how to change every value not equal to X in every rows

Hi,

I have a matrix (20000x365) and I have a vector(20000x1) in which I have the column assiciate with the number that I DON'T want to change (This number change for every rows) .

For every rows I want to change every colums, beside the one that is in the vector, for 0

EX: matrix [18 3 1 11; 8 10 11 3; 9 14 6 1; 4 3 15 21]

Vector [3,1,3,2]

The answers should be [0 0 1 0; 8 0 0 0; 0 0 6 0; 0 3 0 0]

Thanks for your help!

Answer 1

[madhan ravi]

Wanted = zeros(size(matrix));
indices=sub2ind(size(matrix),1:size(matrix,1),Vector);
Wanted(indices)=matrix(indices)

[Walter Roberson]

Should be size(matrix,1) since you are building row indices not column indices.

Note: madhan's strategy is valid, and is equivalent to the strategy I posted a few seconds later. The difference between our code is that I took advantage of knowing how indices are calculated to create code that is slightly more efficient (because it does not call a function to do the calculation.)

[madhan ravi]

Yes your right sir Walter, edited.

[Isabelle Bouret]

Sorry I have an error when I run sub2ind

Error using sub2ind (line 51)

The subscript vectors must all be of the same size.

size(matrix) answers 20000 365

1:size(matrix,1) answers 1:20000

Vector Dimensions 20000,1

[madhan ravi]

See edited answer. size(matrix,1) not 2 also Vector.' - transpose Vector

indices=sub2ind(size(matrix),1:size(matrix,1),Vector.')
% it's easy to figure it out isn't ? 
1:size(matrix,1) % creates a row vector
Vector.' % so if Vector is a column vector transpose it to create a row vector

[Isabelle Bouret]

Thank you!!

I have to modified a llittle bit

Wanted = zeros(size(l_s));
for i=1:L
    if Vector(i,:)==0
     continue
    else 
    indices=sub2ind(size(Matrice),i,Vector(i,:).');
    Wanted(indices)=Matrice(indices);
    end
end 

This work perfectly event though it may not be efficient

[Walter Roberson]

If you are going to loop then

Wanted = zeros(size(l_s));
for i = 1:L
    if Vector(i) ~= 0
        Wanted(i, Vector(i)) = Matrice(i, Vector(i));
    end
end

or

Wanted = zeros(size(l_s));
for i = find(Vector)
    Wanted(i, Vector(i)) = Matrice(i, Vector(i));
end

Answer 2

[Walter Roberson]

rows = size(EX_matrix,1);
ind = (1:rows).' + (Vector(:)-1)*rows;
Output = zeros(size(EX_matrix));
Output(ind) = EX_matrix(ind);