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devel / comp.lang.python / Fwd: Re: sum() vs. loop

Fwd: Re: sum() vs. loop

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From: learn2program@gmail.com (Alan Gauld)
Newsgroups: comp.lang.python
Subject: Fwd: Re: sum() vs. loop
Date: Tue, 12 Oct 2021 23:55:35 +0100
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by: Alan Gauld - Tue, 12 Oct 2021 22:55 UTC

On 10/10/2021 09:49, Steve Keller wrote:
> I have found the sum() function to be much slower than to loop over the
> operands myself:
>
> def sum_products(seq1, seq2):
> return sum([a * b for a, b in zip(seq1, seq2)])
>
> def sum_products2(seq1, seq2):
> sum = 0
> for a, b in zip(seq1, seq2):
> sum += a * b
> return sum
>
> In a program I generate about 14 million pairs of sequences of ints each
> of length 15 which need to be summed. The first version with sum() needs
> 44 seconds while the second version runs in 37 seconds.
>
> Can someone explain this difference?

I'm no expert on Python innards but it looks to me like the first
version loops over the length of the list twice, once to generate
the list of products and the second time to add them together.

The pure Python version only loops once because it does the addition
in transit.

Presumably, especially for large numbers, a single python loop
is faster than 2 C loops?

But that's purely my speculation.

--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
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