using multiple if statement to replace values from another matrix

Hi,

I have two matrix (A and B in attached excel file) which have the same dimension (853x13). I would like to use multiple "if" function as following code;

C = zeros(size(B));
for k = 1:length(B);
    l = 1:length(A);
    if B(k,2) > B(k,3);
        if B(k,4:13) > A(l,4:13);
           C(k,4:13) == A(l,4:13);
        else
           C(k,4:13) = B(k,4:13);
        end
    else
        C(k,4:13) = B(k,4:13);
    end
end

I want matrix "C" to have both values from A and B if it follows the fist line of if condition. However, the new matrix "C" has values only from matrix B. How should I correct it? Since the length of matrix A and B is so huge, I attached excel file of both matrix.

Thanks

Answer 1

[Walter Roberson]

Your line

             C(k,4:13) == A(l,4:13);

is a comparison, not an assignment.

[Phat Pumchawsaun]

I have changed to;

C(k,4:13) = A(l,4:13);

But, there is still be the same problem.

[Walter Roberson]

You have

l = 1:length(A);

so l is a vector.

You have

if B(k,4:13) > A(l,4:13);

l is a vector, as indicated above, so A(l,4:13) is indexing A with subscripts that are both vectors. Since l is 1:length(A) then as long as A has more rows than columns, A(l,4:13) means the same as A(:,4:13) does -- a rectangular portion extracted from an array.

Meanwhile, B(k,4:13) is a vector. So your > operator is comparing a vector to a rectangular array. In all versions before R2016b, that would be an error. In R2016b and later, the result is the same as if you had used

if bsxfun(@gt, B(k,4:13), A(l,4:13))

which in turn is the same as

if repmat(B(k,4:13), length(A), 1) > A(l,4:13)

which repeats the row of B into equivalent size of A's rows. You end up with a rectangular matrix ">" a rectangular matrix. The result of that ">" operator is going to be a rectangular matrix of true and false values.

When "if" is asked to process a vector or matrix of true and false values, then it considers the condition to hold only if all of the values are non-zero (true). In your 2D matrix case, that would be equivalent to

if all(all( repmat(B(k,4:13), length(A), 1) > A(l,4:13) ))

And in your case that does not happen to be the case.

My guess at what you want:

nrow = size(B,1);
for k = 1:nrow
    if B(k,2) > B(k,3)
      C(k,4:13) = min(A(k,4:13), B(k,4:13));
    else
      C(k,4:13) = B(k,4:13);
    end
end

Or in vectorized form with no loop:

C = zeros(size(B));
C(:,4:13) = B(:,4:13);
mask = B(:,2) > B(:,3);
C(mask, 4:13) = min( A(mask,4:13), B(mask,4:13) );