A percentile refers to the value below which a specified percentage of data falls.
Let us understand how we can use the percentile function in NumPy.
Function Syntax
The function takes on syntax as shown below:
Let us discuss function syntax.
Function Parameters
- a – refers to the input array whose percentile we need to calculate.
- q – specifies which percentile we are calculating. Must be a value of 0 to 100 (inclusive).
- axis – defines along which axis we are computing for the percentile.
- out – specifies an output array. This array must have the same shape as the resulting output.
- overwrite_input – allows you to modify the input array.
- method – determines the method for estimating the percentile. Again, check the docs for the accepted values.
- keepdims – reduces the axes with the dimensions of one.
Function Return Value
The percentile() function returns an array with the percentile values of the values along the specified axis.
Example #1
Take the example provided in the code below:
import numpy as np
# create 1D array
arr_1d = np.array([10,14,7,4,3,2,8,1])
# calculate 25th percentile
print(f"25th percentile: {np.percentile(a=arr_1d, q=25)}")
# calculate 50th percentile
print(f"50th percentile: {np.percentile(a=arr_1d, q=50)}")
# 100th percentile
print(f"100th percentile: {np.percentile(a=arr_1d, q=100)}")
The code above uses the percentile function to calculate the 25th, 50th, and 100th percentiles of one-dimensional array.
Since the input is 1D, the function will return a scalar value as shown in the output below:
50th percentile: 5.5
100th percentile: 14.0
Example #2
Consider the code below that calculates the percentiles of a 2d array along the 0 axis.
arr_2d = np.array([[10,14,7,4], [3,2,8,1]])
# calculate 25th percentile
print(f"25th percentile: {np.percentile(a=arr_2d, q=25, axis=0)}")
# calculate 50th percentile
print(f"50th percentile: {np.percentile(a=arr_2d, q=50, axis=0)}")
# 100th percentile
print(f"100th percentile: {np.percentile(a=arr_2d, q=100, axis=0)}")
In the example above, we calculate the 25th, 50th and 100th percentile of a 2D array along the zero axis. The resulting output is as shown:
50th percentile: [6.5 8. 7.5 2.5]
100th percentile: [10. 14. 8. 4.]
Example #3
The code below demonstrates the percentile function with a 2d array along various axes.
arr_2d = np.array([[10,14,7,4], [3,2,8,1]])
# calculate 25th percentile
print(f"25th percentile (axis=1): {np.percentile(a=arr_2d, q=25, axis=1)}")
# calculate 50th percentile
print(f"50th percentile (axis=-1): {np.percentile(a=arr_2d, q=50, axis=-1)}")
# 100th percentile
print(f"100th percentile (axis=None): {np.percentile(a=arr_2d, q=100, axis=None)}")
The code above shows how to calculate the percentiles of a 2d array along various axes.
NOTE: Setting the axis to None flattens the array and calculates the percentile.
50th percentile (axis=-1): [8.5 2.5]
100th percentile (axis=None): 14.0
Conclusion
This article covers how to use the percentile function in NumPy. Hence, tweak the function’s parameters to understand better how the function behaves under various conditions.
Thanks for reading!!
