How to delete elements from ndarray
We can delete elements of ndarray by using delete() function.
delete(arr, obj, axis=None)
- obj can be int, array of ints or slice
- for multi-dimensional arrays we must have to specify the axis, other-wise the
default axis=None will be considered. In this case first the array is flatten to the 1-D
array and deletion will be performed
Example
Python
In [394]:
import numpy as np
help(np.delete) Output
PowerShell
Help on function delete in module numpy:
delete(arr, obj, axis=None)
Return a new array with sub-arrays along an axis deleted. For a one
dimensional array, this returns those entries not returned by
`arr[obj]`.1-D arrays
Python
In [395]:
# To delete a single element of 1-D array at a specified index
a = np.arange(10,101,10)
b = np.delete(a,3) # to delete the element present at 3rd index
print(f"array a : {a}")
print(f"array b : {b}") Output
PowerShell
array a : [ 10 20 30 40 50 60 70 80 90 100]
array b : [ 10 20 30 50 60 70 80 90 100] Example
Python
In [396]:
# To delete elements of 1-D array at a specified indices
a = np.arange(10,101,10)
b = np.delete(a,[0,4,6]) # to delete elements present at indices:0,4,6
print(f"array a : {a}")
print(f"array b : {b}")Output
PowerShell
array a : [ 10 20 30 40 50 60 70 80 90 100]
array b : [ 20 30 40 60 80 90 100] Example
Python
In [397]:
# To delete elements of 1-D array from a specified range using numpy np.s_[]
a = np.arange(10,101,10)
b = np.delete(a,np.s_[2:6]) # to delete elements from 2nd index to 5th index
print(f"array a : {a}")
print(f"array b : {b}") Output
Python
array a : [ 10 20 30 40 50 60 70 80 90 100]
array b : [ 10 20 70 80 90 100] Example
Python
In [398]:
# To delete elements of 1-D array from a specified range using python range
# this is applicable only for 1-D arrays.
a = np.arange(10,101,10)
b = np.delete(a,range(2,6)) # to delete elements from 2nd index to 5th index
print(f"array a : {a}")
print(f"array b : {b}")Output
PowerShell
array a : [ 10 20 30 40 50 60 70 80 90 100]
array b : [ 10 20 70 80 90 100]6
2-D arrays
- Here we have to provide axis.
- If we are not specifying access then array will be flatten(1-D) and then deletion will
be happend.
Example
Python
In [399]:
# without providing the axis. defautl axis=None will be taken
a = np.arange(1,13).reshape(4,3)
b = np.delete(a,1)
print(f"array a : \n {a}")
print(f"array b : \n {b}")Output
PowerShell
array a :
[[ 1 2 3]
[ 4 5 6]
[ 7 8 9]
[10 11 12]]
array b :
[ 1 3 4 5 6 7 8 9 10 11 12]Example
Python
In [400]:
# axis=0. deleting the specifie row
a = np.arange(1,13).reshape(4,3)
b = np.delete(a,1,axis=0) # row at index 1 will be deleted
print(f"array a : \n {a}")
print(f"array b : \n {b}")Output
PowerShell
array a :
[[ 1 2 3]
[ 4 5 6]
[ 7 8 9]
[10 11 12]]
array b :
[[ 1 2 3]
[ 7 8 9]
[10 11 12]] Example
Python
In [401]:
# axis=0. deleting the specified rows
a = np.arange(1,13).reshape(4,3)
b = np.delete(a,[1,3],axis=0) # row at index 1 and index 3 will be deleted
print(f"array a : \n {a}")
print(f"array b : \n {b}") Output
PowerShell
array a :
[[ 1 2 3]
[ 4 5 6]
[ 7 8 9]
[10 11 12]]
array b :
[[1 2 3]
[7 8 9]] Example
Python
In [402]:
# axis=0. deleting the specified range of rows
a = np.arange(1,13).reshape(4,3)
b = np.delete(a,np.s_[0:3],axis=0) # rows from index-0 to index-(3-1) will be deleted
print(f"array a : \n {a}")
print(f"array b : \n {b}") Output
PowerShell
array a :
[[ 1 2 3]
[ 4 5 6]
[ 7 8 9]
[10 11 12]]
array b :
[[10 11 12]] Example
Python
In [403]:
# axis=0. deleting the specified range of rows with step value
a = np.arange(1,13).reshape(4,3)
b = np.delete(a,np.s_[::2],axis=0) # to delete every 2nd row (alternative row) from
index-0
print(f"array a : \n {a}")
print(f"array b : \n {b}") Output
PowerShell
array a :
[[ 1 2 3]
[ 4 5 6]
[ 7 8 9]
[10 11 12]]
array b :
[[ 4 5 6]
[10 11 12]]Example
Python
In [404]:
# axis=1. deleting the specifie column
a = np.arange(1,13).reshape(4,3)
b = np.delete(a,1,axis=1) # column at index 1 will be deleted
print(f"array a : \n {a}")
print(f"array b : \n {b}") Output
PowerShell
array a :
[[ 1 2 3]
[ 4 5 6]
[ 7 8 9]
[10 11 12]]
array b :
[[ 1 3]
[ 4 6]
[ 7 9]
[10 12]] Example
Python
In [405]:
# axis=1. deleting the specified colunns
a = np.arange(1,13).reshape(4,3)
b = np.delete(a,[1,2],axis=1) # columns at index 1 and index 2 will be deleted
print(f"array a : \n {a}")
print(f"array b : \n {b}")Output
PowerShell
array a :
[[ 1 2 3]
[ 4 5 6]
[ 7 8 9]
[10 11 12]]
array b :
[[ 1]
[ 4]
[ 7]
[10]]Example
Python
In [406]:
# axis=1. deleting the specified range of columns
a = np.arange(1,13).reshape(4,3)
b = np.delete(a,np.s_[0:2],axis=1) # rows from index-0 to index-(2-1) will be deleted
print(f"array a : \n {a}")
print(f"array b : \n {b}") Output
PowerShell
array a :
[[ 1 2 3]
[ 4 5 6]
[ 7 8 9]
[10 11 12]]
array b :
[[ 3]
[ 6]
[ 9]
[12]] 3-D arrays
Example
Python
In [407]:
# axis= None
a = np.arange(24).reshape(2,3,4)
b = np.delete(a,3) # flatten to 1-D array and element at 3rd index will be deleted
print(f"array a : \n {a}")
print(f"array b : \n {b}") Output
PowerShell
array a :
[[[ 0 1 2 3]
[ 4 5 6 7]
[ 8 9 10 11]]
[[12 13 14 15]
[16 17 18 19]
[20 21 22 23]]]
array b :
[ 0 1 2 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23] Example
Python
In [408]:
# axis=0. Delete a 2-D array at the specified index
a = np.arange(24).reshape(2,3,4)
b = np.delete(a,0,axis=0) # 2-D array at index 0 will be deleted
print(f"array a : \n {a}")
print(f"array b : \n {b}") Output
PowerShell
array a :
[[[ 0 1 2 3]
[ 4 5 6 7]
[ 8 9 10 11]]
[[12 13 14 15]
[16 17 18 19]
[20 21 22 23]]]
array b :
[[[12 13 14 15]
[16 17 18 19]
[20 21 22 23]]] Example
Python
In [409]:
# axis=1. Delete a row at the specified index from every 2-D array.
a = np.arange(24).reshape(2,3,4)
b = np.delete(a,0,axis=1) # Row at index 0 will be deleted from every 2-D array
print(f"array a : \n {a}")
print(f"array b : \n {b}") Output
PowerShell
array a :
[[[ 0 1 2 3]
[ 4 5 6 7]
[ 8 9 10 11]]
[[12 13 14 15]
[16 17 18 19]
[20 21 22 23]]]
array b :
[[[ 4 5 6 7]
[ 8 9 10 11]]
[[16 17 18 19]
[20 21 22 23]]] Example
Python
In [410]:
# axis=1. Delete a column at the specified index from every 2-D array.
a = np.arange(24).reshape(2,3,4)
b = np.delete(a,1,axis=2) # column at index 1 will be deleted from every 2-D array
print(f"array a : \n {a}")
print(f"array b : \n {b}") Output
PowerShell
array a :
[[[ 0 1 2 3]
[ 4 5 6 7]
[ 8 9 10 11]]
[[12 13 14 15]
[16 17 18 19]
[20 21 22 23]]]
array b :
[[[ 0 2 3]
[ 4 6 7]
[ 8 10 11]]
[[12 14 15]
[16 18 19]
[20 22 23]]] case study
Consider the following array
PowerShell
array([[ 0, 1, 2], [ 3, 4, 5], [ 6, 7, 8], [ 9, 10, 11]])
Delete last row and insert following row in that place:==> [70,80,90] Example
Python
In [411]:
# Solution for the case study
# Step:1 ==> create the required ndarray
a = np.arange(12).reshape(4,3)
# Step:2 ==> Delete the last row.
# We have to mention the axis as axis=0 for rows.
# obj value we can take as -1 , which represents the last row
b = np.delete(a,-1,axis=0)
print("Original Array")
print(f"array a : \n {a}")
print("After deletion the array ")
print(f"array b : \n {b}")
# Step:3 ==> we have to insert the row [70.80.90] at the end
# for this we can use append() function with axis=0
c = np.append(b,[[70,80,90]],axis=0)
print("After insertion the array ")
print(f"array c : \n {c}") Output
PowerShell
Original Array
array a :
[[ 0 1 2]
[ 3 4 5]
[ 6 7 8]
[ 9 10 11]]
After deletion the array
array b :
[[0 1 2]
[3 4 5]
[6 7 8]]
After insertion the array
array c :
[[ 0 1 2]
[ 3 4 5]
[ 6 7 8]
[70 80 90]]Summary
- insert() ==> Insert elements into an array at specified index.
- append() ==> Append elements at the end of an array.
- delete() ==> Delete elements from an array.