padding numpy array/matrix with string

Question

I am trying to read a multi-band image in a python code. My requirement is to form a neighborhood matrix. So I need to pad the matrix with with some number so as to be able to form neighborhood for each element. Ex. a is a matrix, padding with 0

a= |1 2 3 |
   |4 5 6 |
   |7 8 9 |
neighborhood matrix = |0 0 0|
                      |0 1 2|
                      |0 4 5|

I am using numpy.pad(below) for this and it works perfectly with single band. But for multi-band, it converts noDataValue to its equivalent in 0-255 and pads with it, which I do not want.

pixels = np.pad(a, (padding,padding), mode='constant', constant_values=(noDataValue))

where padding = 1, noDataValue = -999.0 but it automatically converting it to 125. And this is happening only for multiband. So any help would be appreciated.

Or

If I can pad matrix with a string, that would be great. I could not find any function that helps padding with String.

Update 1:

Jens Munk · Accepted Answer · 2016-03-30 17:48:58Z

1

convert a to a type which can be the value noDataValue

e.g.

import numpy as np
# ....
a = [[1,2,3],[4,5,6],[7,8,9]]
a = np.array(a).astype(np.float32)
padding = 2
noDataValue = -999.0
pixels = np.pad(a, (padding,padding), mode='constant', constant_values=(noDataValue))

It works here

edited Mar 30, 2016 at 17:48

answered Mar 30, 2016 at 17:27

Jens Munk

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20 Comments

AN_SH Over a year ago

I am getting AttributeError: 'list' object has no attribute 'astype'. Will it pad with any number?

Jens Munk Over a year ago

Sorry, you need to convert to 2D numpy array, 2 secs. Working now?

AN_SH Over a year ago

Well I tried this, a=[[1,2,3],[4,5,6],[7,8,9]] Alpha = np.reshape(a, (-1, 3)) Alpha = Alpha.astype(np.int64) Error: KeyboardInterrupt

Jens Munk Over a year ago

@AN_SH The above works here. You cannot reshape a list. It needs to be a Numpy array

AN_SH Over a year ago

Its working on simple array, which was already working. let me check on multiband array.

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