Python - C program for loop equivalent

Question

I was using the following for loop in my C program:

for (i = 0; i < 5; i++) {
  for (j = i + 1; j < 5; j++) {
    //some operation using the index values
  }
}

What would be the python equivalent for the second for loop, (j = i + 1)? I tried the following but there is an error:

for indexi, i in enumerate(list):
    for indexj = indexi + 1, j in enumerate(list):

How to do it?

Side-note: Don't name your variable list (or any other built-in name). That's just asking for trouble. My answer used mylist as a safe placeholder. — ShadowRanger
– ShadowRanger, Commented Nov 11, 2015 at 9:16
another side-note: rather than stating that "there is an error" it can often be quite helpful to post that error. — umläute
– umläute, Commented Nov 11, 2015 at 9:20
Overwriting the name of a standard-type makes it unreachable for other calls. Do not! — too honest for this site
– too honest for this site, Commented Nov 11, 2015 at 10:28

ShadowRanger · Accepted Answer · 2015-11-11 09:21:15Z

6

If you're trying to get the actual index values for some reason (and/or need to control the inner loop separate from the outer), you'd do:

for i in range(len(mylist)):
    for j in range(i+1, len(mylist)):

# Or for indices and elements:
for i, x in enumerate(mylist):
    for j, y in enumerate(mylist[i+1:], start=i+1):

But if what you really want is unique non-repeating pairings of the elements, there is a better way, itertools.combinations:

import itertools

for x, y in itertools.combinations(mylist, 2):

That gets the values, not the indices, but usually, that what you really wanted. If you really need indices too, you can mix with enumerate:

for (i, x), (j, y) in itertools.combinations(enumerate(mylist), 2):

Which gets the exact index pattern you're looking for, as well as the values. You can also use it to efficiently produce the indices alone as a single loop with:

for i, j in itertools.combinations(range(len(mylist)), 2):

Short answer: itertools is magical for stuff like this.

edited Nov 11, 2015 at 9:21

answered Nov 11, 2015 at 9:10

ShadowRanger

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1 Comment

ShadowRanger Over a year ago

FYI, I followed the convention of i and j referring to indices, while x and y refer to values. Your original question uses i and j for the values, which will trip up maintainers. Obviously, meaningful names are best (and i and j might be better made ix and iy to indicate the index relationship with x and y), but for example code, I'll stick with conventional one-letter names.

Daniel Roseman · Accepted Answer · 2015-11-11 09:11:13Z

3

If you're really just interested in the indexes, you can use range rather than enumerate.

for i in range(5):
    for j in range(i+1, 5):
        print i, j

edited Nov 11, 2015 at 9:11

answered Nov 11, 2015 at 9:09

Daniel Roseman

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