WHILE-DO statement repeats a statement while a condition,
tested at the start of the loop, is met.
WHILE expr DO
expr is evaluated. If its value is zero
(i.e., the condition is false), then the
WHILE-DO statement does
nothing and control flow passes on to the next statement. If the
condition is non-zero (i.e., the condition is true), then the statement
statement is executed and the condition is re-evaluated,
iterating until the condition becomes true.