Question

Python 2.7

Write a function cumsum() that takes a list l as argument and returns the cumulative sum (also known as the prefix sum) of l, which is a list, say cs of the same length as l such that each element cs[i] is equal to the sum of the first i + 1 elements of l, i.e.,

cs[i] == l[0] + l[1] + l[2] + ... + l[i]

You should not modify the argument list l in any way. Also, you do not need to assume that l contains numbers; your function should work with any elements that support +. Practice goal: Although not required, try to solve this without using a nested loop. In this case, you’ll have to write a loop that runs two accumulations in tandem. The first is the standard sum-so-far that we’ve seen several times. The second is an accumulation of the intermediate sum-so-far values into a list (e.g., via an .append() operation); the final value of this list is the desired result.

Answer 1

def cumsum(l):
if all(isinstance(item, int) for item in l) or all(isinstance(item, float) for item in l):
     list = []
     for i in range(len(l)):
         sum = 0;
         for j in range(i+1):
             sum = sum + l[j]
         list.append(sum)
else:
     list = []
     for i in range(len(l)):
         str = "";
         for j in range(i+1):
             str = str + l[j]
         list.append(str)
return list

l1 = [1,2,3,5]
l2 = [2.3, 4.5, 6.7,5.6]
l3 = ['a','b','c','d']
print(cumsum(l1))
print(cumsum(l2))
print(cumsum(l3))