Function that changes the sign of elements in array if the condition between a and b meet

Question

Change sign of elements in numpy array from a to b. I tried this.

import numpy as np
def do_negative(X, a, b):
    lst = []
    for i in X:
      if (a<i<b):
        lst.append(-i)
      else:
        lst.append(i)
    return X
test = np.array(range(9)).reshape(3,3)
do_negative(test, -1, 3).all()

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

Input data: from -1 to 3.

Output should be: np.array([[ 0, -1, -2], [-3, 4, 5], [ 6, 7, 8]])

Shubham Shaswat · Accepted Answer · 2020-03-21 10:20:03Z

1

Try this:

X is 2D numpy array,you have to make it 1D using flatten().Also you returning X instead of returning lst

def do_negative(X, a, b):
  lst = []
  for i in X.flatten():
    if (a < i <= b):
      lst.append(-1*i)
    else:
      lst.append(i)
  return np.array(lst).reshape(3,3)

Remove .all() when calling the function

do_negative(test, -1, 3)

Output:

array([[ 0, -1, -2],
       [-3,  4,  5],
       [ 6,  7,  8]])

answered Mar 21, 2020 at 10:20

Shubham Shaswat

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Comments

mkrieger1 · Accepted Answer · 2020-03-21 10:41:44Z

0

Using for loops to do element-wise operations on arrays is not how NumPy is supposed to be used.

This can be done quite easily with a whole-array operation:

def do_negative(x, a, b):
    result = x.copy()
    result[(a < x) & (x < b)] *= -1
    return result

edited Mar 21, 2020 at 10:41

answered Mar 21, 2020 at 10:30

mkrieger1

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