在Python中使用2个索引变量进行迭代

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

Answer 1

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))

Answer 2

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