MATLAB: How to extract numbers from a column in a matrix dependent on a number in another column

MATLABmatrix manipulation

As stated I need to extract numbers from a column in the following matrix.
I want to log the numbers in column 9 into a vector only when the number in column 4 has the value 2. I tried using an if statment but that got me nowhere and it's been awhile since I have used matlab or done any coding.
Any help is greatly appreciated.

Best Answer

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));
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