Print all rows of an array not equal to zero in Python

Question

I have an array out. I want to print row numbers which have at least one non-zero element. However, I am getting an error. I present the expected output.

import numpy as np

out = np.array([[  0.        ,   0.        ,   0.        ,   0.        ,
          0.        ,   0.        ,   0.        ,   0.        ,
          0.        ,   0.        ,   0.        ,   0.        ],
       [  0.        , 423.81345923,   0.        , 407.01354328,
        419.14952534,   0.        , 212.13245959,   0.        ,
          0.        ,   0.        ,   0.        ,   0.        ],
       [402.93473651,   0.        , 216.08166277, 407.01354328,
          0.        , 414.17017965,   0.        ,   0.        ,
          0.        ,   0.        ,   0.        ,   0.        ],
       [  0.        ,   0.        ,   0.        ,   0.        ,
          0.        ,   0.        ,   0.        ,   0.        ,
          0.        ,   0.        ,   0.        ,   0.        ]])

for i in range(0,len(out)):
    if (out[i]==0):
        print(i)
    else:
        print("None")

The error is

in <module>
    if (out[i]==0):

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

The expected output is

[1,2]

if any(out[i]): or if out[i].any():

Olvin Roght
– Olvin Roght

2022-12-30 12:07:38 +00:00
Commented Dec 30, 2022 at 12:07 — Olvin Roght
– Olvin Roght, Commented Dec 30, 2022 at 12:07

Chrysophylaxs · Accepted Answer · 2022-12-30 12:18:19Z

2

import numpy as np

out = ... # 2d array here

rows = np.where(out.any(axis=1))[0].tolist()

# rows:
# [1, 2]

edited Dec 30, 2022 at 12:18

answered Dec 30, 2022 at 12:10

Chrysophylaxs

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Comments

Nonlinear · Accepted Answer · 2022-12-30 12:15:44Z

1

The issue with your code is that out[i] is an array. When you check if this array is equal to zero, it returns an array of booleans. The if statement then returns an error. The following code should work:

solution = []
for idx, e in enumerate(out):
    if any(e): # if any element of the array is nonzero
        solution.append(idx)
print(solution)

The output is :

[1, 2]

I hope this helps!

edited Dec 30, 2022 at 12:15

answered Dec 30, 2022 at 12:08

Nonlinear

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2 Comments

AEinstein Over a year ago

How do I print the output like [1,2] using your code?

Nonlinear Over a year ago

You need to store the values in a list. I will edit my answer.

buec95 · Accepted Answer · 2023-01-03 06:43:16Z

1

import numpy as np

out = np.array([[  0.        ,   0.        ,   0.        ,   0.        ,
          0.        ,   0.        ,   0.        ,   0.        ,
          0.        ,   0.        ,   0.        ,   0.        ],
       [  0.        , 423.81345923,   0.        , 407.01354328,
        419.14952534,   0.        , 212.13245959,   0.        ,
          0.        ,   0.        ,   0.        ,   0.        ],
       [402.93473651,   0.        , 216.08166277, 407.01354328,
          0.        , 414.17017965,   0.        ,   0.        ,
          0.        ,   0.        ,   0.        ,   0.        ],
       [  0.        ,   0.        ,   0.        ,   0.        ,
          0.        ,   0.        ,   0.        ,   0.        ,
          0.        ,   0.        ,   0.        ,   0.        ]])

for i in range(0,len(out)):
    if (abs(out[i]).sum()!=0):
        print(i)
    else:
        print("None")

This sums up all values of each index and prints you all rows that contain any non-zero values.

edited Jan 3, 2023 at 6:43

answered Dec 30, 2022 at 12:13

buec95

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3 Comments

Chrysophylaxs Over a year ago

What if we have an array like [0, -1, 0, 1]? Better to use numpy's any :)

Nonlinear Over a year ago

You also should avoid using for i in range(0,len(out)) and indexing out after. It would be better to use enumerate.

buec95 Over a year ago

@Chrysophylaxs thanks for mentioning that! I added the abs() function to overcome this issue

Shakir Sadiq · Accepted Answer · 2022-12-30 12:23:43Z

0

We have where and unique attributes in numpy which can help you to achieve this.

get_index = np.where(out != 0)[0]
rows = np.unique(get_index)
print(rows)

answered Dec 30, 2022 at 12:23

Shakir Sadiq

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