fsolve Doesn't Recognize Input Variables

Question

Matthew 2017-3-10

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/329175-fsolve-doesn-t-recognize-input-variables

评论： Matthew 2017-3-15

I'm new to the fsolve command and learning how it works. Following examples from Help, I wrote the following in the child file:

function F = myfun(M)
    F = ((1/M)*(((1+((gamma-1)/2)*M^2)/(1+((gamma-1)/2)))^((gamma+1)/2/(gamma-1))))-(Ae/Ath);

I then wrote the following in the parent file and executed the script:

Ath=0.289;          %Throat area (m^2)
Ae=0.578;           %Exit area (m^2)
gamma=1.37;         %Specific heat ratio
guess=1E-12;
fsolve(@myfun,guess)

However, fsolve doesn't work until I replace the variable references (i.e., gamma, Ae, Ath) in the child file with actual numbers (e.g., replace gamma w/1.37, etc.). Why is this and how do I work around this? Someone said I had to elevate the variables (i.e., gamma, Ae, Ath) to "global variables." But I don't know what that means, how to do it, or why. Any insight would be greatly appreciated.

Thanks, M Ridzon

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Answer 1

Torsten 2017-3-10

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https://ww2.mathworks.cn/matlabcentral/answers/329175-fsolve-doesn-t-recognize-input-variables#answer_258159

在 MATLAB Online 中打开

Ath=0.289;          %Throat area (m^2)
Ae=0.578;           %Exit area (m^2)
gamma=1.37;         %Specific heat ratio
guess=1E-12;
sol=fsolve(@(x)myfun(x,Ath,Ae,gamma),guess)
function F = myfun(M,Ath,Ae,gamma)
    F = ((1/M)*(((1+((gamma-1)/2)*M^2)/(1+((gamma-1)/2)))^((gamma+1)/2/(gamma-1))))-(Ae/Ath);

Best wishes

Torsten.

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Torsten 2017-3-15

在 MATLAB Online 中打开

solution = fsolve(@(x)myfun_Fanno_Flow(x(1),x(2),gamma,P2,Po1),guess,options)

Best wishes

Torsten.

Matthew 2017-3-15

Thank you again! Perfect! I think I am starting to see how fsolve is supposed to be set up.

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Answer 2

Steven Lord 2017-3-10

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/329175-fsolve-doesn-t-recognize-input-variables#answer_258160

That's correct. Functions have their own workspaces where the variables they have defined internally and the variables they received as input arguments when they are called "live". Because you neither defined the variable Ath in your myfun function's body nor passed it in as an input argument to myfun, it doesn't exist in that workspace and so you can't use it. [I'm deliberately omitting some of the more complicated details regarding workspaces and variable scoping; what I've listed is sufficient to answer the question.]

The approach Torsten used to solve this problem and make Ath visible inside the myfun function workspace is the first approach listed on this documentation page. Global variables is another solution, but they can lead to other complications so that documentation page recommends avoiding using them.