CODEWITHSUNDEEP
matlabfor-loopvectorization
JAN
JAN

Reputation: 21895

Vectorizing a MATLAB loop

I'm trying to do my best and not use loops in Matlab, but I find it very hard sometimes .

For example I wrote this code : 

vector = zeros(p,1);

diagDominantFlag = 1;
for row = 1:p
    for col = 1:q
        if(row ~= col)
            vector(row) = vector(row) + A(row,col);
        end
    end
    if(A(row,row) <= vector(row))
        diagDominantFlag = 0;
        break;
    end
end

Is it possible to vectorize that double for loop?

Thanks

Upvotes: 3

Views: 349

Answers (3)

Ansari
Ansari

Reputation: 8218

You can replace the inner loop with

vector(row) = sum(A(row, :)) - A(row, row);

and in fact you can replace the whole thing by

vector = sum(A, 2) - diag(A);

To add the check you have in the outer loop, you can then do something like

f = find(diag(A) <= vector);
if length(f) > 0
    diagDominantFlag = 0;
    vector(f(1):end) = 0;
end

Upvotes: 2

Eitan T
Eitan T

Reputation: 32930

Adding to Ansari's answer, the diagDominantFlag can be calculated by:

diagDominantFlag = all(diag(A) >= (sum(A, 2) - diag(A)));

Thus replacing your double for loop with a one-liner.

Upvotes: 2

Chris A.
Chris A.

Reputation: 6887

Not an answer to your direct question, but, here's a loopless test for diagonal dominance (since you seem interested in that):

all(diag(A) >= (sum(abs(A),2)-abs(diag(A)) )

or for strict diagonal dominance:

all(diag(A) > (sum(abs(A),2)-abs(diag(A)) )

Also, in your above code, make sure you do abs() on the off-diagonal values.

Upvotes: 2

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