How to find the Index of value in Numpy Array ?

In this article, we are going to find the index of the elements present in a Numpy array.

Using where() Method

where() method is used to specify the index of a particular element specified in the condition.

Syntax: numpy.where(condition[, x, y])

Example 1: Get index positions of a given value

Here, we find all the indexes of 3 and the index of the first occurrence of 3, we get an array as output and it shows all the indexes where 3 is present.

Python3

# import numpy package
import numpy as np

# create an numpy array
a = np.array([1, 2, 3, 4, 8, 6, 7, 3, 9, 10])

# display index value of 3
print("All index value of 3 is: ", np.where(a == 3)[0])

print("First index value of 3 is: ",np.where(a==3)[0][0])

Output:

All index value of 3 is:  [2 7]
First index value of 3 is:  2

Example 2: Print First Index Position of Several Values

Here, we are printing the index number of all values present in the values array.

Python3

# import numpy package
import numpy as np

# create an numpy array
a = np.array([1, 2, 3, 4, 8, 6, 2, 3, 9, 10])

values = np.array([2, 3, 10])

# index of first occurrence of each value
sorter = np.argsort(a)

print("index of first occurrence of each value: ",
      sorter[np.searchsorted(a, values, sorter=sorter)])

Output:

index of first occurrence of each value:  [1 2 9]

Example 3: Get the index of elements based on multiple conditions

Get the index of elements with a value less than 20 and greater than 12

Python3

# Create a numpy array
a = np.array([11, 12, 13, 14, 15, 16, 17, 15,
                11, 12, 14, 15, 16, 17, 18, 19, 20])

# Get the index of elements with value less
# than 20 and greater than 12
print("Index of elements with value less\
        than 20 and greater than 12 are: \n",
      np.where((a > 12) & (a < 20)))

Output:

Index of elements with value less than 20 and greater than 12 are: 
 (array([ 2,  3,  4,  5,  6,  7, 10, 11, 12, 13, 14, 15], dtype=int64),)

Get the index of elements in the Python loop

Create a NumPy array and iterate over the array to compare the element in the array with the given array. If the element matches print the index.

Python3

import numpy as np

# create numpy array elements
a = np.array([2, 3, 4, 5, 6, 45, 67, 34])

# display element index where value = 45

index_of_element = -1
for i in range(a.size):
    if a[i] == 45:
        index_of_element = i
        break

if index_of_element != -1:
    print("element index  : ", index_of_element)
else:
    print("The element not present")

Output:

element index  : 5

Using ndenumerate() function to find the Index of value

It is usually used to find the first occurrence of the element in the given numpy array.

Python3

import numpy as np
def ind(array, item):
    for idx, val in np.ndenumerate(array):
        if val == item:
            return idx
    # If no item was found return None, other return types might be a problem due to
    # numbas type inference.

a = np.array([11, 12, 13, 14, 15, 16, 17, 15,
                11, 12, 14, 15, 16, 17, 18, 19, 20])
print(ind(a,11))

Output:

(6,)

Using enumerate() function to find the Index of value

Here we are using the enumerate function and then checking the value with the target value.

Python3

import numpy as np
a = np.array([11, 12, 13, 14, 15, 16, 17, 15,
              11, 12, 14, 15, 16, 17, 18, 19, 20])
print(next(i for i, x in
           enumerate(a) if x == 17))

Output:

6