Numpy: How to apply different formulas in an array based on their values (if elif elif ... else for numpy)?

Question

I have an numpy array of numbers x and some thresholds [A,B,C,D]

I want to apply 5 different formulas to each of the slices of the array, but ideally I don't want to iterate over it (that's why I'm trying to use numpy).

What's the best way to do it? This is what I'm trying to do, is there a better way?

cond_A = np.where(x <= A)
cond_B = np.where((x > A) & (x <= B))
cond_C = np.where((x > B) & (x <= C))
cond_D = np.where((x > C) & (x <= D))
cond_E =np.where(x > D)

x[cond_A] = function_A(x[cond_A])
...
...
x[cond_E]= function_E(x[cond_E])

EDIT: If I try this I'm getting the error:

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

Nuageux · Accepted Answer · 2020-03-26 10:50:23Z

2

You have to use the intersect1d function from numpy in order to apply multiple filters. Here are one example with output in comments, hope this helps

import numpy as np

def function(x):
    return x+10

a = np.arange(10)
print(a) # [0 1 2 3 4 5 6 7 8 9]

mask = np.intersect1d(np.where(a>3), np.where(a<6))
a[mask] = function(a[mask])
print(a) # [ 0  1  2  3 14 15  6  7  8  9]

edited Mar 26, 2020 at 10:50

answered Mar 26, 2020 at 10:37

Nuageux

1,6861 gold badge18 silver badges29 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Yano KX · Accepted Answer · 2020-03-26 10:57:06Z

0

& is the bitwise and operator, I assume you're using and since that's when your reported error happens.

If you want to manipulate arrays in this manner, functional programming concepts like map and filter or list comprehensions will help you. Other answer used numpy functions, here's an answer using list comprehensions:

x = [function_A(i) if cond_A else i for i in myarray]

answered Mar 26, 2020 at 10:57

Yano KX

477 bronze badges

Collectives™ on Stack Overflow

Numpy: How to apply different formulas in an array based on their values (if elif elif ... else for numpy)?

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related