嵌套循环检查两个或多个列表项的pythonic方法

Question

I want to check items against two lists in python which are again put in the one big list In my codes , combinedList is big list and row1 and row2 are sub-list.

I need to check items in row1 and row2 against each other. I got rough idea in psudo code however , since im new to python . is there any good codes for checking against two list for their item without repeating the same pair more than once?

row1 = [a,b,c,d,....]
row2 = [s,c,e,d,a,..]

combinedList = [row1 ,row2]

for ls in combinedList:
        **for i=0 ; i < length of ls; i++
            for j= i+1 ; j <length of ls; j++
                do something here item at index i an item at index j**

Answer 1

I guess you're looking for itertools.product :

>>> from itertools import product
>>> row1 = ['a', 'b', 'c', 'd']
>>> row2 = ['s', 'c', 'e', 'd', 'a']
>>> seen = set()             #keep a track of already visited pairs in this set
>>> for x,y in product(row1, row2):
        if (x,y) not in seen and (y,x) not in seen:
            print x,y
            seen.add((x,y))
            seen.add((y,x))
...         
a s
a c
a e
a d
a a
b s
b c
b e
b d
b a
c s
c c
c e
c d
d s

Update:

>>> from itertools import combinations
>>> for x,y in combinations(row1, 2):
...     print x,y
...     
a b
a c
a d
b c
b d
c d

Answer 2

Use the zip() built-in function to pair values of two lists:

for row1value, row2value in zip(row1, row2):
    # do something with row1value and row2value

If you wanted to combine each element from row1 with each element of row2 instead (the product of the two lists), use itertools.product() instead:

from itertools import product

for row1value, row2value in product(row1, row2):
    # do something with row1value and row2value

zip() simply pairs up the lists producing len(shortest_list) items, product() pairs up each element in one list with each element in the other, producing len(list1) times len(list2) items:

>>> row1 = [1, 2, 3]
>>> row2 = [9, 8, 7]
>>> for a, b in zip(row1, row2):
...     print a, b
... 
1 9
2 8
3 7
>>> from itertools import product
>>> for a, b in product(row1, row2):
...     print a, b
... 
1 9
1 8
1 7
2 9
2 8
2 7
3 9
3 8
3 7