Swap two lists in python

Question

I have a list of 2 lists, which are of equal size, in python like:

list_of_lists = [list1, list2]

In a for loop after doing some processing on both list1 and list2, I have to swap them so that list1 becomes list2 and list2 becomes a list initialized to all zeros. So at the end of the iteration the list_of_lists has to look like:

list_of_lists = [list1 which has contents of list2, list2 which has all zeros]

In C, one could just copy the pointers of list2 and list1 and then point list2 to a list initialized to all zeros. How do I do this in python ?

unutbu · Accepted Answer · 2013-01-02 05:01:43Z

9

It sounds like you are mainly working with list1 and list2 inside the loop. So you could just reassign their values:

list1 = list2
list2 = [0]*len(list2)

Python also allows you to shorten this to a one-liner:

list1, list2 = list2, [0]*len(list2)

but in this case I find the two-line version more readable. Or, if you really want list_of_lists, then:

list_of_lists = [list2, [0]*len(list2)]

or if you want both:

list1, list2 = list_of_lists = [list2, [0]*len(list2)]

edited Jan 2, 2013 at 5:01

answered Jan 2, 2013 at 4:39

unutbu

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ATOzTOA · Accepted Answer · 2013-01-02 04:40:41Z

1

Like this...

list_of_lists = [list_of_lists[1], []]

for i in range(count):
    list_of_lists[1].append(0)

answered Jan 2, 2013 at 4:40

ATOzTOA

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Dave · Accepted Answer · 2021-06-07 20:47:20Z

1

list_of_lists=[ list_of_lists[1], [0,]*len(list_of_lists[1]) ]

The cost of the swap is the same as the pointer swap you mentioned

edited Jun 7, 2021 at 20:47

answered Jan 2, 2013 at 5:08

Dave

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