# Use an index array to construct a new array from a set of choices in Numpy

A new array from the set of choices is constructed using the numpy.ma.choose() method in Python Numpy. Given an array of integers and a list of n choice arrays, this method will create a new array that merges each of the choice arrays. Where a value in index is i, the new array will have the value that choices[i] contains in the same place.

The choices parameter is the choice arrays. The index array and all of the choices should be broadcastable to the same shape.

The mode parameter specifies how out-of-bounds indices will behave −

• 'raise' : raise an error
• 'wrap' : wrap around
• 'clip' : clip to the range

## Steps

At first, import the required library −

import numpy as np

Set choices array −

arr_choices = np.array([[5, 10, 15, 20, 25], [50, 55, 60, 65, 70],
[100, 105, 110, 115, 120], [150, 155, 160, 165, 170], [200, 205, 210, 215, 220]])

Create a new array −

arr = np.array([2, 3, 4, 1, 0])


Displaying the array −

print("Array...",arr)

Displaying the choices array −

print("Choices Array...",arr_choices)


A new array from the set of choices is constructed using the choose() method −

arrRes = np.ma.choose(arr, arr_choices)

The first element will be the first element of the third (2+1) "array" in choices. The second element will be the second element of the fourth (3+1) choice array. The third element will be the third element of the fifth (4+1) choice array, etc −

print("New Array from set of choices...",arrRes)


## Example

import numpy as np

# set choices array
arr_choices = np.array([[5, 10, 15, 20, 25], [50, 55, 60, 65, 70], [100, 105, 110, 115, 120], [150, 155, 160, 165, 170], [200, 205, 210, 215, 220]])

# Create a new array
arr = np.array([2, 3, 4, 1, 0])

# Displaying the array
print("Array...",arr)

# Displaying the choices array
print("Choices Array...",arr_choices)

# A new array from the set of choices is constructed using the choose() method
arrRes = np.ma.choose(arr, arr_choices)

# The first element will be the first element of the third (2+1) "array" in choices,

# The second element will be the second element of the fourth (3+1) choice array

# The third element will be the third element of the fifth (4+1) choice array, etc.
print("New Array from set of choices...",arrRes)

## Output

Array...
[2 3 4 1 0]

Choices Array...
[[ 5 10 15 20 25]
[ 50 55 60 65 70]
[100 105 110 115 120]
[150 155 160 165 170]
[200 205 210 215 220]]

New Array from set of choices...
[100 155 210 65 25]