How can I delete multiple rows of NumPy array? For example, I want to delete the first five rows of x. I'm trying the following code:
import numpy as np
x = np.random.rand(10, 5)
np.delete(x, (0:5), axis=0)
but it doesn't work:
np.delete(x, (0:5), axis=0)
^
SyntaxError: invalid syntax
There are several ways to delete rows from NumPy array.
The easiest one is to use basic indexing as with standard Python lists:
>>> import numpy as np
>>> x = np.arange(35).reshape(7, 5)
>>> x
array([[ 0, 1, 2, 3, 4],
[ 5, 6, 7, 8, 9],
[10, 11, 12, 13, 14],
[15, 16, 17, 18, 19],
[20, 21, 22, 23, 24],
[25, 26, 27, 28, 29],
[30, 31, 32, 33, 34]])
>>> result = x[5:]
>>> result
array([[25, 26, 27, 28, 29],
[30, 31, 32, 33, 34]])
You can select not only rows but columns as well:
>>> x[:2, 1:4]
array([[1, 2, 3],
[6, 7, 8]])
Another way is to use "fancy indexing" (indexing arrays using arrays):
>>> x[[0, 2, 6]]
array([[ 0, 1, 2, 3, 4],
[10, 11, 12, 13, 14],
[30, 31, 32, 33, 34]])
You can achieve the same using np.take:
>>> np.take(x, [0, 2, 6], axis=0)
array([[ 0, 1, 2, 3, 4],
[10, 11, 12, 13, 14],
[30, 31, 32, 33, 34]])
Yet another option is to use np.delete as in the question. For selecting the rows/columns for deletion it can accept slice objects, int, or array of ints:
>>> np.delete(x, slice(0, 5), axis=0)
array([[25, 26, 27, 28, 29],
[30, 31, 32, 33, 34]])
>>> np.delete(x, [0, 2, 3], axis=0)
array([[ 5, 6, 7, 8, 9],
[20, 21, 22, 23, 24],
[25, 26, 27, 28, 29],
[30, 31, 32, 33, 34]])
But all this time that I've been using NumPy I never needed this np.delete, as in this case it's much more convenient to use boolean indexing.
As an example, if I would want to remove/select those rows that start with a value greater than 12, I would do:
>>> mask_array = x[:, 0] < 12 # comparing values of the first column
>>> mask_array
array([ True, True, True, False, False, False, False])
>>> x[mask_array]
array([[ 0, 1, 2, 3, 4],
[ 5, 6, 7, 8, 9],
[10, 11, 12, 13, 14]])
>>> x[~mask_array] # ~ is an element-wise inversion
array([[15, 16, 17, 18, 19],
[20, 21, 22, 23, 24],
[25, 26, 27, 28, 29],
[30, 31, 32, 33, 34]])
For more information refer to the documentation on indexing: https://docs.scipy.org/doc/numpy/reference/arrays.indexing.html
If you want to delete selected rows you can write like
np.delete(x, (1,2,5), axis = 0)
This will delete 1,2 and 5 th line, and if you want to delete like (1:5) try this one
np.delete(x, np.s_[0:5], axis = 0)
by this you can delete 0 to 4 lines from your array.
np.s_[0:5] --->> slice(0, 5, None) both are same.
Pass the multiple row numbers to the list argument. General Syntax:
np.delete(array_name,[rownumber1,rownumber2,..,rownumber n],axis=0)
Example: delete first three rows in an array:
np.delete(array_name,[0,1,2],axis=0)
