Most efficient way to calculate this sum in python

Question

Let's say I have a list "A" of length about 40, consisting of integers from 0 to 5, and a list "Score". I want to calculate the sum of Score[A[i]].

Of course, I can do:

sum = 0 
for x in A do:
    sum += Score[x]

But is there a faster way? I know that numpy can do multiplication of lists, but this requires some sort of indexing.

You could try total = sum(Score[x] for x in A) sum() is a builtin function. — zondo
– zondo, Commented Feb 3, 2016 at 14:46
(Because you tagged this [numpy]) If Score is a NumPy array you'd use Score[A].sum(). No particular reason to use this over native Python methods if you don't need other NumPy functions though. — Alex Riley
– Alex Riley, Commented Feb 3, 2016 at 14:49

cromod · Accepted Answer · 2016-05-19 15:40:55Z

2

I see those solutions :

print sum(Score[x] for x in A)
print sum(map(lambda x: Score[x], A))

edited May 19, 2016 at 15:40

answered Feb 3, 2016 at 15:07

cromod

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4 Comments

Alex Riley Over a year ago

Both Score and A are lists so I don't think your code works as it's written.

Alex Riley Over a year ago

No worries - that works now, although it's probably faster to use just a generator expression in sum, i.e. sum(Score[x] for x in A)

CaptainCodeman Over a year ago

Thanks for this. But what if Score is different for each element, in other words, it's the sum of Score[i][A[i]]. Would I write sum(Score[i][A[i]]), or is there a faster way?

cromod Over a year ago

Except numpy, I don't see any faster way than sum and a generator expression. I read numpy is faster but you must define your array with numpy. See DrV's answer here.

user5597655 · Accepted Answer · 2016-02-03 14:47:49Z

1

The Python function "sum" is pretty efficient. It avoids memory overhead (I believe it is written in C) and should be a bit faster.

It would look like this

intSum = sum(array)

answered Feb 3, 2016 at 14:47

user5597655

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