While - do
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Much of the power of a computer is its ability to do repetitive tasks. In programming it is frequently necessary to make tasks execute over and over. This is called looping. XPL0 has four kinds of looping statements each of which repeatedly execute a block of code until certain conditions are met.
The "while" statement is a conditional looping structure. As long as the condition is met, the following statement or block is repeatedly executed. This statement has the form:
while BOOLEAN EXPRESSION do STATEMENT
while Guess # Number do
As long as the variables Guess and Number are not equal, the code within the begin-end block is repeated. The program tests the condition at the beginning of the "while" statement. If the condition is false, the block in the loop is ignored. If the condition is true, the block is executed and the code loops back to retest the condition. The condition must eventually become false, otherwise the loop continues forever. The following block diagram illustrates the operation of the while loop: