Error using sym/subsref

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?

Answer 1

[Walter Roberson]

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.

[Rohit Tamrakar]

Thanks Walter this worked.

I got this as answer.

ans =

47.8960

2.2849

-2.1809

I want the the value of "m" to be selected as 2.28 i.e the smallest positive integer among these three values. What code I have to use for selecting 2.28 value?

[Walter Roberson]

min(S(S>0))

[Rohit Tamrakar]

Thanks Walter :)

Answer 2

[Mathias Andersen]

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

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

[Rohit Tamrakar]

How can I get the value for variable "m" ?

[Walter Roberson]

S is the output of solve(). If two or more variables were being solved for and a single output were given, then S would be a structure with one field for each variable being solved for, and then S.m would exist.

S.m would also be created if 'returnconditions', true is specified.

[Mathias Andersen]

S contains the values for m

[Walter Roberson]

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.