access the elements in a matrix given the column indexes

For example x = [1 2 3 4; 5 6 7 8; 9 10 11 12]

Given the row indexes vector y = [1 3 4]; I'd like to get the associated elements : x(1, y(1)) = 1, x(2, y(2)) = 7, x(3, y(3)) = 12. [1 7 12]

Don't use loop.

THanks

Answer 1

[dpb]

iy=[1 3 4];
ix=[1:length(iy)];
y=x(sub2ind(size(x),ix',iy');

[Matt Kindig]

I believe that first line is not valid Matlab syntax.

[dpb]

typo, yes...should be obvious. Corrected, thanks.

Answer 2

[Jan]

how about

diag( x(:, y) )

[zhang]

doesn't work for y = [1 2 4];

[Jan]

result for diag( x(:, [1 2 4]) ) is [1 6 12]'

y(1) is 1 -> x(1, 1) is 1

y(2) is 2 -> x(2, 2) is 6

y(3) is 4 -> x(3, 4) is 12

maybe I didn't quite get your question, to me it looks correct?

[zhang]

Yes, you're right. I'm just curious why does this work? can you give a little bit more explanation?

[Jan]

in x(:, y) the colon operator returns all rows of the matrix x. the second argument 'y' does 'column selection', i.e. it picks only the interesting columns from x:

x(:, [1 2 4])

ans =

     1     2     4
     5     6     8
     9    10    12

the values you are looking for obviously appear as diagonal elements in the result and can be extracted with the diag() function.

the reason why the intersting values are on the diagonal is pretty simple, allthough I admit, that one might must give it a second thought :)

I hope that helped!