Comparing two lists element-wise in python

Question

I have two list

first= (1,2,3,4,5,6)
last=(6,5,4,3,2,1)

I need to compare the corresponding values only. I have used the below code and getting 36 results as the 1st element in first is comparing with all the six elements of last list.

for x in first:
    for y in last:
        if x>y:
            print("first is greater then L2",y)
        elif x==y:
            print("equal")
        else:
            print("first is less then L2",y)

irst= (1,2,3,4,5,6)
last=(6,5,4,3,2,1)
for x in first:
    for y in last:
        if x>y:
            print("first is greater then L2",y)
        elif x==y:
            print("equal")
        else:
            print("first is less then L2",y)

output:

L1 is less then L2 6
L1 is less then L2 5
L1 is less then L2 4
L1 is less then L2 3
L1 is less then L2 2
go dada
L1 is less then L2 6
L1 is less then L2 5
L1 is less then L2 4
L1 is less then L2 3
go dada
L1 is greater then L2 1
L1 is less then L2 6
L1 is less then L2 5
L1 is less then L2 4
go dada
L1 is greater then L2 2
L1 is greater then L2 1
L1 is less then L2 6
L1 is less then L2 5
go dada
L1 is greater then L2 3
L1 is greater then L2 2
L1 is greater then L2 1
L1 is less then L2 6
go dada
L1 is greater then L2 4
L1 is greater then L2 3
L1 is greater then L2 2
L1 is greater then L2 1
go dada
L1 is greater then L2 5
L1 is greater then L2 4
L1 is greater then L2 3
L1 is greater then L2 2
L1 is greater then L2 1
y

I need results by comparing the corresponding elements only. Which means there should be only six outputs.

This is why R beats python. ```x = c(1, 2, 3) x[x %in% c(2, 3)] = (2, 3)``` — Frank, Mar 03 '21 at 19:07

glhr · Accepted Answer · 2019-05-04T19:39:28.787

12

first and last are tuples, not lists (lists elements are within square brackets like [1,2,3]).

You can use zip(first,last) to create a list of pairs from the two tuples:

[(1, 6), (2, 5), (3, 4), (4, 3), (5, 2), (6, 1)]

then iterate over the tuples and compare each pair:

first = (1,2,3,4,5,6)
last = (6,5,4,3,2,1)

for l1,l2 in zip(first,last):
    if l1 < l2:
        print("l1 < l2")
    elif l1 > l2:
        print("l2 > l1")
    elif l1 == l2:
        print("l1 == l2")

Output:

l1 < l2
l1 < l2
l1 < l2
l2 > l1
l2 > l1
l2 > l1

Another approach would be to iterate over the indices, however this approach is less Pythonic:

for i in range(len(first)):
    l1 = first[i]
    l2 = last[i]
    if l1 < l2:
        print("l1 < l2")
    elif l1 > l2:
        print("l2 > l1")
    elif l1 == l2:
        print("l1 == l2")

edited May 04 '19 at 19:39

answered May 04 '19 at 19:33

glhr

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If you want to know if they are all equal all( [l[0] == l[1] for l in list(zip(a,b))] ) == True. Explanation: inside to out. zip together, then turn the zip into a list of tuples. Compare them using list comprehension. Finally the all checks that they are ALL equal – brian_ds Mar 12 '21 at 22:31
if I have a case where the lists are of different length, how do i cycle through the iteration by going back to the first element of the short list to match the next element of the long list? – Shanely Jul 23 '21 at 06:04

score 3 · Answer 2 · answered May 04 '19 at 19:33

3

You should combine the two tuples into a list of two-element, pairwise tuples with zip:

for x, y in zip(first, last):
    if x < y: ... # Your code

answered May 04 '19 at 19:33

DYZ

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Comparing two lists element-wise in python

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