Replacing values in a matrix based on values in a column vector

1 view (last 30 days)
I have a column vector (with dimensions m x 1) which contains values from 1 to 10.
I want to make a matrix yMat with dimension m x 10, such that, in corresponding rows of y and yMat, the value at the column of yMat corresponding to the value in y is equal to 1.
All other values in yMat should be 0.
For example, if
y = [2; 4; 1; 8]
then
yMat = [0 1 0 0 0 0 0 0 0 0; 0 0 0 1 0 0 0 0 0 0; 1 0 0 0 0 0 0 0 0 0; 0 0 0 0 0 0 0 1 0 0]
The following code works, but is there any way to do this without using a for loop?
yMat = zeros(m, 10); % m is the number of rows in column vector y
for i = 1:m
yMat(i, y(i)) = 1;
end

Accepted Answer

Digaamber Dhamija
Digaamber Dhamija on 22 Jun 2020
Edited: Digaamber Dhamija on 22 Jun 2020
Just in case anyone else has a similar problem, I found an even simpler solution.
y = [2; 4; 1; 8];
yMat = 1:10 == y;
which produces a logical array yMat.
You can get numeric values by doing:
yMat = double(1:10 == y);
producing
yMat =
0 1 0 0 0 0 0 0 0 0
0 0 0 1 0 0 0 0 0 0
1 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 1 0 0

More Answers (1)

Mara
Mara on 21 Jun 2020
I came up with this. But not sure if this the easiest way to do it.
yMat = zeros(length(y)*10, 1);
yMat((y-1)*length(y)+ (1:length(y))') =1;
yMat = reshape(yMat, length(y), []);
  2 Comments
Digaamber Dhamija
Digaamber Dhamija on 21 Jun 2020
Edited: Digaamber Dhamija on 21 Jun 2020
The formula
(y-1)*length(y) + (1:length(y))'
seems to be doing something similar to calculating the linear indices for the original dimensions of yMat.
It would also have worked if you did this
yMat = zeros(length(y), 10);
yMat((y-1)*length(y) + (1:length(y))') = 1;
Mara
Mara on 21 Jun 2020
True, it does not have to be a vector for the indexing to work. Thanks for the feedback! I did not see that you received another answer already, it seems to be more intuitive than this one, anyways.

Sign in to comment.

Products


Release

R2020a

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!