CODEWITHSUNDEEP
pythonnumpy
Calvin Jones
Calvin Jones

Reputation: 367

Accessing elements in array Python

This is my first time handling multidimensional arrays and I'm having problems accessing elements. I'm trying to get the red pixels of a picture but just the first 8 elements within the array. Here's the code

import Image
import numpy as np

im = Image.open("C:\Users\Jones\Pictures\1.jpg")
pix = im.load() 

r, g, b = np.array(im).T
print r[0:8]

Upvotes: 2

Views: 2204

Answers (5)

auraham
auraham

Reputation: 1741

I think it could be more simple. This example uses a random matrix (this will be your r matrix):

In [7]: from pylab import *                 # convention

In [8]: r = randint(0,10,(10,10))           # this is your image

In [9]: r
array([[7, 9, 5, 5, 6, 8, 1, 4, 3, 4],
       [5, 4, 4, 4, 2, 6, 2, 6, 4, 2],
       [1, 4, 9, 9, 2, 6, 1, 9, 0, 6],
       [5, 9, 0, 7, 9, 9, 5, 2, 0, 7],
       [8, 3, 3, 9, 0, 0, 5, 9, 2, 2],
       [5, 3, 7, 8, 8, 1, 6, 3, 2, 0],
       [0, 2, 5, 7, 0, 1, 0, 2, 1, 2],
       [4, 0, 4, 5, 9, 9, 3, 8, 3, 7],
       [4, 6, 9, 9, 5, 9, 3, 0, 5, 1],
       [6, 9, 9, 0, 3, 4, 9, 7, 9, 6]])

Then, extract first 8 columns and do something

In [17]: r_8 = r[:,:8]              # extract columns

In [18]: r_8
Out[18]: 
array([[7, 9, 5, 5, 6, 8, 1, 4],
       [5, 4, 4, 4, 2, 6, 2, 6],
       [1, 4, 9, 9, 2, 6, 1, 9],
       [5, 9, 0, 7, 9, 9, 5, 2],
       [8, 3, 3, 9, 0, 0, 5, 9],
       [5, 3, 7, 8, 8, 1, 6, 3],
       [0, 2, 5, 7, 0, 1, 0, 2],
       [4, 0, 4, 5, 9, 9, 3, 8],
       [4, 6, 9, 9, 5, 9, 3, 0],
       [6, 9, 9, 0, 3, 4, 9, 7]])

In [19]: r_8 = r_8 * 2              # do something

In [20]: r_8
Out[20]: 
array([[14, 18, 10, 10, 12, 16,  2,  8],
       [10,  8,  8,  8,  4, 12,  4, 12],
       [ 2,  8, 18, 18,  4, 12,  2, 18],
       [10, 18,  0, 14, 18, 18, 10,  4],
       [16,  6,  6, 18,  0,  0, 10, 18],
       [10,  6, 14, 16, 16,  2, 12,  6],
       [ 0,  4, 10, 14,  0,  2,  0,  4],
       [ 8,  0,  8, 10, 18, 18,  6, 16],
       [ 8, 12, 18, 18, 10, 18,  6,  0],
       [12, 18, 18,  0,  6,  8, 18, 14]])

Now, this is the trick. Replace the first 8 columns in r using hstack:

In [21]: r = hstack((r_8, r[:,8:]))             # it replaces the FISRT 8 columns, note the indexing notation 

In [22]: r
Out[22]: 
array([[14, 18, 10, 10, 12, 16,  2,  8,  3,  4],    # it does not touch the last 2 columns
       [10,  8,  8,  8,  4, 12,  4, 12,  4,  2],
       [ 2,  8, 18, 18,  4, 12,  2, 18,  0,  6],
       [10, 18,  0, 14, 18, 18, 10,  4,  0,  7],
       [16,  6,  6, 18,  0,  0, 10, 18,  2,  2],
       [10,  6, 14, 16, 16,  2, 12,  6,  2,  0],
       [ 0,  4, 10, 14,  0,  2,  0,  4,  1,  2],
       [ 8,  0,  8, 10, 18, 18,  6, 16,  3,  7],
       [ 8, 12, 18, 18, 10, 18,  6,  0,  5,  1],
       [12, 18, 18,  0,  6,  8, 18, 14,  9,  6]])

Upvotes: 1

Shokodemon
Shokodemon

Reputation: 5

EDIT: as to what DSM pointed out, OP is infact using a numpy array.

i retract my answer as nneonneo's correct

Upvotes: 0

cyphar
cyphar

Reputation: 2890

You can do it like this:

r[0][:8]

Note, however, that this will not work if the first row has less than 8 pixels. To fix that, do this:

from itertools import chain
r = list(chain.from_iterable(r))
r[:8]

or (if you don't want to import an entire module):

r = [val for element in r for val in element]
r[:8]

Upvotes: 1

auraham
auraham

Reputation: 1741

do you want all rows too? Try this r[:,:8]

only want the first row? Try this r[0,:8]

Upvotes: 1

nneonneo
nneonneo

Reputation: 179392

Since you're dealing with images, r is a 2-D array. To get the first 8 pixels in the image, try

r.flatten()[:8]

This will wrap around automatically if the first row has less than 8 pixels.

Upvotes: 3

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