Why did this error message appear?

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Jason Chen
Jason Chen 2021-11-14
评论： Jason Chen 2021-11-15
Unable to perform assignment because value of type 'sym' is not convertible to 'double'.
line10.Unable to convert expression containing symbolic variables into double array. Apply 'subs' function first to
substitute values for variables.
  1. m=600:1200;
  2. r=ones(20,601);t=ones(20,601);
  3. n0=1;
  4. n1=1.75;d1=125;
  5. n2=2.5;d2=87.5;
  6. n3=1.45;
  7. k=2*pi./m;
  8. e1=n1*d1*k;e2=n2*d2*k;
  9. x1= sym('x1',[1,601]);
  10. r(1,:)=(n0+n1)/(2*n1)+(n1-n0)/(2*n1)*x1;
  11. t(1,:)=(n1-n0)/(2*n1)+(n1+n0)/(2*n1)*x1;
  12. for i=2:20
  13. if rem(i,2)==0
  14. t(i,:)=(n1+n2)/(2*n2)*t(i-1)*exp(2*e1*1i)+(n1-n2)/(2*n2)*r(i-1)*exp(-2*e1*1i);
  15. r(i,:)=(-n1+n2)/(2*n2)*t(i-1)*exp(2*e1*1i)+(n2+n1)/(2*n2)*r(i-1)*exp(-2*e1*1i);
  16. else
  17. t(i,:)=(n1+n2)/(2*n1)*t(i-1)*exp(2*e2*1i)+(n2-n1)/(2*n1)*r(i-1)*exp(-2*e2*1i);
  18. r(i,:)=(-n2+n1)/(2*n1)*t(i-1)*exp(2*e2*1i)+(n2+n1)/(2*n1)*r(i-1)*exp(-2*e2*1i);
  19. end
  20. end
  21. solve(r(20,:),x);
  22. T=t(20,:).*conj(t(20,:))*n3;
  23. plot(m,T);

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回答（1 个）

Steven Lord
Steven Lord 2021-11-14
Ran in:
Since r is a double array, anything that you want to assign into it must be a double array or convertible into a double array.
In this example:
r = 1:10
r = 1×10
1 2 3 4 5 6 7 8 9 10
syms x y
z = x.^2+y.^2
z = 
what is the double precision value of z? Without having values for x and y we cannot convert z into a double array, and so trying to store z inside r won't work.
r(1) = z
Unable to perform assignment because value of type 'sym' is not convertible to 'double'.

Caused by:
Error using symengine
Unable to convert expression containing symbolic variables into double array. Apply 'subs' function first to substitute values for variables.
You could make r a symbolic array from the start, or you could substitute values for x and y into z (thus making z a plain number.)
rsym = sym(1:10)
  1 个评论
Jason Chen
Jason Chen 2021-11-15
I try to make r a symbolic array from the start,but how can i make it a complex array at last and plot a image？
error symengine
Unable to convert expression containing symbolic variables into double array. Apply 'subs' function first to
substitute values for variables.
sym/double (line 709 )
Xstr = mupadmex('symobj::double', S.s, 0);
(line 26 )
double(t);
  1. m=600:1200;
  2. r=sym('r',[20,601]);
  3. t=sym('t',[20,601]);
  4. n0=1;
  5. n1=1.75;d1=125;
  6. n2=2.5;d2=87.5;
  7. n3=1.45;
  8. k=2*pi./m;
  9. e1=n1*d1*k;e2=n2*d2*k;
  10. x1= sym('x1',[1,601]);
  11. r(1,:)=(n0+n1)/(2*n1)+(n1-n0)/(2*n1)*x1(1,:);
  12. t(1,:)=(n1-n0)/(2*n1)+(n1+n0)/(2*n1)*x1(1,:);
  13. for i=2:20
  15. if rem(i,2)==0
  16. t(i,:)=(n1+n2)/(2*n2)*t(i-1)*exp(2*e1*1i)+(n1-n2)/(2*n2)*r(i-1)*exp(-2*e1*1i);
  17. r(i,:)=(-n1+n2)/(2*n2)*t(i-1)*exp(2*e1*1i)+(n2+n1)/(2*n2)*r(i-1)*exp(-2*e1*1i);
  18. else
  19. t(i,:)=(n1+n2)/(2*n1)*t(i-1)*exp(2*e2*1i)+(n2-n1)/(2*n1)*r(i-1)*exp(-2*e2*1i);
  20. r(i,:)=(-n2+n1)/(2*n1)*t(i-1)*exp(2*e2*1i)+(n2+n1)/(2*n1)*r(i-1)*exp(-2*e2*1i);
  21. end
  22. end
  23. for i=1:601
  24. solve(r(20,i),x1(1,i));
  25. end
  26. double(t);
  27. T=t(20,:).*conj(t(20,:))*n3;
  28. plot(m,T);

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