parallelizing for-loop containing if-statement

Question

I have to traverse a 2D-array and perform some operations on them, depending on the results of an if-statement. I have made a small example of the loop here:

N=128;    

A = rand(N,N);
B = rand(N,N);

sqr = @(x) x.^2;

for xi=1:N
    for yi=1:N
        a = A(xi,yi);
        b = B(xi,yi);

        if( abs(a-b)<1 )
            result=2.0;
        else
            result = sqr(a-b);
        end

        res_matrix(xi,yi) = result;
    end
end

I want to parallelize this for-loop. I have read the MathWorks-page on parfor, and the way I would parallelize it is just by making the outer-loop into a parfor.

Is this the best speed-up I can get, or should I structure my loop differently?

Here is a more detailed version of my loop:

for xi= 1:N
    for yi= 1:N

        a  = A(:,xi,yi);   %a is a vector
        b  = B(:,xi,yi);   %b is a vector
        D  = a-b;

        if( max(D./a)<1e-3 )
            test_var=2.0;  
        else

            F_min = F(a, b, 0); %F is some function, such as a Newton-Raphson solver etc...
            F_max = F(a, b, 1); %F is some function, such as a Newton-Raphson solver etc...          

            if( F_min*F_max>0.0 )
                test_var=2.0;
            else
                test_var = F(a, b, 2);
            end
        end

        var(1, xi,yi) = test_var;
    end
end

Changing directly for loops into par-for loops is good when, you want to repeat the same process again and again (for instance when gathering statistical information). Otherwise you need to manage the resource assignment to each different worker. — NKN
– NKN, Commented Aug 24, 2015 at 13:16
a minimal reproducible example of your "detailed version" with inputs and outputs would help others help you — m.s.
– m.s., Commented Aug 24, 2015 at 14:59

NKN · Accepted Answer · 2015-08-24 13:31:57Z

3

I think you can reduce the code (replace the two for-loops and the if statement) into something like this:

C = abs(A-B);
D = ones(N);
R = sqrt(C);
idxC = bsxfun(@gt,C,D);
R(idxC) = 2.0;

first you calculate the condition abs(A,B) and then apply the comparison to all the elements of the matrix.

edited Aug 24, 2015 at 13:31

answered Aug 24, 2015 at 13:25

NKN

6,4526 gold badges37 silver badges54 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

BillyJean Over a year ago

thanks, but my example in my OP was very artificial, I'm not actually taking the square root of anything, it was just a simple example to show the overall structure...

BillyJean Over a year ago

I posted a more detailed version of my loop, it is possible to vectorize/optimize it?

josoler · Accepted Answer · 2015-08-26 15:45:25Z

2

If you want a speed-up you may be interested in the vectorized form:

[s1, s2, s3] = size(A);     % Size of A and B
C=A-B;                      % Difference between  A and B

%// Upper threshold condition
idx_under_threshold = squeeze(max(C,[],1)<threshold);

%// Functions to compute operations involving function F
fmin = @(idx1, idx2) F(A(:,idx1,idx2),B(:,idx1,idx2),0);
fmax = @(idx1, idx2) F(A(:,idx1,idx2),B(:,idx1,idx2),1);
fvalue = @(idx1, idx2) F(A(:,idx1,idx2),B(:,idx1,idx2),2);

%// Combined indices of A and B
idA = repmat((1:s2)',1,s3);    %'
idB = repmat(1:s3, s2, 1);

%// Computation of fmin, fmax and fvalue over all the elements
Fmin = cell2mat(arrayfun(fmin, idA, idB, 'UniformOutput', false));
Fmax = cell2mat(arrayfun(fmax, idA, idB, 'UniformOutput', false));
Fvalue = cell2mat(arrayfun(fvalue, idA, idB, 'UniformOutput', false));

%// Second condition: ( abs(a-b)<1 )
idx_2 = idx_under_threshold | (Fmin.*Fmax < 5);

%// Inizalization of the result to 'result = sqr(a-b)'
Var = Fvalue;

%// Handling cases where 'result = 2.0'
Var(idx_2) = 2;

You can place that piece of code instead of the for loop

edited Aug 26, 2015 at 15:45

answered Aug 24, 2015 at 13:25

josoler

1,44310 silver badges15 bronze badges

1 Comment

BillyJean Over a year ago

I posted a more detailed version of my loop, it is possible to vectorize/optimize it?

Collectives™ on Stack Overflow

parallelizing for-loop containing if-statement

2 Answers 2

2 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related