How to call an element in a numpy array?

Question

This is a really simple question, but I didnt find the answer.
How to call an element in an numpy array?

import numpy as np

arr = np.array([[1,2,3,4,5],[6,7,8,9,10]])

print arr(0,0)

The code above doesn’t work.

Asked By: kame

||

Source

Answer 1

Just use square brackets instead:

print arr[0,0]

Answered By: carl

Answer 2

TL;DR:

Using slicing:

>>> import numpy as np
>>> 
>>> arr = np.array([[1,2,3,4,5],[6,7,8,9,10]])
>>> 
>>> arr[0,0]
1
>>> arr[1,1]
7
>>> arr[1,0]
6
>>> arr[1,-1]
10
>>> arr[1,-2]
9

In Long:

Hopefully this helps in your understanding:

>>> import numpy as np
>>> np.array([ [1,2,3], [4,5,6] ])
array([[1, 2, 3],
       [4, 5, 6]])
>>> x = np.array([ [1,2,3], [4,5,6] ])
>>> x[1][2] # 2nd row, 3rd column 
6
>>> x[1,2] # Similarly
6

But to appreciate why slicing is useful, in more dimensions:

>>> np.array([ [[1,2,3], [4,5,6]], [[7,8,9],[10,11,12]] ])
array([[[ 1,  2,  3],
        [ 4,  5,  6]],

       [[ 7,  8,  9],
        [10, 11, 12]]])
>>> x = np.array([ [[1,2,3], [4,5,6]], [[7,8,9],[10,11,12]] ])

>>> x[1][0][2] # 2nd matrix, 1st row, 3rd column
9
>>> x[1,0,2] # Similarly
9

>>> x[1][0:2][2] # 2nd matrix, 1st row, 3rd column
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: index 2 is out of bounds for axis 0 with size 2

>>> x[1, 0:2, 2] # 2nd matrix, 1st and 2nd row, 3rd column
array([ 9, 12])

>>> x[1, 0:2, 1:3] # 2nd matrix, 1st and 2nd row, 2nd and 3rd column
array([[ 8,  9],
       [11, 12]])

Answered By: alvas

Answer 3

If you are using numpy and your array is an np.array of np.array elements like:

A = np.array([np.array([10,11,12,13]), np.array([15,16,17,18]), np.array([19,110,111,112])])

and you want to access the inner elements (like 10,11,12 13,14.......) then use:

A[0][0] instead of A[0,0]

For example:

>>> import numpy as np
>>>A = np.array([np.array([10,11,12,13]), np.array([15,16,17,18]), np.array([19,110,111,112])])
>>> A[0][0]
>>> 10
>>> A[0,0]
>>> Throws ERROR

(P.S.: Might be useful when using numpy.array_split())

Answered By: imflash217

Answer 4

Also, you could try to use ndarray.item(), for example, arr.item((0, 0))(rowid+colid to index) or arr.item(0)(flatten index), its doc https://docs.scipy.org/doc/numpy/reference/generated/numpy.ndarray.item.html

Answered By: 张春天

Answer 5

Use numpy. array. flatten() to convert a 2D NumPy array into a 1D array
print(array_2d)
array_1d = array_2d. flatten() flatten array_2d
print(array_1d)

Answered By: Fan Yang

How to call an element in a numpy array?

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