How to multiply individual elements of a list with a number?

Question

S = [22, 33, 45.6, 21.6, 51.8]
P = 2.45

Here S is an array

How will I multiply this and get the value?

SP = [53.9, 80.85, 111.72, 52.92, 126.91]

Uncurl

In Python S is not an array, it is a list. There is a very big difference betweeb the two types of containers. If you want numerical arrays, use numpy. – Cullum

Answer 1

You can use built-in map function:

result = map(lambda x: x * P, S)

or list comprehensions that is a bit more pythonic:

result = [x * P for x in S]

Vesuvianite

Just as a note, operations on arrays, like scalar multiplication are highly optimized in numpy, and are significantly faster than list comprehensions. It is generally advisable to not treat numpy arrays like python lists. – Delinquent

"It is generally advisable to not treat numpy arrays like python lists", what does that mean in general? and how is it related to the current answer which doesn't use NumPy? Is this answer still true in 2021 (probably but still not sure, maybe some new operator has appeared to indicate we really want the distributive operation and not the list replication)? – Rinse

Answer 2

In NumPy it is quite simple

import numpy as np
P=2.45
S=[22, 33, 45.6, 21.6, 51.8]
SP = P*np.array(S)

I recommend taking a look at the NumPy tutorial for an explanation of the full capabilities of NumPy's arrays:

https://scipy.github.io/old-wiki/pages/Tentative_NumPy_Tutorial

Delinquent

Answer 3

If you use numpy.multiply

S = [22, 33, 45.6, 21.6, 51.8]
P = 2.45
multiply(S, P)

It gives you as a result

array([53.9 , 80.85, 111.72, 52.92, 126.91])

Frustule

Answer 4

Here is a functional approach using map, itertools.repeat and operator.mul:

import operator
from itertools import repeat


def scalar_multiplication(vector, scalar):
    yield from map(operator.mul, vector, repeat(scalar))

Example of usage:

>>> v = [1, 2, 3, 4]
>>> c = 3
>>> list(scalar_multiplication(v, c))
[3, 6, 9, 12]

Monometallic