使用map函数在列表中的奇数位置获取列表元素

Question

This is the code that I have been trying. It reads a .txt file which contains two lists. which are respectively read by D1 and D2. In D2, I need the elements at odd position only. The code that I have tried is given below and it doesn't give me the required output. Here is the code:

import cv2
import numpy


fo = open("Test_input0.txt","r")
D1=[map(int, fo.readline().split())]
D2=[map(int, fo.readline().split())]
d2 = D2[1::2]


print d2

The output it gives is:

[ ]

The .txt file content is in the following format:

1 2 1 3 7 6 2 9 8 1 5 4
5 10 6 11 15 15                

Answer 1

map() returns a list, which you then wrap in another list with [] . When you try

d2 = D2[1::2]

you are trying to slice from index 1 a list containing a single item. Therefore you will get an empty list returned.

You can correct it by removing the surrounding [] :

D1= map(int, fo.readline().split())
D2= map(int, fo.readline().split())

Or, generally considered to be more Pythonic, a list comprehension:

D1 = [int(x) for x in fo.readline().split()]
D2 = [int(x) for x in fo.readline().split()]

Now D2[1::2] should give you the items in D2 at odd indexes.

You can even combine both operations in one list comprehension:

D2 = [int(x) for x in fo.readline().split()[1::2]]

which has the advantage of converting only the required values to integers.

Answer 2

This should work:

fo = open("Test_input0.txt","r")
D1 = [int(x) for x in next(fo).split()]
D2 = [int(x) for x in next(fo).split()]
D2 = D2[1::2]

The only problem you had is the extra [] :

Change:

D1=[map(int, fo.readline().split())]

into:

D1 = map(int, fo.readline().split())

And it should work too.