Iterate with 2 Indexes Variable in Python

Question

Is there a better or cleaner way to achieve the results below? I have 4 conditions and half of the code is filled with "while-loop", i += 1, and j += 1.

i = 0
j = 0
limit1 = 2
limit2 = 4

while (i < limit1 and j < limit2):
    lists.append ('First index: %d, Second index: %d' % (i, j))
    j += 1
    lists.append ('First index: %d, Second index: %d' % (i, j))
    i += 1
    j += 1

for i in lists:
    print (i)

Results:

First index: 0  Second index: 0
First index: 0  Second index: 1
First index: 1  Second index: 2
First index: 1  Second index: 3

The name lists says that that variable consists of multiple lists, but it doesn't. It's just a list of ints. It'd probably be a good idea to use a different name. — user2357112
– user2357112, Commented Jun 17, 2015 at 1:57
@user2357112 Well, it's a list of strings that could possibly be described as "lists". — jangler
– jangler, Commented Jun 17, 2015 at 2:03
Whoops, list of strings, not list of ints. In any case, calling strings lists is a bad idea; they're not lists, and they behave very differently from lists. — user2357112
– user2357112, Commented Jun 17, 2015 at 2:04

user2357112 · Accepted Answer · 2015-06-17 02:14:17Z

2

It's quite straightforward to just compute j from i:

for i in xrange(limit1):
    l.append('First index: %d, Second index: %d' % (i, 2*i))
    l.append('First index: %d, Second index: %d' % (i, 2*i+1))

This assumes limit2 is twice limit1. If that isn't always the case, you can add an additional check:

for i in xrange(limit1):
    if 2*i >= limit2:
        break
    l.append('First index: %d, Second index: %d' % (i, 2*i))
    l.append('First index: %d, Second index: %d' % (i, 2*i+1))

or compute which limit to use up front:

for i in xrange(min(limit1, (limit2 + 1)//2)):

though as you can see, the limit computation may be error-prone.

Note that if limit2 isn't a multiple of 2, your code may emit an entry for j == limit2:

>>> lists = []
>>> i = 0
>>> j = 0
>>> limit1 = 2
>>> limit2 = 3
>>> while (i < limit1 and j < limit2):
...     lists.append ('First index: %d, Second index: %d' % (i, j))
...     j += 1
...     lists.append ('First index: %d, Second index: %d' % (i, j))
...     i += 1
...     j += 1
...
>>> for i in lists:
...     print (i)
...
First index: 0, Second index: 0
First index: 0, Second index: 1
First index: 1, Second index: 2
First index: 1, Second index: 3

If this isn't desired, we can rearrange the loop to go by j instead of i:

for j in xrange(min(limit2, limit1*2)):
    l.append('First index: %d, Second index: %d' % (j//2, j))

edited Jun 17, 2015 at 2:14

answered Jun 17, 2015 at 1:55

user2357112

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3 Comments

dting Over a year ago

This doesn't necessarily break when j is greater than limit2.

user2357112 Over a year ago

@DTing: Indeed. It assumes that limit2 == 2*limit1. I can make that assumption explicit and add a version where that assumption isn't made.

dreamzboy Over a year ago

This is exactly what I'm looking for. Thank you for the detail break down. Time to review those Summation class materials.

jangler · Accepted Answer · 2015-06-17 02:01:47Z

1

lists = ['First index: %d, Second index %d' % (j//2, j) for j in range(limit2) if j//2 < limit1]

edited Jun 17, 2015 at 2:01

answered Jun 17, 2015 at 1:51

jangler

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3 Comments

user2357112 Over a year ago

This does something completely different from what's desired.

jangler Over a year ago

You're right. I guess I'm not sure why what's desired is desired, but I'll edit my answer.

user2357112 Over a year ago

Better. It's important to note that if limit2 isn't a multiple of 2, this code won't emit an entry for j == limit2, while the question's code often will. Whether this is a bug in the original code is something only the questioner can tell us.

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