conditionally stop iteration of for loop in python

Question

In a typical C-like language a for loop gives you more control over iteration. I'm wondering how to do the equivilent

for(int i = 0; i < A.length; i++) {
  do_things(A[i]);
  if (is_true(i)) {
    i--;
  }
}

in Python?

In other languages I opt to use their container based for loop constructs, but they typically have vanilla for loops that I can use for situations like this. How do I get more "control" when iterating in Python?

I imagine the answer to this is very basic, but the search terms are muddied with other questions.

You have to use the equivalent while-loop, or write an iterator with this capability. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Aug 24, 2017 at 23:53
I recommend that you play with the Python iteration capabilities. As many people have learned, you can alter the iteration variable within the loop. However, evaluation of array size and other side effects may surprise you, as Python has some different optimizations and variable handling. — Prune
– Prune, Commented Aug 24, 2017 at 23:56
Also, please note that making such changes in the iteration parameter is a dangerous practice. In the development groups I've joined over the past 10-15 years, the code you posted would fail review. — Prune
– Prune, Commented Aug 24, 2017 at 23:58
Your example doesn't stop the iteration, that's done with a break statement, which exists in Python. — Barmar
– Barmar, Commented Aug 24, 2017 at 23:59
@2c2c Use continue instead then. Decrementing i is the wrong action. — Chris
– Chris, Commented Aug 25, 2017 at 0:17

Chris · Accepted Answer · 2017-08-25 00:40:03Z

1

The best equivalent in Python would be a while-loop:

i = 0
while i < A.length: # If `A` is a regular Python container type, use `len()`
    do_things(A[i])
    if  is_true(i):
        i -= 1
    i += 1

Note however, as said in the comments, iterating like this over a container is more often than not, a bad idea. You should review your code and make sure you actually need this behavior.

EDIT

It's just a sloppy explanation of a loop that doesnt continue to the next element until some condition is met. I didn't explain clear enough I guess

Use continue instead then. Decrementing i is the wrong action:

i = 0
while i < A.length:
    do_things(A[i])
    if not is_true(i):
        continue
    i += 1

Better yet, you can ditch the while-loop all together and use enumerate:

for i, el in enumerate(A):
    if not is_true(el):
        continue
    # do work

edited Aug 25, 2017 at 0:40

answered Aug 24, 2017 at 23:56

Chris

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2 Comments

mgilson Over a year ago

A.length won't usually work (too much Javascript?). You probably meant len(A) :-)

Chris Over a year ago

@mgilson Well. not exactly. I wasn't exact;y sure what A was. Whether it was a custom type, or a simple array. I opted to keep accuracy to the original code while translating, but your probably right. So I'll ad a note.

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