How to sum elements in functional way

Question

I am trying to write a function which maps elements of a list to get sum of the element and the previous elements in the list in a functional style using python e.g. :

func([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]) = [0, 1, 3, 6, 10, 15, 21, 28, 36, 45]

I have tried using recursion, but get RuntimeError: maximum recursion depth exceeded with a long list.:

def recursion_way(inlist, accu, summ):
    if len(inlist) == 0:
         return accu
    else:
        return recursion_way(inlist[1:], accu + [summ + inlist[0]], summ + inlist[0])

@user1879805: You can edit the question and add your recursion there :) (it'll be much clearer than in the comments) — Savir
– Savir, Commented Dec 5, 2012 at 17:07
@Martijn Pieters: You're absolutely right... I deleted my comment. The recursion_way is working fine for just 10 elements (the day I learn to read, I'll conquer the world!!) :) — Savir
– Savir, Commented Dec 5, 2012 at 17:11
@user1879805 Just so that you know, the maximum recursion limit by default in python is 1000. — Trufa
– Trufa, Commented Dec 5, 2012 at 17:13
@Trufa: default recursion limit - it's reconfigurable at runtime. — Eric
– Eric, Commented Dec 5, 2012 at 17:16

Eric · Accepted Answer · 2012-12-05 17:09:50Z

7

Does a comprehension count?

>>> [sum(l[:i]) for i, _ in enumerate(l)]
[0, 0, 1, 3, 6, 10, 15, 21, 28, 36]

or perhaps using reduce:

reduce(
    lambda (sums, last), x: (sums+[x+last], x+last),
    l, ([], 0)
)[0]

Or another way:

reduce(lambda sums,x: sums+[x+sums[-1]], l[1:], l[:1])

edited Dec 5, 2012 at 17:09

answered Dec 5, 2012 at 17:04

Eric

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Comments

kreativitea · Accepted Answer · 2012-12-05 17:08:29Z

3

How about reduce? Slow, but interesting, imo.

def func(p):
    for i in xrange(1, len(p)+1):
        yield reduce(lambda x, y: x + y, p[0:i])

>>> list(func(p))
[0, 1, 3, 6, 10, 15, 21, 28, 36, 45]

answered Dec 5, 2012 at 17:08

kreativitea

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2 Comments

NPE Over a year ago

The for loop uses mutable data and as such probably doesn't qualify as "functional programming".

Eric Over a year ago

Your reduce(lambda x, y: x + y, p[0:i]) is the same as sum(p[:i]), which IMO is much more readable

Ramón J Romero y Vigil · Accepted Answer · 2012-12-05 17:05:41Z

1

Can you use numpy?

import numpy
numpy.cumsum([0,1,2,3,4,5,6,7,8,9])
array([ 0,  1,  3,  6, 10, 15, 21, 28, 36, 45])

answered Dec 5, 2012 at 17:05

Ramón J Romero y Vigil

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2 Comments

Eric Over a year ago

NameError: name 'cumsum' is not defined

Ramón J Romero y Vigil Over a year ago

Changed my answer to use numpy

NPE · Accepted Answer · 2012-12-05 17:05:56Z

1

Here is cumulative sum done in the style of functional programming:

def func(l):
   if len(l) < 2: return l
   sub = func(l[:-1])
   return sub + [sub[-1] + l[-1]]

print func([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

answered Dec 5, 2012 at 17:05

NPE

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Comments

jadkik94 · Accepted Answer · 2012-12-05 17:16:01Z

1

Here's what I got, using recursion:

def f(L, n=0):
    # If the list is empty, that means we went through all the elements there
    if len(L)>0:
        # n is the last element in the sum list. Add to it the first remaining element
        n = n+L[0]
        # Return a list containing the newest item and those of the remaining elements
        return [n] + f(L[1:], n)
    else:
        # It it is empty, there are no more sums to calculate
        return []

answered Dec 5, 2012 at 17:16

jadkik94

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Comments

ckb · Accepted Answer · 2012-12-05 17:06:50Z

-2

l = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
reduce(lambda x, y: x+y, l)

answered Dec 5, 2012 at 17:06

ckb

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1 Comment

David Wolever Over a year ago

You've implemented sum, not the "rolling sum" the OP is looking for.

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