CODEWITHSUNDEEP
pythonlist
user1855165
user1855165

Reputation: 289

Python list of list count elements

Hello I am trying to count all the elements in a list of lists. For example 

a = [[1,2],[3,4],[10,3,8]]

I want to return:

7

I tried count and size with no luck. Thanks in advance.

Upvotes: 1

Views: 2921

Answers (6)

Saeed
Saeed

Reputation: 2089

You can flatten that list and then use len() on the flattened list:

a = [[1,2],[3,4],[10,3,8]]

flattened_a = [item for sublist in a for item in sublist]

len(flattened_a)

Upvotes: 0

Fuji Komalan
Fuji Komalan

Reputation: 2047

list_count = 0

mainList = [[1,2],[3,4],[10,3,8]]
for inner_list in mainList:
  list_count += len(inner_list)

print(list_count)

Upvotes: 0

thefourtheye
thefourtheye

Reputation: 239443

a = [[1,2],[3,4],[10,3,8]]
print(sum(map(len, a)))

Output

7

This can be written as a generator expression, like this

print(sum(len(item) for item in a))

The simplest method which would work even for multilevel nested lists, goes like this

def get_size(current_item):
    if isinstance(current_item, list):
        return sum(get_size(item) for item in current_item)
    else:
        return 1

a = [[1,2],[3,4],[10,3,8],[[1, 2, 3], [2, 3]]]
print get_size(a)    # 12

Upvotes: 8

inspectorG4dget
inspectorG4dget

Reputation: 113915

def myLen(L):
    if not L:
        return 0
    elif not isinstance(L[0], list):
        return 1 + myLen(L[1:])
    else:
        return myLen(L[0]) + myLen(L[1:])

Output:

>>> myLen([[1,2],[3,4],[10,3,8]])
7

Upvotes: 0

Adam Smith
Adam Smith

Reputation: 54163

For academic purposes, if your list is more than one level deep (e.g. [ [1,2,3], [4,5,[6,7,8]]], one of your elements contains a list) you'll want to do something like:

def count_elements(target):
    count = 0
    for element in target:
        if type(element) is list:
            count += count_elements(element)
        else:
            count += 1
    return count

But the problem as described by OP is easier-solved with one of the other answers. Just mentioning that those solutions are not easily scalable.

Upvotes: 1

shx2
shx2

Reputation: 64298

You can either sum the lengths (like @thefourtheye suggested), or iterate over all elements, and count:

sum(len(b) for b in a)
sum(1 for b in a for c in b)

The first way is clearly better, but the second is nice for demonstrating some things you can do with list comprehensions, and would also work on a more general structure, of iterable of iterables (which do not have __len__ defined).

Upvotes: 0

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