While

while bool-expr do statements od;

The while loop executes the statement sequence statements while the condition bool-expr evaluates to true.

First bool-expr is evaluated. If it evaluates to false execution of the while loop terminates and the statement immediately following the while loop is executed next. Otherwise if it evaluates to true the statements are executed and the whole process begins again.

The difference between the while loop and the repeat until loop (see Repeat) is that the statements in the repeat until loop are executed at least once, while the statements in the while loop are not executed at all if bool-expr is false at the first iteration.

If bool-expr does not evaluate to true or false an error is signalled and a break loop (see Break Loops) is entered. As usual you can leave the break loop with quit;. If you enter return false;, execution continues with the next statement immediately following the while loop. If you enter return true;, execution continues at statements, after which the next evaluation of bool-expr may cause another error.

    gap> i := 0;;  s := 0;;
    gap> while s <= 200  do
    >        i := i + 1;  s := s + i^2;
    >    od;
    gap> s;
    204        # first sum of the first i squares larger than 200 

Previous Up Top Next
Index