嵌套for循环遍历文本文件中的每个组合

Question

Below is the code I have:

with open('test.txt') as f:
    for i in f:

        for j in f:

            pw_n= i+j
            print(pw_n)

Sample data from test.txt is:

USA
Canada
Mexico
China
Russia

Output Currently:

USA
CANADA

USA
MEXICO

USA
China

USA
Russia

The above is great but I would like it to go through all iterations. So, after it finishes USA, I would like it to go to Canada then Mexico, etc... Ex.:

Canada
USA

Canada
Canada

Canada
Mexico
..........

Any help regarding this would be great. I've tried to increment 'i' by 1 but that wasn't working.

Thanks!

Answer 1

Take a look at https://docs.python.org/3.4/library/itertools.html#itertools.permutations

>>> list(permutations(['USA', 'Canada', 'Mexico'], 2)) 
[
    ('USA', 'Canada'), 
    ('USA', 'Mexico'), 
    ('Canada', 'USA'), 
    ('Canada', 'Mexico'), 
    ('Mexico', 'USA'), 
    ('Mexico', 'Canada')
]

Answer 2

Here is one solution

with open('test.txt') as f:
    lines = f.readlines()

for i, val in enumerate(lines):
    for j in range(i+1, len(lines)):
        print("%s%s" %(val, lines[j]))
    print("")

Answer 3

If you want to stay as close as possible to your current code, you should use file.readlines() :

with open('test.txt') as f:
    lines = f.readlines()
    for i in lines:
        for j in lines:
            pw_n = i + j
            print(pw_n)

As an addition, I would recommend changing lines = f.readlines() to:

lines = [x.rstrip('\n') + '\n' for x in f.readlines()]

That list comprehension will force all words to end with a '\\n' (in your file Russia doesn't have any '\\n' at the end and so it is shown differently on the output, this fixes that).

If you are looking for a better code you should use itertools.permutations(iterable, r=None) , where r is the number of items per combinations (in this case we need 2)

from itertools import permutations
with open('test.txt') as f:
    for x, y in permutations(f.readlines(), 2):
        print(x + y)

You may also improve this code by adding:

for x, y in permutations((x.rstrip('\n') + '\n' for x in f.readlines()), 2):

Here a list comprehension isn't needed, instead, we can use a generator expression, which is more memory friendly.

Since you are iterating over each permutation inside a for loop, in no moment you need to store it as a list, that means you don't need to store on memory all the permutations at the same time, instead, they are calculated as you use them. The same applies to the generator expression, you don't store it in opposition with the list comprehension from the first code.