How to break while loop in an inner for loop in python?

Question

while doesn't break when i>10 in for loop:

i = 0
x = 100
while i<=10:
    for a in xrange(1, x+1):
        print "ok"
        i+=1

and it prints "ok" 100 times. How to break the while loop when i reaches 10 in for loop?

Solution 1

Until the inner loop "returns", the condition in the outer loop will never be re-examinated. If you need this check to happen every time after i changes, do this instead:

while i<=10:
    for a in xrange(1, x+1):
        print "ok"
        i+=1
        if i > 10:
            break

That break will only exit the inner loop, but since the outer loop condition will evaluate to False, it will exit that too.

Solution 2

i = 0
x = 100
def do_my_loops():
  while i<=10:
    for a in xrange(1, x+1):
      print "ok"
      i+=1
      if time_to_break:
        return
do_my_loops()

where time_to_break is the condition you're checking.

Or in general:

def loop_container():
  outer_loop:
    inner_loop:
      if done:
        return

loop_container()

Solution 3

The problem is that the outer loop's condition won't be checked until the inner loop finishes - and at this point i is already 100. From my perspective, the correct way to write this and get the desired output would be to put a guard inside the inner loop and break it when i reaches 10.

for a in xrange(1, x+1):
    if i < 10:
        print "ok"
        i+=1
    else:
        break

If there is some other reason why you want to break an outer loop while you're inside the inner loop, maybe you should let us in on the details to better understand it.

Comments

[Matt Briançon]

Just be sure not to add anything after the for loop or that will execute after the inner loop breaks.

[alwbtc]

@Matt Briançon Yes?? I thought while will break as soon as for breaks?

[mgibsonbr]

@alwbtc while won't break unless either of these happens: a) you call break while in its body (strictly in its body, not in the loops inside it); b) the body finished executing, and the condition evaluates to False; c) you call continue while in its body (strictly, also) and the condition evaluates to False.