# Return the Lower triangle of an array and zero the main diagonal as well in Numpy

To return the lower triangle of an array, use the numpy.tril() method in Python Numpy The 1st parameter is the input array. The 2nd parameter is the 'k' i.e. the diagonal above which to zero elements. Here,

• k = 0 (the default) is the main diagonal, k < 0 is below it and k > 0 is above.
• The k = -1 value is to zero the main diagonal as well

The function returns a copy of an array with elements above the k-th diagonal zeroed. For arrays with ndim exceeding 2, tril will apply to the final two axes.

## Steps

At first, import the required library −

import numpy as np

Create a 2d array −

arr = np.array([[36, 36, 78, 88], [92, 81, 98, 45], [22, 67, 54, 69 ], [69, 80, 80, 99]])

Displaying our array −

print("Array...",arr)

Get the datatype −

print("Array datatype...",arr.dtype)


Get the dimensions of the Array −

print("Array Dimensions...",arr.ndim)

Get the shape of the Array −

print("Our Array Shape...",arr.shape)

Get the number of elements of the Array −

print("Elements in the Array...",arr.size)

To return the lower triangle of an array, use the numpy.tril() method. The 2nd parameter is the 'k' i.e. the diagonal above which to zero elements −

print("Result...",np.tril(arr, k = -1))


## Example

import numpy as np

# Create a 2d array
arr = np.array([[36, 36, 78, 88], [92, 81, 98, 45], [22, 67, 54, 69], [69, 80, 80, 99]])

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

# Get the datatype
print("Array datatype...",arr.dtype)

# Get the dimensions of the Array
print("Array Dimensions...",arr.ndim)

# Get the shape of the Array
print("Our Array Shape...",arr.shape)

# Get the number of elements of the Array
print("Elements in the Array...",arr.size)

# To return the lower triangle of an array, use the numpy.tril() method in Python Numpy
# The 1st parameter is the input array
# The 2nd parameter is the 'k' i.e. the diagonal above which to zero elements.
# k = 0 (the default) is the main diagonal, k < 0 is below it and k > 0 is above.' array
# The k = -1 value is to zero the main diagonal as well
print("Result...",np.tril(arr, k = -1))

## Output

Array...
[[36 36 78 88]
[92 81 98 45]
[22 67 54 69]
[69 80 80 99]]

Array datatype...
int64

Array Dimensions...
2

Our Array Shape...
(4, 4)

Elements in the Array...
16

Result...
[[ 0 0 0 0]
[92 0 0 0]
[22 67 0 0]
[69 80 80 0]]