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How to apply a function (like the sum) among all elements of a list?

asked 2017-10-14 14:55:18 -0500

descudero gravatar image

In magma you have the possibility of invoking

&+[x0, x1, x2]

and that will return the sum of the elements in the array. You can also replace the sum operator by another binary operator that is associative. I'm wondering if SageMath has a similar feature, I've been looking in the documentation and apparently the best option is to use the built-in function sum(), but this doesn't generalize to any other binary operation.

Thanks for the help!

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answered 2017-10-15 02:10:58 -0500

slelievre gravatar image

updated 2017-10-15 02:51:16 -0500

The easiest are sum and product (corresponding to + and *): if a is a list (or a tuple, or any iterable) of numbers or symbolic expressions, sum(a) gets you the sum of its elements, prod(a) their product.

Here is an example with numbers.

sage: sum([1, 2, 3, 4, 5])
sage: prod([1, 2, 3, 4, 5])

Here is an example with symbolic expressions.

sage: x, y, z = SR.var("x y z")
sage: sum([x, y, z])
x + y + z
sage: prod([x, y, z])

Of course you could mix the two.

sage: sum([x, y, z, 1, 2, 3])
x + y + z + 6
sage: prod([x, y, z, 5, 6])

Next are all and any (for the and and or operators).

sage: all(is_prime(k) for k in (3, 5, .. 11))
sage: any(is_prime(k) for k in (3, 5, .. 11))

For a more general operator, use reduce.

sage: a = [720, 3, 4, 5]
sage: reduce(lambda x, y: x // y, a)

Optionally, include a start value outside the list.

sage: a = [3, 4, 5]
sage: reduce(lambda x, y: x // y, a, 720)
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Thanks a lot for the help! All of this makes sense, and it's very useful.

descudero gravatar imagedescudero ( 2017-10-15 09:02:36 -0500 )edit

reduce is not really compatible with python3

FrédéricC gravatar imageFrédéricC ( 2017-10-15 11:09:54 -0500 )edit

In Python3, reduce is no longer a built-in. Instead, use functools.reduce or an explicit for loop.

slelievre gravatar imageslelievre ( 2017-10-15 14:09:01 -0500 )edit

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Asked: 2017-10-14 14:55:18 -0500

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Last updated: Oct 15 '17