Code that was working under R2012b and do not work any more under R2016a: fcts solve and eval

Question

Charles Praplan 2017-3-8

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/328718-code-that-was-working-under-r2012b-and-do-not-work-any-more-under-r2016a-fcts-solve-and-eval

评论： Stephen23 2017-3-14

Hi, I have following short code that was working under Matlab R2012b (see result below) and do not work any more under R2016a!

Could you examine this!

Thank you

fm    = 1000;
BW_Hz = 100;
delta_OMEGA = BW_Hz/fm;
ai = sqrt(2);
bi = 1;
syms x;
y=bi^2*x^8 -delta_OMEGA^2*bi*x^6  +(delta_OMEGA^2*ai^2-2*bi^2-2*delta_OMEGA^2*bi)*x^4 -delta_OMEGA^2*bi*x^2 + bi^2;
alphas1b=solve(y,x)
alphas1c=eval(alphas1b)

R2012b : OK --------

alphas1b =
                                              1.0360030147808337582053932803524
                                             0.96524815635941929264411150626075
                                            -0.96524815635941929264411150626075
                                             -1.0360030147808337582053932803524
  - 0.035333235097304614354828859673636 + 0.99937558630254651692473420806315*i
  - 0.035333235097304614354828859673636 - 0.99937558630254651692473420806315*i
     0.035333235097304614354828859673636 + 0.99937558630254651692473420806315*i
     0.035333235097304614354828859673636 - 0.99937558630254651692473420806315*i
alphas1c =
     1.0360          
     0.9652          
    -0.9652          
    -1.0360          
    -0.0353 + 0.9994i
    -0.0353 - 0.9994i
     0.0353 + 0.9994i
     0.0353 - 0.9994i

R2016a : ERRORS --------

alphas1b =
 -root(z^4 - z^3/100 - 2*z^2 - z/100 + 1, z, 1)^(1/2)
 -root(z^4 - z^3/100 - 2*z^2 - z/100 + 1, z, 2)^(1/2)
 -root(z^4 - z^3/100 - 2*z^2 - z/100 + 1, z, 3)^(1/2)
 -root(z^4 - z^3/100 - 2*z^2 - z/100 + 1, z, 4)^(1/2)
  root(z^4 - z^3/100 - 2*z^2 - z/100 + 1, z, 1)^(1/2)
  root(z^4 - z^3/100 - 2*z^2 - z/100 + 1, z, 2)^(1/2)
  root(z^4 - z^3/100 - 2*z^2 - z/100 + 1, z, 3)^(1/2)
  root(z^4 - z^3/100 - 2*z^2 - z/100 + 1, z, 4)^(1/2)
Error using evalin
Undefined function or variable 'z'.
Error in sym/eval (line 11)
s = evalin('caller',vectorize(map2mat(char(x))));

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Answer 1

Michelle Wu 2017-3-13

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/328718-code-that-was-working-under-r2012b-and-do-not-work-any-more-under-r2016a-fcts-solve-and-eval#answer_258517

This is due to a change in the design of function 'solve' starting in R2014b. The solver now returns exact representations of the solutions. To get numeric results, you may want to try function 'vpasolve' instead of 'solve' in the newer release.

As a workaround, please replace line 'alphas1b=solve(y,x)' in your code with the following code to call the numeric solver:

alphas1b=vpasolve(y,x)