Traversing an image matrix columnwise

Hi guys i wanna traverse a matrix of an image column wise ie. it should consider first column and visit all the rows of that column and so on using for loops. Plz help me if u know the logic or code.

Answer 1

[Josh]

If I understand your question, the solution is a nested for loop along the lines of

[rows,cols] = size(Matrix);
for col = 1:cols
    for row = 1:rows
        work with Matrix(row,col);
    end
end

[MechtEngineer]

Exactly the same as mine above, except neater!

[Karan Ratnaparkhi]

thnx josh n mark..

Answer 2

[MechtEngineer]

I think you would want the following, assuming an m x n matrix A(row x column) with position of each array point (i,j)

for j = 1:size(A,2)  % for each column
   for i = 1:size(A,1)  % for each row
      % put your operation in here - I am just adding 1 to each pixel as an eg.
      A(i,j) = A(i,j) + 1;
   end
end

Answer 3

[David Young]

Linear indexing uses columnwise ordering, so as long as you don't need the row and column indexes explicitly, you can do

for k = 1:numel(A)
  % operate on A(k)
end

It might be even better to consider whether you can vectorise the operation and avoid the loops altogether.

[MechtEngineer]

Impressive solution! That really shortens the code required, and does the same thing.

And yes, as David Young says above, if you vectorise (or make the desired operations into matrices to add, subtract, multiply, correlate, etc) the operation, it will be quicker than "for" loops - I tried it recently, and my code ran 5 times faster!