How to extract odd values and even values from a 500x500 black image ?

I want to extract the even and odd numbers from 500x500 black matrix to turn them white without using loops or conditions statement.

[DGM]

If black is zero and the array is all black, then which instances of zero are even and which ones are odd?

[Maha Almuaikel]

pardon me I was not clear, I mean the odd and even index from the zeros matrix

[Image Analyst]

You're still not clear. Zero is neither an odd number or an even number.

Answer 1

[Image Analyst]

Do you have any other numbers other than 0? If so you can use rem(M, 2). Observe:

M = magic(5)

M = 5×5

17 24 1 8 15 23 5 7 14 16 4 6 13 20 22 10 12 19 21 3 11 18 25 2 9

mCopy = M;
oddIndexes = rem(mCopy, 2) == 1

oddIndexes = 5×5 logical array
   1   0   1   0   1
   1   1   1   0   0
   0   0   1   0   0
   0   0   1   1   1
   1   0   1   0   1

oddNumbers = M(oddIndexes) % Extract the odd numbers.

oddNumbers = 13×1

17 23 11 5 1 7 13 19 25 21

% Turn the odd numbers to white (255)
mCopy(oddIndexes) = 255

mCopy = 5×5

255 24 255 8 255 255 255 255 14 16 4 6 255 20 22 10 12 255 255 255 255 18 255 2 255

evenIndexes = rem(mCopy, 2) == 0

evenIndexes = 5×5 logical array
   0   1   0   1   0
   0   0   0   1   1
   1   1   0   1   1
   1   1   0   0   0
   0   1   0   1   0

evenNumbers = M(evenIndexes)

evenNumbers = 12×1

4 10 24 6 12 18 8 14 20 2

% Turn the even numbers to white (255)
mCopy = M; % Reset mCopy so there are no 255's in there anymore.
mCopy(evenIndexes) = 255

mCopy = 5×5

17 255 1 255 15 23 5 7 255 255 255 255 13 255 255 255 255 19 21 3 11 255 25 255 9

Answer 2

[DGM]

Consider the matrix:

A = randi([10 99],5,5)

A = 5×5

37 23 33 22 34 58 58 16 79 86 11 39 73 93 97 11 37 92 79 19 32 19 47 48 85

% get elements from odd linear indices
oddelems = A(1:2:numel(A))

oddelems = 1×13

37 11 32 58 37 33 73 47 79 79 34 97 85

% get elements from even linear indices
evenelems = A(2:2:numel(A))

evenelems = 1×12

58 11 23 39 19 16 92 22 93 48 86 19

That's linear indices. Maybe you want to address odd or even rows/columns:

% get elements from odd rows
oddrows = A(1:2:size(A,1),:)

oddrows = 3×5

37 23 33 22 34 11 39 73 93 97 32 19 47 48 85

% get elements from even rows
evenrows = A(2:2:size(A,1),:)

evenrows = 2×5

58 58 16 79 86 11 37 92 79 19

... and so on for columns

EDIT: I'm dumb. I forgot you wanted to set pixels to white.

The above still holds. You're still trying to address your array. Just apply that to an assignment:

B = zeros(5)

B = 5×5

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

B(1:2:numel(B)) = 1 % set odd linear indices to 1

B = 5×5

1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

... and so on