Array as input for symbolic function

Question

cdlapoin 2022-2-27

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1659585-array-as-input-for-symbolic-function

评论： cdlapoin 2022-2-27

采纳的回答： Torsten

在 MATLAB Online 中打开

this is an quation for the Mach number as a function of location and area ratio

so I create an array of area ratios and want to examine M

A_ratio = (0:1:100);

gamma = 1.4;

syms M

eq1 = A_ratio == ((gamma+1)/2)^((gamma+1)/(2*(gamma-1)))*((1+(gamma-1)/2*M^2)^((gamma+1)/(2*(gamma-1)))/M)

eq1 =

var = vpa(eq1)

var =

double(var)

ans = 1×101

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

here the symbolic output of vpa looks correct (non-zero), but when I try to change back from symbolic to numeric array the result is all zeros. is double() the wrong command for this? I am referencing: https://www.mathworks.com/help/symbolic/conversion.html

is there a smarter way to do this?

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Answer 1

Torsten 2022-2-27

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1659585-array-as-input-for-symbolic-function#answer_905770

在 MATLAB Online 中打开

gamma = 1.4;
syms A_ratio M
eq1 = A_ratio == ((gamma+1)/2)^((gamma+1)/(2*(gamma-1)))*((1+(gamma-1)/2*M^2)^((gamma+1)/(2*(gamma-1)))/M);
sol = solve(eq1,M);
solnum = subs(sol,A_ratio,1)   %e.g.

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cdlapoin 2022-2-27

Thank you,

I didn't expect to get 6 roots, and none of them matched what I know about how the output should look. In the end it was because of a missing (-) sign in my definition of eq1. Once that was corrected I could easily see that the real part of the first root was the correct value I was looking for.

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