CODEWITHSUNDEEP
matlabmatrixmultidimensional-arrayreplacevectorization
Andi
Andi

Reputation: 4899

Replace values from a certain row onwards column-wise

I have a 10x5 double matrix mat. I also have a 1x5 row vector start_rows. In mat I would like to replace all numbers from specific rows onwards using start_rows. I could use a loop and replace all the numbers column by column. However, I'm sure there is some vectorized solution.

mat = nan(10, 5);
start_rows = [3,5,1,7,2];

% How to avoid that loop
for idx = 1 : numel(start_rows)
    mat(start_rows(idx):end, idx) = 1;
end

Upvotes: 1

Views: 89

Answers (1)

Dev-iL
Dev-iL

Reputation: 24179

This can be solved comparing an array of the following form to your start_rows vector:

     1     1     1     1     1
     2     2     2     2     2
     3     3     3     3     3
     4     4     4     4     4
     5     5     5     5     5
     6     6     6     6     6
     7     7     7     7     7
     8     8     8     8     8
     9     9     9     9     9
    10    10    10    10    10

which will return a logical array for when the condition is met (this uses broadcasting AKA implicit expansion).

If mat always contains zeros, and you're replacing with ones:

(1:size(mat,1)).'+ mat >= start_rows;

If mat is nonzero:

(1:size(mat,1)).'+ 0*mat >= start_rows;             % option 1
(1:size(mat,1)).'+ zeros(size(mat)) >= start_rows;  % option 2

If replacing with values other than 1 (or true):

((1:size(mat,1)).'+ 0*mat >= start_rows) * newVal;

Upvotes: 2

Related Questions