Problem with for-loop and trapz

Hello,

I have a problem with using a function generated by a for-loop in the trapz-function. The loop looks like:

for a = 1:1:m
  for b = 1:1:m
      y = y + a .* b .* exp(-(b.^2 + a.^2)./(4 .* m.^2 .* sigma.^2)) .* besseli(0,sigma) .* b .* a ./(2 .* m.^2 .* sigma.^2); 
  end
end

I use the resulting y in the trapz-function

trapz(sigma,y)

with a defined 'sigma' there is the error: Undefined function 'max' for input arguments of type 'sym'.

But if I just copy the result of the loop:

f = (50.*exp(-1./(2.*sigma.^2)).*besseli(0, sigma))./sigma.^2 + (576.*exp...
trapz(sigma,f)

it gives me the right result. How can I do it without copying the result from the loop? Thanks for your help!

[Sean de Wolski]

What is sigma? Can you give us full reproduction steps?

[Erik]

syms a b sigma;
m = 10;

for a = 1:1:m
  for b = 1:1:m
      y = y + a .* b .* exp(-(b.^2 + a.^2)./(4 .* m.^2 .* sigma.^2)) .* besseli(0,sigma) .* b .* a ./(2 .* m.^2 .* sigma.^2)
  end
end

sigma = 0.1:0.1:1;
trapz(sigma,y)

Answer 1

[Sean de Wolski]

You need to subs-stitute the values in for sigma:

syms a b sigma y;
m = 10;
for a = 1:1:m
  for b = 1:1:m
    y = y + a .* b .* exp(-(b.^2 + a.^2)./(4 .* m.^2 .* sigma.^2)) .* besseli(0,sigma) .* b .* a ./(2 .* m.^2 .* sigma.^2);
  end
end
sigma_values = 0.1:0.1:1;

subs(y,sigma,sigma_values);
trapz(y)

[Erik]

I tried that, but for some reason "trapz(y)" is just zero.

[Sean de Wolski]

I don't really know what you're trying to do. You have sigma in your equation for y. If you want to substitute this with the values in sigma_values, use subs like I did. If you then want to integrate this with respect to sigma values, you can do that as well:

syms a b sigma y;
m = 10;
y = sym.empty;
for a = 1:1:m
for b = 1:1:m
    y = y + a .* b .* exp(-(b.^2 + a.^2)./(4 .* m.^2 .* sigma.^2)) .* besseli(0,sigma) .* b .* a ./(2 .* m.^2 .* sigma.^2);
end
end
sigma_values = 0.1:0.1:1;

Z = trapz(sigma_values,subs(y,sigma,sigma_values))
double(Z);

But I have no clue what you want so it's just a shot in the dark.