Breaking out of nested loops

Question

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  gt  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  gt  50                  raise BreakIt except BreakIt      pass

User · Accepted Answer

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

User · Answer

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  gt  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  gt  50                      return     inner func

User · Answer

If you re going to raise an exception  you might raise a StopIteration exception   That will at least make the intent obvious

User · Answer

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

User · Answer

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

User · Answer

Sometimes I use a boolean variable  Naive  if you want  but I find it quite flexible and comfortable to read  Testing a variable may avoid testing again complex conditions and may also collect results from several tests in inner loops       x loop must break   False     for x in range 10           for y in range 10               print x y             if x y  gt  50                  x loop must break   True                 break         if x loop must break  break

User · Answer

for x in xrange 10       for y in xrange 10           print x y         if x y  gt  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

User · Answer

Use itertools product   from itertools import product for x  y in product range 10   range 10         do whatever you want     break   Here s a link to itertools product in the python documentation   http   docs python org library itertools html itertools product  You can also loop over an array comprehension with 2 fors in it  and break whenever you want to    gt  gt  gt    x  y  for y in   y1    y2   for x in   x1    x2            x1    y1      x2    y1          x1    y2      x2    y2

[python] Breaking out of nested loops

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