CODEWITHSUNDEEP
matlabmatrixvectorvectorizationsubtraction
Giuseppe Trapasso
Giuseppe Trapasso

Reputation: 189

Matlab - subtract a vector to each row of a matrix without for statement

I have a piece a code for an exam, what They want me to do in order to achieve a better grade is to implement the same thing without a second "for" statement.

The code is:

piv = 1:n;                  %// piv: position vector
for k = 1:n-1               %// for each column :
   if ((max(abs(A(piv(k:n),k)))) > eps(normA))    %// if pivot is non zero
       [~, I] = max(A(piv(k:n),k));       %// find the max index
       I = I + (k-1);                 
       piv([k,I]) = piv([I,k]);       %// swap pivot elements
       A(piv(k+1:n),k) = A(piv(k+1:n),k)/A(piv(k),k);    %// calculate the multipliers and save them in the column 
       for j = k+1:n
            A(piv(j),k+1:n) = A(piv(j),k+1:n) - (A(piv(k),k+1:n)*A(piv(j),k));   %// multiply for multipliers and subtract them by the row 
       end 
    end
end

This is the Gauss factorizing method but doesn't matter, the matter is I need to have the same result without the second for e and the j variable.

Upvotes: 3

Views: 461

Answers (1)

Divakar
Divakar

Reputation: 221524

You can certainly kill the innermost loop with bsxfun. I am leaving it to you to explain to your prof on how it does what it does. Going through the bsxfun docs would be a good idea and in this process you might learn some vectorization techniques. Here's the implementation -

parte2 = bsxfun(@times,A(piv(k),k+1:n),A(piv(k+1:n),k))
A(piv(k+1:n),k+1:n) = A(piv(k+1:n),k+1:n) - parte2

Upvotes: 3

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