CODEWITHSUNDEEP
pythonfunctionintegersum
Jessica Smith
Jessica Smith

Reputation: 63

Summation Program

I am trying to create a simple function that will take the summation numbers given an integer from 0. For example, If the user inputs the integer 3, the results would be 6, since 0 + 1 + 2 + 3 = 6.

What is the best/most efficient way to do this?

Eventually I want to create a loop that will do this for a list, where the user inputs the list [4, 5, 6] and the function would return [10, 15, 16].

Thanks in advance!

Upvotes: 0

Views: 1002

Answers (5)

Guoliang
Guoliang

Reputation: 895

time complexity: O(n)

def get_sum(a):
   return [n * (n + 1) / 2 for n in a]


if __name__ == '__main__':
   print get_sum([4, 5, 6])

result: [10, 15, 21]

https://en.wikipedia.org/wiki/Triangular_number

Upvotes: 1

Tom Karzes
Tom Karzes

Reputation: 24052

There exists a closed form solution which has constant time (rather than linear time), which will be vastly faster than any iterative solution. The // divide operator forces an integer result (in case you're using Python 3).

def mysum(n):
    return n*(n+1)//2

If you want to apply this to a list, you can use map:

def mysum2(li):
    return map(mysum, li)

Or you can do it directly, without using mysum (this will be fastest):

def mysum2(li):
    return [n*(n+1)//2 for n in li]

The closed-form solutions will work on much larger values than the iterative solutions. For example:

>>> mysum2([4, 5, 6, 10000000000000])
[10, 15, 21, 50000000000005000000000000]
>>>

The solutions that use sum will never return a result for a case like this.

Upvotes: 2

AmanRaj
AmanRaj

Reputation: 329

X = [4,6,7]
X = [x*(x+1)/2 for x in X]

Upvotes: 1

Jerrybibo
Jerrybibo

Reputation: 1325

The summation function would be useful here. 

def findsum(x):
    # The 0 is optional, but it makes it clear that you are starting from 0.
    return sum(range(0, x + 1))

So for example:

>>> findsum(6)
21

Upvotes: 0

mobiusklein
mobiusklein

Reputation: 1423

For a single input, you can use

def summer(n):
     # Create a list of numbers from 0 to n.
     # The range function is an end-exclusive range
     return sum(range(n+1))

For multiple inputs, use the map function

def multisummer(ns):
    return map(summer, ns)

Upvotes: 1

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