comparing two lists and finding indices of changes

Question

I’m trying to compare two lists and find the position and changed character at that position. For example, these are two lists:

list1 = ['I', 'C', 'A', 'N', 'R', 'U', 'N']
list2 = ['I', 'K', 'A', 'N', 'R', 'U', 'T']

I want to be able to output the position and change for the differences in the two lists. As you can see, a letter can be repeated multiple times at a different index position. This is the code that I have tried, but I can’t seem to print out the second location accurately.

for indexing in range(0, len(list1)):
    if list1[indexing] != list2[indexing]:
        dontuseindex = indexing
        poschange = indexing + 1
        changecharacter = list2[indexing]
for indexingagain in range(dontuseindex + 1, len(list1)):
    if list1[indexingagain] != list2[indexingagain]:
        secondposchange = indexingagain + 1
        secondchangecharacter = list2[indexingagain]

Is there a better way to solve this problem or any suggestions to the code I have?

My expected output would be:

2    K
7    T

Asked By: interstellar

||

Source

Answer 1

for index, (first, second) in enumerate(zip(list1, list2)):
    if first != second:
        print(index, second)

Output:

1 K
6 T

If you want the output you gave, we need to count from 1 instead of the usual 0:

for index, (first, second) in enumerate(zip(list1, list2), start=1):

Answered By: Peter Wood

Answer 2

Another possibility to save all the not-equal elements with the index is with a list comprehensions:

list1 = ['I', 'C', 'A', 'N', 'R', 'U', 'N']
list2 = ['I', 'K', 'A', 'N', 'R', 'U', 'T']

# Append index, element1 and element2 as tuple to the list if they are not equal
changes = [(i, list1[i], list2[i]) for i in range(len(list1)) if list1[i] != list2[i]]
print(changes)
#prints [(1, 'C', 'K'), (6, 'N', 'T')]

Not exactly what you specified as output but it’s close.

You could print the specified output with a loop:

for i in changes:
    print(i[0] + 1, i[1])
# 2 K
# 7 T

In the comments several alternative ways of designing the list comprehension were suggested:

Using enumerate and zip:

changes = [(i, e1, e2) for i, (e1, e2) in enumerate(zip(list1, list2)) if e1 != e2]

Using enumerate with start index and zip:

changes = [(i, e1, e2) for i, (e1, e2) in enumerate(zip(list1, list2), 1)  if e1 != e2]

Using zip and itertools.count:

import itertools
changes = [(i, e1, e2) for i, e1, e2 in zip(itertools.count(), list1, list2)) if e1 != e2]

Using zip and itertools.count with start-index:

changes = [(i, e1, e2) for i, e1, e2 in zip(itertools.count(1), list1, list2)) if e1 != e2]

All of them producing the same result as the original but using different (better) python features.

Answered By: MSeifert

Answer 3

list1 = ['I', 'C', 'A', 'N', 'R', 'U', 'N']
list2 = ['I', 'K', 'A', 'N', 'R', 'U', 'T']
[i for i, x in enumerate(zip(list1,list2)) if x[0]!=x[1]]

Output:

[1, 6]

Answered By: Sam S.

comparing two lists and finding indices of changes

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