If your matrix is A, then
output = log(A(A(:,4)==2,9));
In case it is not obvious what that one line of code is going, work from the "inside out":
- A(:,4) == 2 -- Checks to see if the 4th column is equal to 2, returning a vector of true/false booleans.
- A(A(:,4)==2,9) -- That vector is then used as a logical index into A, grabbing only the rows that are true, and only the 9th column
- Then take the (natural) logarithm of those values
If you want to take the log "in place", then do this:
idx = A(:,4)==2;
A(idx,9) = log(A(idx,9));
