在Python中同时向后和向前迭代两个列表

Question

I have two lists:

list1 = [2,1,0]
list2 = [0,1,2]

I have tried this one.

for i,j in zip(list1,list2):
    print (i,j) //This prints out 2 0, 1 1 and 0 2

But I want to print out 2 2, 1 1 and 0 0. So I have to loop the second loop backwards while I loop the first one forwards. I know how to do each of these in separate for loop , but I cannot figure out how to do this in one single for loop .

Answer 1

You're looking for the reversed function.

for i, j in zip(list1, reversed(list2):
    print(i, j)

Alternatively, depending on the data type, you can use fancy slicing . This generally works on data types that implement slicing (eg lists, tuples, numpy arrays), but won't work on a lot of other iterators.

for i, j in zip(list1, list2[::-1]):
    print(i, j)

The list2[::-1] slice means to slice the entire list with an increment of -1, which means that it will move backward through the list instead of forwards.

Answer 2

Here's another option, which is very basic, but may get the job done:

for i,j in enumerate(list1):
    print j, list2[-i-1]

This will work as expected under the assumption both lists are the same length, otherwise, it will simply iterate for the number of elements in the first list.