Question

In keeping with the “There’s only one obvious way to do it”, how do you get the magnitude of a vector (1D array) in Numpy?

def mag(x): 
    return math.sqrt(sum(i**2 for i in x))

The above works, but I cannot believe that I must specify such a trivial and core function myself.

Answer 1

The function you’re after is numpy.linalg.norm. (I reckon it should be in base numpy as a property of an array — say x.norm() — but oh well).

import numpy as np
x = np.array([1,2,3,4,5])
np.linalg.norm(x)

You can also feed in an optional ord for the nth order norm you want. Say you wanted the 1-norm:

np.linalg.norm(x,ord=1)

And so on.