You can use the following methods to find the index position of specific values in a NumPy array:
Method 1: Find All Index Positions of Value
np.where(x==value)
Method 2: Find First Index Position of Value
np.where(x==value)[0][0]
Method 3: Find First Index Position of Several Values
#define values of interest vals = np.array([value1, value2, value3]) #find index location of first occurrence of each value of interest sorter = np.argsort(x) sorter[np.searchsorted(x, vals, sorter=sorter)]
The following examples show how to use each method in practice.
Method 1: Find All Index Positions of Value
The following code shows how to find every index position that is equal to a certain value in a NumPy array:
import numpy as np #define array of values x = np.array([4, 7, 7, 7, 8, 8, 8]) #find all index positions where x is equal to 8 np.where(x==8) (array([4, 5, 6]),)
From the output we can see that index positions 4, 5, and 6 are all equal to the value 8.
Method 2: Find First Index Position of Value
The following code shows how to find the first index position that is equal to a certain value in a NumPy array:
import numpy as np #define array of values x = np.array([4, 7, 7, 7, 8, 8, 8]) #find first index position where x is equal to 8 np.where(x==8)[0][0] 4
From the output we can see that the value 8 first occurs in index position 4.
Method 3: Find First Index Position of Several Values
The following code shows how to find the first index position of several values in a NumPy array:
import numpy as np #define array of values x = np.array([4, 7, 7, 7, 8, 8, 8]) #define values of interest vals = np.array([4, 7, 8]) #find index location of first occurrence of each value of interest sorter = np.argsort(x) sorter[np.searchsorted(x, vals, sorter=sorter)] array([0, 1, 4])
From the output we can see:
- The value 4 first occurs in index position 0.
- The value 7 first occurs in index position 1.
- The value 8 first occurs in index position 4.
Additional Resources
The following tutorials explain how to perform other common operations in NumPy:
How to Map a Function Over a NumPy Array
How to Convert NumPy Array to List in Python
How to Calculate the Magnitude of a Vector Using NumPy
