How to break out of multiple loops?

Question

Given the following code (that doesn\'t work):

while True:
    #snip: print out current state
    while True:
        ok = get_input(\"Is this ok? (y/n)\")
          


        

Answer 1

You can define a variable( for example break_statement ), then change it to a different value when two-break condition occurs and use it in if statement to break from second loop also.

while True:
    break_statement=0
    while True:
        ok = raw_input("Is this ok? (y/n)")
        if ok == "n" or ok == "N": 
            break
        if ok == "y" or ok == "Y": 
            break_statement=1
            break
    if break_statement==1:
        break

Answer 2

keeplooping=True
while keeplooping:
    #Do Stuff
    while keeplooping:
          #do some other stuff
          if finisheddoingstuff(): keeplooping=False

or something like that. You could set a variable in the inner loop, and check it in the outer loop immediately after the inner loop exits, breaking if appropriate. I kinda like the GOTO method, provided you don't mind using an April Fool's joke module - its not Pythonic, but it does make sense.

Answer 3

Factor your loop logic into an iterator that yields the loop variables and returns when done -- here is a simple one that lays out images in rows/columns until we're out of images or out of places to put them:

def it(rows, cols, images):
    i = 0
    for r in xrange(rows):
        for c in xrange(cols):
            if i >= len(images):
                return
            yield r, c, images[i]
            i += 1 

for r, c, image in it(rows=4, cols=4, images=['a.jpg', 'b.jpg', 'c.jpg']):
    ... do something with r, c, image ...

This has the advantage of splitting up the complicated loop logic and the processing...

Answer 4

There is a hidden trick in the Python while ... else structure which can be used to simulate the double break without much code changes/additions. In essence if the while condition is false, the else block is triggered. Neither exceptions, continue or break trigger the else block. For more information see answers to "Else clause on Python while statement", or Python doc on while (v2.7).

while True:
    #snip: print out current state
    ok = ""
    while ok != "y" and ok != "n":
        ok = get_input("Is this ok? (y/n)")
        if ok == "n" or ok == "N":
            break    # Breaks out of inner loop, skipping else

    else:
        break        # Breaks out of outer loop

    #do more processing with menus and stuff

The only downside is that you need to move the double breaking condition into the while condition (or add a flag variable). Variations of this exists also for the for loop, where the else block is triggered after loop completion.

Answer 5

Solutions in 2 Ways

With an example: Are these two matrices equal/same?
matrix1 and matrix2 are same size, n, 2 dimentional matrices.

First Solution, without a function

same_matrices = True
inner_loop_broken_once = False
n = len(matrix1)

for i in range(n):
    for j in range(n):

        if matrix1[i][j] != matrix2[i][j]:
            same_matrices = False
            inner_loop_broken_once = True
            break

    if inner_loop_broken_once:
        break

---

Second Solution, with a function
This is the final solution for my case

def are_two_matrices_the_same (matrix1, matrix2):
    n = len(matrix1)
    for i in range(n):
        for j in range(n):
            if matrix1[i][j] != matrix2[i][j]:
                return False
    return True

Have a nice day!

Answer 6

# this version breaks up to a certain label

break_label = None
while True:
    # snip: print out current state
    while True:
        ok = get_input("Is this ok? (y/n)")
        if ok == "y" or ok == "Y":
            break_label = "outer"   # specify label to break to
            break
        if ok == "n" or ok == "N":
            break
    if break_label:
        if break_label != "inner":
            break                   # propagate up
        break_label = None          # we have arrived!
if break_label:
    if break_label != "outer":
        break                       # propagate up
    break_label = None              # we have arrived!

#do more processing with menus and stuff