python combine 'while loop' with 'for loop' to iterate through some data

Question

I am trying to combine a while loop with a for loop to iterate through some list but I am getting infinite loops. My code:

l=[0,2,3,4]
lo=0
for i in range(len(l)):
     while (True):
          lo+=1
     if lo+l[i]>40:
         break
     print(lo)

This code results in an endless loop. I want an output when the condition lo+ l[i] is greater than 40; it should stop looping and print the final lo output as result. I tried every method of indentation of the print line, but could not get what I wanted. Thanks in advance.

Solution 1

You get infinite loop because you wrote the infinite loop. You've probably thought that the break statement will somehow "magically" know that you don't want to end just the for loop, but also the while loop. But break will always break only one loop - the innermost one. So that means that your code actually does this:

while (True):               # <- infinite while loop
    lo += 1
    for i in range(len(l)): # <- for loop
        if not l[i] < 3:
            break           # <- break the for loop
        print(lo)
    # while loop continues

If you want to end both loops, you have to do it explicitly - for example, you can use a boolean variable:

keep_running = True
while (keep_running):
    lo += 1
    for i in range(len(l)):
        if not l[i] < 3:
            # this will effectively
            # stop the while loop:
            keep_running = False
            break
        print(lo)

Solution 2

You don't need the external infinite loop and you don't have to manage the index yourself (see enumerate in the documentation).

l = [0,1,2,3,4]
for index, value in enumerate(l):
    if value >= 3:
         break
    print(index)

I changed the condition if value not < 3: to if value >= 3: for improved readability.

Comments

[Chris]

Try go trough in on paper, step by step and you will see what really happens, or add logs/prints to see what happens in each step.

[Jamie Cockburn]

It's not really clear what you are trying to achieve. What output do you want?

[Mike Vella]

You should always try and avoid breaks, they make code generally harder to understand.

[jack]

i am trying to get the lo output as the l[i]>3 condition is not met.