Teaching Kids Programming – Implement the Accumulate Function in Python

ACMer

4 years ago

Teaching Kids Programming: Videos on Data Structures and Algorithms

The accumulate function is provided in itertools and allows us to return a generator containing the sum of previous items. For example:

from itertools import accumulate
a = [1, 2, 3, 4, 5]
print(accumulate(a))
<itertools.accumulate object at 0xa770d0>
list(accumulate(a))
[1, 3, 6, 10, 15]

from itertools import accumulate
a = [1, 2, 3, 4, 5]
print(accumulate(a))
<itertools.accumulate object at 0xa770d0>
list(accumulate(a))
[1, 3, 6, 10, 15]

We can define an accumulate function that returns a list:

def accumulate(a):
  for i in range(1, len(a)):
     a[i] += a[i - 1]
  return a

def accumulate(a):
  for i in range(1, len(a)):
     a[i] += a[i - 1]
  return a

We can also make it return a generator:

def accumulate(a):
  for i in range(1, len(a)):
     a[i] += a[i - 1]
  yield from a

def accumulate(a):
  for i in range(1, len(a)):
     a[i] += a[i - 1]
  yield from a

If we want to not modify the original list:

def accumulate(a):
  b = [a[0]]
  for i in range(1, len(a)):
     b.append(a[i] + b[-1])
  yield from b

def accumulate(a):
  b = [a[0]]
  for i in range(1, len(a)):
     b.append(a[i] + b[-1])
  yield from b

Time complexity is O(N) where N is the number of elements in the list.

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