I have vector of indexes. How to set ones at given index in each row?

Question

I have a vector y with integer values from 1 to 10:

octave> size(y)
ans =

   5000      1

I created zeros array y1:

octave> size(y1)
ans =

   5000     10

I need to set 1 in each row of y2, in the element with index equal to value in y. So in first row, when I have:

octave> y(1)
ans =  10

I need to have:

octave> y1(1,:)
ans =

   0   0   0   0   0   0   0   0   0   1

Something reverse to [w, y] = max(y2, [], 2); I have in other places of my code.

Is there a simple one-line trick to do that? And if not, how can I iterate over both arrays simultaneously?

Answer 1

I'm not sure if you mean to do this in y1 or y2. In any case, try this (on y1 for example):

y = randi(10,5000,1);
y1 = zeros(size(y,1), 10);
y1(sub2ind(size(y1), (1:size(y1,1))', y)) = 1;

Answer 2

You can use this trick

y1 = eye(10)(y,:);

or it's two-step version

y1 = eye(10);
y1 = y1(y,:);

Explanation

In first step you create a identity matrix

  >> y1 = eye(10)    

    y1 =    

 Diagonal Matrix

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

In the second step you use y as index in the indentity matrix. This step literally copy rowsfrom identity matrix and create desired matrix.

>> y = [1,1,2,2,5,10,10];
>> y1 = y1(y,:)
y1 =

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

Answer 3

Don't know a one line trick but you can do it in a one simple loop:

for i=1:length(y)
     y1(i,y(i))=1;
end