How to perform element-wise multiplication of two

Question

I want to perform an element wise multiplication, to multiply two lists together by value in Python, like we can do it in Matlab.

This is how I would do it in Matlab.

a = [1,2,3,4]
b = [2,3,4,5]
a .* b = [2, 6, 12, 20]

A list comprehension would give 16 list entries, for every combination x * y of x from a and y from b. Unsure of how to map this.

If anyone is interested why, I have a dataset, and want to multiply it by Numpy.linspace(1.0, 0.5, num=len(dataset)) =).

Answer 1

you can multiplication using lambda

foo=[1,2,3,4]
bar=[1,2,5,55]
l=map(lambda x,y:x*y,foo,bar)

Answer 2

Can use enumerate.

a = [1, 2, 3, 4]
b = [2, 3, 4, 5]

ab = [val * b[i] for i, val in enumerate(a)]

Answer 3

create an array of ones; multiply each list times the array; convert array to a list

import numpy as np

a = [1,2,3,4]
b = [2,3,4,5]

c = (np.ones(len(a))*a*b).tolist()

[2.0, 6.0, 12.0, 20.0]

Answer 4

Yet another answer:

-1 ... requires import
+1 ... is very readable

import operator
a = [1,2,3,4]
b = [10,11,12,13]

list(map(operator.mul, a, b))

outputs [10, 22, 36, 52]

Answer 5

The map function can be very useful here. Using map we can apply any function to each element of an iterable.

Python 3.x

>>> def my_mul(x,y):
...     return x*y
...
>>> a = [1,2,3,4]
>>> b = [2,3,4,5]
>>>
>>> list(map(my_mul,a,b))
[2, 6, 12, 20]
>>>

Of course:

map(f, iterable)

is equivalent to

[f(x) for x in iterable]

So we can get our solution via:

>>> [my_mul(x,y) for x, y in zip(a,b)]
[2, 6, 12, 20]
>>>

In Python 2.x map() means: apply a function to each element of an iterable and construct a new list. In Python 3.x, map construct iterators instead of lists.

Instead of my_mul we could use mul operator

Python 2.7

>>>from operator import mul # import mul operator
>>>a = [1,2,3,4]
>>>b = [2,3,4,5]
>>>map(mul,a,b)
[2, 6, 12, 20]
>>>

Python 3.5+

>>> from operator import mul
>>> a = [1,2,3,4]
>>> b = [2,3,4,5]
>>> [*map(mul,a,b)]
[2, 6, 12, 20]
>>>

Please note that since map() constructs an iterator we use * iterable unpacking operator to get a list. The unpacking approach is a bit faster then the list constructor:

>>> list(map(mul,a,b))
[2, 6, 12, 20]
>>>

Answer 6

You can try multiplying each element in a loop. The short hand for doing that is

ab = [a[i]*b[i] for i in range(len(a))]