You should use for whenever you can, because  it creates  the structure of your  loops clear. Sometimes, however, you require to do an operation various times, but you don't want to know in advance how many  times it requires  to be done.  In such conditions, you can use a while instruction statement, of the form:

while :

...

In  order  to  evaluate a while statement, the  interpreter  evaluates , placing a Boolean  value.  If the value  is 0, it skips  all the statements and  calculation moves  on to the next  statement in the code.  If the value  is 1 or true, then  the instructions are performed, and  the

is calculated again.  If it is False, execution of the loop is ended, and if it is

True, it goes around again.

It will generally be the case that you initialize a variable before the while statement, modify that variable in the course of executing the loop, and check some property of that variable in the Boolean expression. Consider that you need to write  a procedure that  takes an argument n and  returns the largest  power of 2 that is shorter  than  n. You may do it like this:

def pow2Smaller(n):

p = 1

while p*2 < n:

p = p*2

return p