python中的嵌套循环产生笛卡尔积

Question

I am trying to loop through two list in python to get a output like this.

list1 = [a, b, c]
list2 = [1, 2, 3]

So it should loop for each value of list1 over list 2. list1[a] will loop 3 times over list2 and so on till it reaches c. However, I am not getting the desired output.

i=''
list1 = ['ABC ' + str(i)+ ' '+'1' + ' ' + '5', 'CDE '+ str(i)+ ' '+  '1'+ ' ' + '5']
list2=['123','234','456']
for c in list1:
         for i in list2:
               print c

Output:

ABC  1 5
ABC  1 5
ABC  1 5
CDE  1 5
CDE  1 5
CDE  1 5

Desired out put should be like below:

ABC 123 1 5
CDE 123 1 5
ABC 234 1 5
CDE 234 1 5
ABC 456 1 5
CDE 456 1 5

Please guide me what I am missing here, am new with this.

Answer 1

You need to interpolate the list2 value inside the loop. You could the itertools.product() function instead of nesting your loops; do swap the order of the lists if you want the values of list1 to alterate:

from itertools import product

list1 = ['ABC {} 1 5', 'CDE {} 1 5']
list2 = ['123','234','456']
for l2, l1 in product(list2, list1):
    print l1.format(l2)

The str.format() method fills the {} in the list1 strings with your l2 values, letting you avoid having to rebuild list2 from scratch each time:

>>> for l2, l1 in product(list2, list1):
...     print(l1.format(l2))
...
ABC 123 1 5
CDE 123 1 5
ABC 234 1 5
CDE 234 1 5
ABC 456 1 5
CDE 456 1 5

Another alternative is for list1 to contain tuples of values to be used, and only when printing you convert everything to one string by using print() as a function (Python 3, or a from __future__ import in Python 2):

from __future__ import print_function

list1 = [('ABC', 1, 5), ('CDE', 1, 5)]
list2 = ['123','234','456']
for l2, (a, b, c) in product(list2, list1):
    print(a, l2, b, c)

It depends on what else you want to do with the values produced.

Note that if the 1 and 5 numbers are otherwise unchanged between the values of list1 , there is no need at all to do a complicated dance with string templates or tuples. You'd just reference those inside the loop:

b, c = 1, 5
list1 = ['ABC', 'CDE']
list2 = ['123','234','456']
for l2, l1 in product(list2, list1):
    print '{} {} {} {}'.format(l1, l2, b, c)

I still used string formatting just to show another form of combining several values into one string.

Answer 2

You need to move the construction of list1 into your for loop. Reorganize your lines as follows:

list2 = ['123', '234', '456']

for i in list2:
    list1 = ['ABC ' + str(i) + ' ' + '1' + ' ' + '5',
             'CDE ' + str(i) + ' ' + '1' + ' ' + '5']
    for c in list1:
        print c

this will print

ABC 123 1 5
CDE 123 1 5
ABC 234 1 5
CDE 234 1 5
ABC 456 1 5
CDE 456 1 5

That being said, this can be improved using string formatting :

for i in list2:
    list1 = ['ABC {0} 1 5', 'CDE {0} 1 5']
    for c in list1:
        print c.format(i)