I have a matrix, for example
A = [ 1 2 3; 4 5 6; 7 8 9] ;
and a vector of size 1x3 which specifies which element in each row is the one I'm looking for - i.e. If
vector = [ 1 2 1 ]
then the desired output is
[ 1 5 7 ]
since 1 is the 1'st element in the 1'st row, 5 is the 2'nd in the 2'nd row, and 7 is the 1'st element in the 3'rd row.
How do I achieve this? Couldn't find a built in function to do this, which surprised me.
MATLAB provides the SUB2IND function to convert rows/columns subscripts to linear indices:
>> A = [1 2 3; 4 5 6; 7 8 9];
>> idx = sub2ind(size(A),1:3,[1 2 1]); %# rows: [1 2 3], cols: [1 2 1]
>> A(idx)
1 5 7
First of all, the indexes in Matlab go from top to bottom.
So in your case A[1] = 1 , A[2] = 4 , A[3] = 7
That said, it would be easier to work on A' , because its a bit more trivial.
B = A';
B((vector + [0:2].* 3))
Here's a variation of Yochai's answer but without the transpose (this is also basically what SUB2IND does in Amro's answer):
output = A((1:3)+3.*(vector-1));
Or for an array A of an arbitrary size:
nRows = size(A,1);
output = A((1:nRows)+nRows.*(vector-1));
