Problem with for-loop and trapz

Question

Erik 2013-10-1

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/88807-problem-with-for-loop-and-trapz

评论： Sean de Wolski 2013-10-8

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!

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Sean de Wolski 2013-10-1

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

Erik 2013-10-7

编辑：Erik 2013-10-8

在 MATLAB Online 中打开

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)

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Answer 1

Sean de Wolski 2013-10-8

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/88807-problem-with-for-loop-and-trapz#answer_98988

在 MATLAB Online 中打开

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)

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Erik 2013-10-8

编辑：Erik 2013-10-8

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

Sean de Wolski 2013-10-8

在 MATLAB Online 中打开

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);