Comparing two lists in Python

Question

So to give a rough example without any code written for it yet, I’m curious on how I would be able to figure out what both lists have in common.

Example:

listA = ['a', 'b', 'c']
listB = ['a', 'h', 'c']

I’d like to be able to return:

['a', 'c']

How so?

Possibly with variable strings like:

john = 'I love yellow and green'
mary = 'I love yellow and red'

And return:

'I love yellow and'

Asked By: Matthew

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Source

Answer 1

Use set intersection for this:

list(set(listA) & set(listB))

gives:

['a', 'c']

Note that since we are dealing with sets this may not preserve order:

' '.join(list(set(john.split()) & set(mary.split())))
'I and love yellow'

using join() to convert the resulting list into a string.

—

For your example/comment below, this will preserve order (inspired by comment from @DSM)

' '.join([j for j, m in zip(john.split(), mary.split()) if j==m])
'I love yellow and'

For a case where the list aren’t the same length, with the result as specified in the comment below:

aa = ['a', 'b', 'c']
bb = ['c', 'b', 'd', 'a']

[a for a, b in zip(aa, bb) if a==b]
['b']

Answered By: Levon

Answer 2

If the two lists are the same length, you can do a side-by-side iteration, like so:

list_common = []
for a, b in zip(list_a, list_b):
    if a == b:
        list_common.append(a)

Answered By: CosmicComputer

Answer 3

Intersect them as sets:

set(listA) & set(listB)

Answered By: Ry-

Answer 4

i think this is what u want ask me anything about it i will try to answer it

    listA = ['a', 'b', 'c']
listB = ['a', 'h', 'c']
new1=[]
for a in listA:
    if a in listB:
        new1.append(a)

john = 'I love yellow and green'
mary = 'I love yellow and red'
j=john.split()
m=mary.split()
new2 = []
for a in j:
    if a in m:
        new2.append(a)
x = " ".join(new2)
print(x)

Answered By: Margouma Saleh

Answer 5

If order doesn’t matter, but you want to print out a nice diff:


def diff(a, b):
  if len(set(a) - set(b)) > 0:
    print(f"Items removed: {set(a) - set(b)}")
  if len(set(b) - set(a)) > 0:
    print(f"Items added: {set(b) - set(a)}")
  if set(a) == set(b):
    print(f"They are the same")

diff([1,2,3,4], [1,2,3])
# Items removed: {4}

diff([3,4], [1,2,3])
# Items removed: {4}
# Items added: {1, 2}

diff([], [1,2,3])
# Items added: {1, 2, 3}

diff([1,2,3], [1])
# Items removed: {2, 3}

listA = ['a', 'b', 'c']
listB = ['a', 'h', 'c']
diff(listA, listB)
# Items removed: {'b'}
# Items added: {'h'}

john = 'I love yellow and green'
mary = 'I love yellow and red'
diff(john, mary)
# Items removed: {'g'}

Answered By: ubershmekel

Comparing two lists in Python

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