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Sometimes we just need to check if a condition is False. We can use the Boolean expression NOT to test for a false condition.

Consider this simple Python (3.x) program that prints out all numbers from 0 to 100:

stop = False
count = 0
while not(stop):
	print(count)
	count = count + 1
	if count > 100:
		stop = True

This program uses selection to determine whether to carry on printing out the value of ‘count’:

• The program examines the condition of the Boolean expression while not(stop), in line 3.
• If the Boolean value of ‘stop’ is False, then the condition is True.
• The program loops, incrementing the value of count by 1 each time.
• Once the value of ‘count’ is greater than 100, the value of ‘stop’ is changed to the Boolean value ‘T���ܱ�’. The condition of the Boolean expression while not(stop), in line 3, is now False and the program stops iterating as a result.

Using a NOT expression can sometimes be quite confusing, but it can sometimes also be quite useful.