CODEWITHSUNDEEP
python
rookie_senior
rookie_senior

Reputation: 89

Compare occurrences of shared items between lists

Ok, for a fun project I'm working on in order to learn some python I'm hitting a wall with what should be a basic task: I need to compare lists for the times items shared among the lists occur in each list. Using shared_items = set(alist).intersection(blist) gives me the items sharedbetwen the lists, but it does not tell me, how often those items occur in each list. I tried loops like this for example:

def the_count(alist,blist):
    c = 0
    for x in alist:
        for y in blist:
            if x == y:
                c += 1
    return c

but that doesn't do the trick.

Another attempt was to use Counter:

c = Counter(alist)
b = Counter(blist)

But trying to loop over the Counter results failed too, last try was

a = Counter(alist)
b = Counter(blist)
for key, val in a:
   if key in b:
    val1 = b[key]
    if val < val1:
        print b[key]
    else:
        print a[key]

Upvotes: 1

Views: 1002

Answers (4)

Fabio Teixeira
Fabio Teixeira

Reputation: 76

Using list (dict) as jrd1 pointed comprehension:

>>> list1 = [0, 1, 2, 3, 1, 2, 3, 4, 3, 2]
>>> list2 = [1, 4, 3, 5, 2, 1, 0, 2, 7, 8]
>>> {i:(list1.count(i), list2.count(i)) for i in set(list1) & set(list2)}
{0: (1, 1), 1: (2, 2), 2: (3, 2), 3: (3, 1), 4: (1, 1)}

Upvotes: 1

Bharadwaj Yarlagadda
Bharadwaj Yarlagadda

Reputation: 61

Best way is to get the unique items from two lists and check the count of those distinct numbers in each list.

for distinct_num in set(alist + blist):
    print(alist.count(distinct_num))
    print(blist.count(distinct_num))

Upvotes: -1

jrd1
jrd1

Reputation: 10726

You almost had it using the set intersection. Since that gives you the common elements amongst both lists, all you have to do now is loop over that and count the elements. One way could be:

list1 = [0, 1, 2, 3, 1, 2, 3, 4, 3, 2]
list2 = [1, 4, 3, 5, 2, 1, 0, 2, 7, 8]
shared = set(list1).intersection(list2)

# Now, loop over the elements and create a dictionary using a generator.
# The key will be the shared element, and the value would be a tuple
# which corresponds to the counts of the first list and the second list, respectively
counts = {num:(list1.count(num), list2.count(num)) for num in shared}

counts now contains:

{
    0: (1, 1), 
    1: (2, 2), 
    2: (3, 2), 
    3: (3, 1), 
    4: (1, 1)
}

This can further be abstracted into a function similar to:

def count_shared_elements(list1, list2):
    shared = set(list1).intersection(list2)
    return {num:(list1.count(num), list2.count(num)) for num in shared}

Upvotes: 2

zython
zython

Reputation: 1288

Take a look at the answers linked in the question comments, another way to do this would be like this:

for a in alist:
    c+= blist.count(a)

Upvotes: -1

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