我该如何使用列表理解功能？

Question

a='abcdcdc'
list_=[x*3 for x in a]
print list_

This is printing this OK!!!! :

['aaa', 'bbb', 'ccc', 'ddd', 'ccc', 'ddd', 'ccc']

But How can i print this ? :

['abc','cdc','cdc']

Answer 1

You can use:

>>> [a[i:i+3] for i in range(0,len(a) - 1,2)]
['abc', 'cdc', 'cdc']

This means i will get the following values:

>>> list(range(0,len(a) - 1,2))
[0, 2, 4]

(as the list comprehension loops over i ). This means that range(0,len(a) - 1,2) starts at 0 and in increments of 2 it'll increase until it reaches the highest value but no more than len(a) - 1 . There is a longer explanation in the Python 3 documentation about range() which can be helpful to read.

This means it'll slice a into the desired segments (the notation a[i:j] means it'll take the slice from i to j of a ). It'll first take the slice a[0:0 + 3] , then the slice a[2:2 + 3] and lastly the slice a[4:4 + 3] which are the strings that you're looking for.

We're subtracting 1 from the length of a as otherwise we don't end up with the result that we want:

>>> [a[i:i+3] for i in range(0,len(a),2)]
['abc', 'cdc', 'cdc', 'c']

Answer 2

[a[i]+a[i+1]+a[i+2] for i in range(0,len(a)-1,2)]

Answer 3

Using list comprehension here might actually be counter to the zen of Python ("simple is better than complex").

Though the other list comprehensions answers are correct, it takes a while to understand why they work.

On the other hand, a code like the following isn't hard to figure out at first glance:

a='abcdcdc'

for i in range(0, len(a), 2):
    b = a[i:i+3]
    if len(b) == 3:
        print(b)