Error using sym/subsref

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Rohit Tamrakar 2020-5-4

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/522857-error-using-sym-subsref

评论： Rohit Tamrakar 2020-5-4

采纳的回答： Walter Roberson

在 MATLAB Online 中打开

I have this program to get the value of "m"

clc
syms m
eqns = (72*m^2)-(1.5*m^3) == 358;
S=solve (eqns,m,'Real',true)
double (S.m);

However in the output I get this error

S =

32*cos(atan(2167511^(1/2)/5965)/3) + 16

16*3^(1/2)*sin(atan(2167511^(1/2)/5965)/3) - 16*cos(atan(2167511^(1/2)/5965)/3) + 16

16 - 16*3^(1/2)*sin(atan(2167511^(1/2)/5965)/3) - 16*cos(atan(2167511^(1/2)/5965)/3)

Error using sym/subsref

Too many output arguments.

Error in Untitled (line 5)

double (S.m);

Can any body help me with this?

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采纳的回答

Walter Roberson 2020-5-4

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When there is only a single variable being solved for, and ReturnConditions is not set, then solve() does not return a structure containing the one variable: it returns the value of the variable. In terms of your code, that means you would use

double(S) %instead of double(S.m)

Or you could instead

S=solve (eqns, m, 'Real', true, 'returnconditions', true);

and then S.m would exist.

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Walter Roberson 2020-5-4

在 MATLAB Online 中打开

min(S(S>0))

Rohit Tamrakar 2020-5-4

Thanks Walter :)

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Mathias Andersen 2020-5-4

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Double(S) will produce a matrix with the solutions.

S.m is not a variable stated anywhere in the script

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Mathias Andersen 2020-5-4

编辑：Mathias Andersen 2020-5-4

S contains the values for m

Walter Roberson 2020-5-4

Not when you solve for a single variable and you do not have ReturnConditions true: in that case, MATLAB cannot tell that apart from the case where you want the non-struct version of the solution.

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