# Deleting diagonal elements of a numpy array

Approach #1

One approach with masking -

A[~np.eye(A.shape[0],dtype=bool)].reshape(A.shape[0],-1)


Sample run -

In [395]: A
Out[395]:
array([[1, 2, 3],
[4, 5, 6],
[7, 8, 9]])

In [396]: A[~np.eye(A.shape[0],dtype=bool)].reshape(A.shape[0],-1)
Out[396]:
array([[2, 3],
[4, 6],
[7, 8]])


Approach #2

Using the regular pattern of non-diagonal elements that could be traced with broadcasted additions with range arrays -

m = A.shape[0]
idx = (np.arange(1,m+1) + (m+1)*np.arange(m-1)[:,None]).reshape(m,-1)
out = A.ravel()[idx]


Approach #3 (Strides Strikes!)

Abusing the regular pattern of non-diagonal elements from previous approach, we can introduce np.lib.stride_tricks.as_strided and some slicing help, like so -

m = A.shape[0]
strided = np.lib.stride_tricks.as_strided
s0,s1 = A.strides
out = strided(A.ravel()[1:], shape=(m-1,m), strides=(s0+s1,s1)).reshape(m,-1)


Runtime test

Approaches as funcs :

def skip_diag_masking(A):
return A[~np.eye(A.shape[0],dtype=bool)].reshape(A.shape[0],-1)

m = A.shape[0]
idx = (np.arange(1,m+1) + (m+1)*np.arange(m-1)[:,None]).reshape(m,-1)
return A.ravel()[idx]

def skip_diag_strided(A):
m = A.shape[0]
strided = np.lib.stride_tricks.as_strided
s0,s1 = A.strides
return strided(A.ravel()[1:], shape=(m-1,m), strides=(s0+s1,s1)).reshape(m,-1)


Timings -

In [528]: A = np.random.randint(11,99,(5000,5000))

...: %timeit skip_diag_strided(A)
...:
10 loops, best of 3: 56.1 ms per loop
10 loops, best of 3: 82.1 ms per loop
10 loops, best of 3: 32.6 ms per loop


Solution steps:

• Delete the location of the diagonal elements which is at the location range(0, len(x_no_diag), len(x) + 1)
• Reshape your array to (num_rows, num_columns - 1)

The function:

import numpy as np

def remove_diag(x):
x_no_diag = np.ndarray.flatten(x)
x_no_diag = np.delete(x_no_diag, range(0, len(x_no_diag), len(x) + 1), 0)
x_no_diag = x_no_diag.reshape(len(x), len(x) - 1)
return x_no_diag


Example:

>>> x = np.random.randint(5, size=(3,3))
array([[0, 2, 3],
[3, 4, 1],
[2, 4, 0]])
>>> remove_diag(x)
array([[2, 3],
[3, 1],
[2, 4]])