Take a lineout of a matrix, defined by a function

Let M be an m by n matrix, let f:(1:m)->(1:n)

how can I create a vector that contains all the points that f hits?

Example:

Let:

M =  [1 2 3;
      4 5 6;
      7 8 9]

and let j = f(i) = i

that is, my output vector should be v = [1 5 9]

or let j = f(i) = 4-i

in this case, v = [7 5 3]

or, more complicated, let j = f(i) = round(1+sqrt(i))

then v = [4 5 9]

Answer 1

[Thorsten]

 i = 1:size(M,1);

 f = @(i) i;
 M(sub2ind(size(M), i, f(i)))

 f = @(i) 4 - i;
 M(sub2ind(size(M), i, f(i)))

 f = @(i) round(1+sqrt(i));
 M(sub2ind(size(M), i, f(i)))

[Noah Chrein]

This works! Thank you.