how to find input value of a function knowing one of the outputs?

Hello everyone!

i've got a matlab function defined as

[Q_hw,GUE,COP]=getGUE(T_hwi,T_set,T_ext,Q_gas)

What i need to do now is to find the correct Q_gas so that Q_hw is equal to a set value (let's say equale to 5). I assume to know T_hwi,T_set,T_ext and the set value of the output Q_hw but i have no info about GUE,COP.

Can anyone help me?

Thank you!

FP

Answer 1

[dpb]

Q_Htgt=5;                                             % the target solution value
fnQ=@(QG) getGUE(T_hwi,T_set,T_ext,QG)-Q_Htgt;        % define solver function
Q_guess = YourStartGuessValue;                        % need a starting value in neighborhood
Q=fzer0(@fnQ,Q_guess);                                % see if can find a zero
[~,GUE_tgt,COP_tgt]=getGUE(T_hwi,T_set,T_ext,Q);      % solve for other values given the zero

NB: The values of the other arguments to getGUE and the target Q_H values are embedded in the anonymous function definition for fnQ; if you need to solve for another set of parameters or target value, those values must be redefined and then the definition of the anonymous function re-executed to make it reflect the changes.

See doc fzero for more details, examples with alternate ways to handle extra parameters.