Problem with vpa integral

Question

Aviral Srivastava 2024-3-15

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2094831-problem-with-vpa-integral

评论： Walter Roberson 2024-3-15

Vu(k,T) =  ((gama_q.*(g2).*(T^2)))./(2*k);
Vii_u(k,T) = diff(Vu ,k );
rho_u(k,T) = (-Vii_u).*(Vu^2)*l1;
%Fi_u  =  matlabFunction(rho_u.*l1);
Fii( 1 ) = vpaintegral(rho_u, k_min_q ,4*k_min_q);

P.S. I am using vpaintegral becasue int does not yield a functional expression and fails to calculate integral. I tried numerical approach but the error is above the bounds. Once I get Fii as a function of T then I need to differenciate Fii w.r.t T.

k and T are sym variables;

g2 is another sym expression of the following form

g2(k) = N./log(1 + ((k.^2)./lambda^2));

I want Fii(1) to be an integrak done w.r.t k so that the resultant expression is a function of only T.

I am getting the follwing error:

The following error occurred converting from symfun to double:

Unable to convert expression into double array.

Error in m_t_u_B2_transition (line 61)

Fii( 1 ) = vpaintegral(rho_u, k_min_q ,4*k_min_q);

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Steven Lord 2024-3-15

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2094831-problem-with-vpa-integral#answer_1425931

在 MATLAB Online 中打开

This type of error indicates you're assiging a symbolic expression into an element of a double array. When you do this, MATLAB will attempt to convert the symbolic expression into double so it "fits" in the double array. Sometimes this works.

sqrt2 = sqrt(sym(2))

sqrt2 =

doubleArray = zeros(1, 5)

doubleArray = 1×5

0 0 0 0 0

doubleArray(2) = sqrt2

doubleArray = 1×5

0 1.4142 0 0 0

Sometimes MATLAB cannot perform that conversion.

syms x

symArray = zeros(1, 5, 'sym')

symArray =

symArray(2) = sqrt2

symArray =

symArray(4) = x^2

symArray =

doubleArray(4) = x^2

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.

Two potential ways to resolve this error could be to substitute numeric values into the symbolic expression, so instead of a value like "x^2" it has a numeric value (if I substituted x = 3 into the symbolic expression x^2, MATLAB would store the value 9 in doubleArray(4)) or to define the array as symbolic at the start rather than double (like I did with symArray above.)

5 个评论
显示 3更早的评论隐藏 3更早的评论

Torsten 2024-3-15

We might be able to help if you supply executable code that reproduces the error message you get. The code from above is incomplete because sym declarations and/or parameters are missing.