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pythonalgorithmlistsum
enrico.bacis
enrico.bacis

Reputation: 31504

Sum lists with different lengths

What's the best way to sum two or more lists even if they have different lengths?

For example I have:

lists = [[1, 2], [0, 3, 4], [5]]

and the result should be:

result = [6, 5, 4]

Upvotes: 12

Views: 5720

Answers (4)

wahmad
wahmad

Reputation: 1

This worked well for lists with a thousand and a bit elements. checking with if statements weather to append took ages and ages but this ran pretty fast.i had a function,"longestlist" , that identified and returned the longest list but that's pretty easy to write so using that output...

def listfiller(a,b,c,d)
  longest = longestlist(a,b,c,d)
  for i in range(len(longest)-len(a)):
    a.append(0)
  for i in range(len(longest)-len(b)):
    b.append(0)
    print(b)
  for i in range(len(longest)-len(c)):
    c.append(0)
  for i in range(len(longest)-len(d)):
    d.append(0)
  return(a,b,c,d)

Upvotes: 0

N S
N S

Reputation: 21

#You can do the same without using predefined header files.
def sumlists(a,b,c):
    sumlist = []
    while(len(a)!=len(b) or len(a)!=len(c)):
        if len(a)>len(b):
            b.append(0)
        if len(a)>len(c):
            c.append(0)
        elif len(b)>len(a):
            a.append(0)
        if len(b)>len(c):
             c.append(0)
        elif len(c)>len(a):
            a.append(0)
        if len(c)>len(b):
             b.append(0)
    for i,j,k in zip(a,b,c):
         sumlist.append(i+j+k)
    return sumlist
 print(sumlists([1,2],[0,3,4],[5]))

Upvotes: 2

enrico.bacis
enrico.bacis

Reputation: 31504

The best I came up with is the following:

result = [sum(filter(None, i)) for i in map(None, *lists)]

It's not so bad, but I have to add NoneTypes and then filter them in order to sum up.

Upvotes: 0

Ashwini Chaudhary
Ashwini Chaudhary

Reputation: 250961

You can use itertools.izip_longest(), and use a fillvalue equal to 0

In [6]: [sum(x) for x in itertools.izip_longest(*lists, fillvalue=0)]
Out[6]: [6, 5, 4]

for Python < 2.6:

In [27]: ml = max(map(len, lists))

In [28]: ml       #length of the longest list in lists
Out[28]: 3

In [29]: [sum(x) for x in zip(*map(lambda x:x+[0]*ml if len(x)<ml else x, lists))]
Out[29]: [6, 5, 4]

Upvotes: 19

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