Using fzero with multiple parameters

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Dave
Dave 2013-1-20
评论： Steven Lord 2023-6-16
Having trouble debugging error...
I have 2 scripts:
-function
-script that calls function for use
I'm pasting both scripts along with the error I received and where exactly I think the problem is (according to the error).
Function:
function F=EXPLICITFUNCTION(x)
F = x(1) + x(2) + x(3) - T2;
x(1)*exp(m*L) + x(2)*exp(-m*L) + x(3) - T1;
-k*(x(1)*m*exp(m*L)-x(2)*m*exp(m*L)) + k*(m*(x(1)-x(2))) - x(4);
P*h*((x(1)*(exp(L*m) - 1))/m - (x(2)*(1/exp(L*m) - 1))/m) - x(4);
end
Script:
%______________________________
clear all
clc
%______________________________
T2 = 375; %........................K
T1 = 450; %........................K
D = 0.003; %........................m
P = pi*D; %........................m
A_Cond = (pi/4)*(0.003-0.002); %.........m^2
h = 257.799; %......................W/(m^2*K)
k = 70; %......................W/(m*K)
L = 0.03; %......................m
m = sqrt((h*P)/(k*A_Cond)); %.......(1/m)
x0= [15;12;450;12];
fzero(@(x) MMAE322SUPERFUNCTION(x, T2, T1, m, L, k, P, h), x0)
The error I receive is:
-Error using fzero (line 413)
Second argument must be a scalar or vector of length 2.
-Error in MMAE322SCRIPT (line 19)
fzero(@(x) MMAE322SUPERFUNCTION(x, T2, T1, m, L, k, P, h), x0)
Somehow I think it's because of my guess, x0. I need 4 guesses, one for each unknown variable, but I don't know how to interpret the error. I tried for the sake of debugging by erasing 2 guess in my x0 and of course that didn't work..
I've done this before with a single parameter and single guess but anymore than 1 parameter I get a similar error.
Any help is greatly appreciated! :)

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采纳的回答

Walter Roberson
Walter Roberson 2013-1-20
fzero() is strictly for functions with one parameter. You need fminsearch()
  8 个评论
显示 6更早的评论
Robert Buckles
Robert Buckles 2023-6-15
编辑：Robert Buckles 2023-6-15
Not wrong in the sense of applicability, just incomplete, and wrong to say that fzero cannot accept a function with multiple arguments. fzero can be used, so long as only one argument is being varied at a time. It is a lower-level function.
fminsearch is nice and easy until you encounter a situation where it doesn't converge. Even for a function as simple a poly2 fit, where you want to find the minimum, fminsearch can take you into outer space (and take forever to get there). fzero is more direct as a thresholding function, i.e. where does a function (or gradient of the function in this case) cross through zero? You can specify a [start stop] domain with fzero whereas fminsearch only has a start value, and this is the problem and why it goes crazy sometimes. If you need multiple varied parameters, you evaluate one parameter at a time with fzero and wrap it in a smart hunting routine. Of course we hope for built-in intelligence.
Steven Lord
Steven Lord 2023-6-16
The documentation for fzero states that its first input must be a "Function to solve, specified as a handle to a scalar-valued function or the name of such a function. fun accepts a scalar x and returns a scalar fun(x)." The original question passed in a function that did not satisfy that requirement, requiring passing in a non-scalar vector for x and returning a non-scalar vector as output.
That function can accept additional fixed parameters, but it can only solve a system of one equation in one unknown. To solve a system with multiple equations or multiple unknowns you may wish to switch to the fsolve function in Optimization Toolbox instead as others have mentioned.

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更多回答（2 个）

Tobin Fricke
Tobin Fricke 2019-1-12
Use fsolve from the Optimization Toolbox.
  1 个评论
Robert Buckles
Robert Buckles 2023-6-15
I would if I had paid for the toolbox. Even with corporate dollars, I find the kilobuck pricetag for each toolbox a little daunting. Where to shave costs...

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Robert Buckles
Robert Buckles 2021-3-6
Extra arguments (other than the one being zeroed) can be passed to the function. The extra arguments come after the options argument, but you cannot leave out the option. Use [ ] for the options if none are desired.
The posted syntax is a bit wrong. Try this:
x0 = 15; % but is preferrable to use a range [0, 30]
fzero(@(x,T2,T1,m,L,k,P,h) MMAE322SUPERFUNCTION(x, T2, T1, m, L, k, P, h), ...
x0, [], T2, T1, m, L, k, P, h);
% would be better to place the other arguments as a vector and pass the vector, parsing it within the function
  2 个评论
Walter Roberson
Walter Roberson 2021-3-6
Yup, but
x0= [15;12;450;12];
from the original question is not a scalar.
John D'Errico
John D'Errico 2021-3-6
编辑：John D'Errico 2021-3-6
Sadly, this misses the point completely. While it shows how to pass in extra ARGUMENTS, the problem as posted had a vector of unknowns. And while I could probably have flagged your answer as incorrect, I did not.

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