Breaking out of nested loops

user_33996 0
Viewed 521711

Is there an easier way to break out of nested loops than throwing an exception? (In Perl, you can give labels to each loop and at least continue an outer loop.)

for x in range(10):
    for y in range(10):
        print x*y
        if x*y > 50:
            "break both loops"

I.e., is there a nicer way than:

class BreakIt(Exception): pass

try:
    for x in range(10):
        for y in range(10):
            print x*y
            if x*y > 50:
                raise BreakIt
except BreakIt:
    pass
nested-loops python loops
8 Answers
for x in xrange(10):
    for y in xrange(10):
        print x*y
        if x*y > 50:
            break
    else:
        continue  # only executed if the inner loop did NOT break
    break  # only executed if the inner loop DID break

The same works for deeper loops:

for x in xrange(10):
    for y in xrange(10):
        for z in xrange(10):
            print x,y,z
            if x*y*z == 30:
                break
        else:
            continue
        break
    else:
        continue
    break
MizardX_223640

It has at least been suggested, but also rejected. I don't think there is another way, short of repeating the test or re-organizing the code. It is sometimes a bit annoying.

In the rejection message, Mr van Rossum mentions using return, which is really sensible and something I need to remember personally. :)

unwind_281690

If you're able to extract the loop code into a function, a return statement can be used to exit the outermost loop at any time.

def foo():
    for x in range(10):
        for y in range(10):
            print(x*y)
            if x*y > 50:
                return
foo()

If it's hard to extract that function you could use an inner function, as @bjd2385 suggests, e.g.

def your_outer_func():
    ...
    def inner_func():
        for x in range(10):
            for y in range(10):
                print(x*y)
                if x*y > 50:
                    return
    inner_func()
    ...
user_250500

You can also refactor your code to use a generator. But this may not be a solution for all types of nested loops.

JS_187860

In this particular case, you can merge the loops with a modern python (3.0 and probably 2.6, too) by using itertools.product.

I for myself took this as a rule of thumb, if you nest too many loops (as in, more than 2), you are usually able to extract one of the loops into a different method or merge the loops into one, as in this case.

Tetha_176630
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