How to vectorize this condition switch code

Dear all,

I have one code like this

function y = intefai1(a,m1,m)
% fai(m1,x)*fai(m,x)
if m1 >= 0
   if m >= 0
       y = alpha1(a,m1,m);
   else
       y = -gam(a,m1,abs(m));
   end
else
   if m >= 0
       y = -gam(a,m,abs(m1));
   else
       y = betass(a,abs(m),abs(m1));
   end
end

How can i avoid for loop with logical operation? Thanks

[Matt J]

What do you mean avoid a for-loop? You don't have a for-loop now.

[Xianjie]

Sorry, I want to how to vectorize it. Because now in my code, m and n are matrix. So please help me on it.

[Harshvardhan]

where is n?

[Xianjie]

Sorry. Now in these codes, there is no n. Only want to vectorize this code and also it can fit for n and n1.

Answer 1

[Dr. Seis]

From your more recent question, I found:

M=10;N=10;P=4;
for m1=-P:M
    for n1=-P:N
        s=(m1+P)*(N+1+P)+n1+P+1;
        for m=-P:M
            for n=-P:N
                t=(m+P)*(N+1+P)+n+P+1;
                A2(s,t)=intefai1(a,m1,m);
            end
        end
    end
end

I am still not sure what a is (so I will assume it is a constant like your m1 and m). So instead of those for loops you could just:

[m,m1]=meshgrid(-P:M); % m and m1 are now matrices !! 
A2 = zeros(size(m));
A2(m1 >= 0 & m >= 0) = ... 
      alpha( a, m1(m1 >= 0 & m >= 0), m(m1 >= 0 & m >= 0) );

And similarly for the other conditions.

Now... I realize that this produces a (P+M+1)x(P+M+1) matrix instead of a ((P+M+1)*(P+N+1))x((P+M+1)*(P+N+1)) matrix. Once you have these values for the smaller matrix you can populate the larger matrix by essentially copying/replicating... you don't want to have to do the same computations (P+M+1)*(P+N+1) times if you only need to do it once and then just replicate.

Answer 2

[Jan]

Does the method work, which I have provided in your former thread http://www.mathworks.com/matlabcentral/answers/52831-how-to-vectorize-these-codes ? An equivalent approach should work here also.

[Dr. Seis]

I guess the bigger issue might have been how to construct m and m1 (or m and n in your case) such that the logical indexing scheme would produce the same result as the multi-for-loop-if-else configuration.

The information above I took from their more recent question here: http://www.mathworks.com/matlabcentral/answers/53399-this-calculate-error-is-why

[Jan]

@Elige: Thanks. I loose the overview in this stack of threads.