How to compare list and return match and non match in python?

Question

how to compare the 2 list of same length and return the matches and non matches elements have to append as a single element with space

l1=["a","b","c"]
l2=["a","d","c"]

result=[]
for i in l1:
    for j in l2:
        if i == j:
            match = i
            result.append(match)
        else:
            non_match = i + " "+ j
            result.append(non_match)

print(result)

Actual Output:
['a', 'a d', 'a c', 'b a', 'b d', 'b c', 'c a', 'c d', 'c']

Expected Output:
["a","b d","c"]

Answer 1

As long as order of the items in the output doesn't matter, you could do this:

Output = list(map(lambda x: " ".join(set(x)), zip(List1, List2)))

>>> Output
['a', 'd b', 'c']

The logic can be broken down at follows:

1: zip the two lists together:

# display the zipped lists:
>>> list(zip(List1, List2))
[('a', 'a'), ('b', 'd'), ('c', 'c')]

2: Turn each tuple in the resulting list into a set (to get unique values):

# display the result of calling set on the zipped lists
>>> list(map(set, zip(List1, List2)))
[{'a'}, {'d', 'b'}, {'c'}]

3: concatenate the members of each set with join

Output = list(map(lambda x: " ".join(set(x)), zip(List1, List2)))

Answer 2

Loop through both lists, appending a space & the List2 element if the corresponding elements are not equal to each other.

[List1[i] + (f" {List2[i]}" if List1[i] != List2[i] else '') for i in range(len(List1))]

I'll add @grind's answer as well for completeness. I think we both like it a bit better. As mentioned it doesn't need indexes and the formatting also includes the concatenation of left & right which I consider an improvement too.

[left if left == right else f'{left} {right'} for left, right in zip(List1, List2)]

The first one will throw an IndexError if the lengths of the two lists are different. The second will result in a new list which has a length equal to the shorter of the two input lists.

Answer 3

Other answers tell other methods, but I solve your issue. The issue with you code is because you are running through the python list completely two times. You can use zip function from python here. I have solved your code for you.

l1=["a","b","c"]
l2=["a","d","c"]

result=[]
for i,j in zip(l1, l2):
    if i == j:
        match = i
        result.append(match)
    else:
        non_match = i + " "+ j
        result.append(non_match)

print(result)

Answer 4

Try:

[' '.join((a, b)) if a != b else a
 for a, b in zip(l1, l2)]

zip(l1, l2) can let you iterate l1, l2 simultaneously, ''.join((a, b)) if a != b else a is a Conditional Expression which expresses what you want. Conditional Expression part can be evaluated to value, which will eventually aggregate into the result you want through the list comprehension.