Multiply elements of inner lists as a list comprehension

Question

Could this be done in single line using list comprehension?

lst = [[1, 2, 3], [1, 2, 3, 4], [5, 6], [9]]
products = ?? (Multiple each list elements)

Desired output = [6, 24, 30, 9]

I tried something like:

products = [l[i] * l[i + 1] for l in lst for i in range(len(l) - 1)]

but didn't work.

Answer 1

You can use reduce() to apply multiplication to a list of integers, together with operator.mul() to do the actual multiplication:

from functools import reduce

from operator import mul

products = [reduce(mul, l) for l in lst]

In Python 3, reduce() has been moved to functools.reduce(), hence the supporting import statement. As functools.reduce exists since Python 2.6, it is simply easier to import it from there if you need to keep your code compatible with both Python 2 and 3.

Demo:

>>> from operator import mul
>>> lst = [[1, 2, 3], [1, 2, 3, 4], [5, 6], [9]]
>>> [reduce(mul, l) for l in lst]
[6, 24, 30, 9]

operator.mul() can be replaced with lambda x, y: x * y but why have a dog and bark yourself?

Answer 2

Another approach using numpy

>>> from numpy import prod
>>> [prod(x) for x in lst] 
[6, 24, 30, 9]

Ref - Documentation on prod

Answer 3

Try:

products = [reduce(lambda x, y: x * y, l) for l in lst]