How to find first non-zero value in every column of a NumPy array?

Learn, how to find first non-zero value in every column of a NumPy array in Python? By Pranit Sharma Last updated : December 23, 2023

NumPy is an abbreviated form of Numerical Python. It is used for different types of scientific operations in python. Numpy is a vast library in python which is used for almost every kind of scientific or mathematical operation. It is itself an array which is a collection of various methods and functions for processing the arrays.

Problem statement

Suppose that we are given a numpy array and we need to find the indices of the first index (for every column) where the value is non-zero.

Finding the first non-zero value in every column of a NumPy array

For this purpose, we will define a function inside which we can use numpy.argmax() along that axis (zeroth axis for columns here) on the mask of non-zeros to get the indices of first matches.

Let us understand with the help of an example,

Python code to find first non-zero value in every column of a NumPy array

# Import numpy
import numpy as np

# Creating a numpy array
arr = np.array([[1,1,0],[1,-1,0],[1,0,0],[1,1,0]])

# Display original array
print("Original Array:\n",arr,"\n")

# Defining a function
def fun(arr, axis, invalid_val=-1):
    mask = arr!=0
    return np.where(mask.any(axis=axis), mask.argmax(axis=axis), invalid_val)

# Calling the function
res = fun(arr, 0)

# Display result
print("Result:\n",res,"\n")