The while loop has the following syntax:
while <<boolean expression>>:
<<statement>>
<<statement>>
<<statement>>
.
.
.The statements under the while clause get executed repeatedly as long as the boolean expression evaluates to True.
For example, the following program prints “y” to the standard output until someone kill it, just like the yes utility in Unix.
while True:
print "y"
Infinite loops are rarely useful. Hence, most while loop’s condition involves variables whose values change every time the loop is run.
Here’s an example of a variable used in a while loop whose value changes because of the user’s input.
x = 0
sum = 0
while x >= 0:
x = float(raw_input("Enter a number (a negative number to stop): "))
sum += x
print "The sum of all non-negative numbers entered is", str(sum) + "."
Here’s an example of a counter variable, which is a variable holding an integer value which is increased by one in each iteration.
i = 0
while i < 10:
print i
i += 1
The counter variable lets us know which iteration we are in. So, based on its value, we can do something different in each loop.