Why do unexpected imaginary values appear when I try to fill my matrix with a loop?

The entire code is too big to put here, but I have

Ntot   = Nq*Nw;        
matrix = zeros(Ntot,4);
Nline = 0;             

for iq=1:Nq     
    for iw=1:Nw 
        Nline = Nline+1;             
        matrix(Nline,1) = q(iq);       
        matrix(Nline,2) = iw;         
        matrix(Nline,3) = pq(iq)/Nw;  
        matrix(Nline,4) = function(q(iq), w(iw,:), u(iw,:), r(iw,:));
    end
end

with q and pq vectors with only real numbers. The function in line 4 is also working. The first loop (of iq, so for several lines) goes as expected, but in the second loop suddenly unwanted and unexpected imaginary numbers appear in ALL the columns. Can anyone help me with this?

[Evelyn]

ps Nq and Nw are just numbers

[Azzi Abdelmalek]

What is function?

[Star Strider]

More specifically, we need to know what this line:

matrix(Nline,4) = function(q(iq), w(iw,:), u(iw,:), r(iw,:));

is doing. What is ‘function’ and what is it returning?

(One caution: function is a reserved word in MATLAB. Please do not use that name for your own functions or variables.)

[Walter Roberson]

Speculation: the function on the 4th line is not okay, and is using assignin('caller')

[Evelyn]

I changed the name of the function to a more general name, the real name is 'max_meerpeil' and it uses a value of q and three rows, from w, u and r, to calculate one value. I tried it with a random value and random rows, and it works as it should. For the first loop of iq (so for several lines) it also works as it should, just as the other lines. In the second loop of iq it starts to act weird.

[Evelyn]

Walter (or someone else), do you have a suggestion how to get the rows out of w, r and u otherwise?

Answer 1

[Matt J]

Trap the line that generates complex values by Setting A Conditional Breakpoint. Logical places to do this would be all the lines in the loop that assign to "matrix". The relevant condition would be

 nnz(imag(matrix))>0

Do similarly inside whatever "function" is now called until you find the exact line.