check if value exists at index in numpy array

Question

This is probably an easy question but I can't seem to find the answer anywhere.

Let's say I have a numpy array like below:

x = np.array([[2,3],[1,0],[5,6]])

I am looking for a way to say if a value exists in x do something.

For example

if x[2][1] exists do something: This happens because it exists (and has a 6 value)

if x[4][2] exists do something: This does not happen because it does not exist (but does not throw an error)

Thank You!

miraunpajaro · Accepted Answer · 2021-04-09 21:08:33Z

1

Just use x.shape(). This returns a tuple with the dimensions, so in your example, you only need to check if 4<x.shape()[0] and 2<x.shape()[1].

edited Apr 9, 2021 at 21:08

answered Apr 9, 2021 at 20:25

miraunpajaro

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hpaulj · Accepted Answer · 2021-04-09 20:25:07Z

1

In [114]: x = np.array([[2,3],[1,0],[5,6]])
In [115]: x[2][1]
Out[115]: 6
In [116]: x[2,1]                # more idiomatic indexing
Out[116]: 6

Indexing out of bounds does raise an error:

In [117]: x[4][2]
Traceback (most recent call last):
  File "<ipython-input-117-74efee77abf6>", line 1, in <module>
    x[4][2]
IndexError: index 4 is out of bounds for axis 0 with size 3

In [118]: x[4,2]
Traceback (most recent call last):
  File "<ipython-input-118-97c89dcf326d>", line 1, in <module>
    x[4,2]
IndexError: index 4 is out of bounds for axis 0 with size 3

answered Apr 9, 2021 at 20:25

hpaulj

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3 Comments

kdbaseball8 Over a year ago

I know the shape of my array it is 28x28. What I actually trying to do is write a blurring function that takes the average of the 8 digits around it and itself. The problem is the edges do not have 8 digits around them so I need something that accounts for this.

kdbaseball8 Over a year ago

I was trying to write two loops where if the value index didn't exist it would just skip it.

hpaulj Over a year ago

If you are indexing with slices, you can end up with an 'empty' window, e.g. arr[26:29, 28:32]. That is you can 'slice' off the edge without warning (that's true for Python lists as well). Sounds like this is a bigger task, one involving convolving and other image processing. If numpy doesn't have the tools, you may want to look in scipy or other sci-kit.

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