FSOLVE requires all values returned by functions to be of data type double

Question

Alka Sharma on 22 Jan 2020

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https://uk.mathworks.com/matlabcentral/answers/501436-fsolve-requires-all-values-returned-by-functions-to-be-of-data-type-double

Commented: Alka Sharma on 23 Jan 2020

clc;
clear;
clear all;
close all;
% defining inputs
N = 2;
Mf = 100;
Xf = 0.045;
Xn = 0.1;
Tf = 30;
To = 110;
deltaT = 40;
Cf=4.18;
Do=30.0;
% entering U values
for I=1:N
    prompt='enter u values';
 x= input (prompt) ;
u(I)= x;
end
% entering Cp values
 for i=1:N
     prompt='enter cp values';
     x=input(prompt);
     cp(i)=x;
 end
 % overall material balance
Bn=Mf*Xf/Xn ;
D=Mf-Bn;
disp(Bn)
disp(D)
% sum of i/ui
usum=0;
for i=1:N
    usum=usum+(1/u(i))
end
disp(usum)
% calculating deltT for each effect
delT(1)=deltaT/(u(1)*usum);
disp(delT(1));
for i=2:N
 delT(i)=u(1)*delT(1)/u(i);
end
% calculating temperature for each effect
for i=1:N
    x=0;
    for j=1:i
        x = x+delT(j)
    end
    T(i)=To-x;
end
% latent heat of vaporization
for i=1:N
   lamda(i)=2589.583+.9156*T(i)-4.8343*10^-2 *T(i)^2 ;
end
lamdao=2589.583+.9156*To-4.8343*10^-2 *To^2 ;
disp(lamda)
% Enthalpy Balance
for i=1:N
    Bo(i)=0;
end
 
[B,fval] = fsolve(@(B)@ourfun,Bo);
%disp (B);
function F = ourfun(B) 
 
F(1)=Do*lamdao + B(1)*lamda(1)+Mf*(Cf*(Tf-T(1))+lamda(1));
F(2)=B(1)*(cp(1)*(T(1)-T(2))-lamda(1)-lamda(2))+B(2)*lamda(2)+Mf*lamda(1);
 
for i=3:N
F(i)=B(i-1)*(Cp(i-1)*(T(i-1)-T(i))-lamda(i-1)-lamda(i))+B(i)*lamda(i)+B(i-2)*lamda(i-1); 
end
end

Result:

Error using fsolve (line 287)
FSOLVE requires all values returned by functions to be of data type double.
 
Error in MEE (line 63)
[B,fval] = fsolve(@(B)@ourfun,Bo);

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Answer 1

Stephen23 on 22 Jan 2020

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https://uk.mathworks.com/matlabcentral/answers/501436-fsolve-requires-all-values-returned-by-functions-to-be-of-data-type-double#answer_411415

Edited: Stephen23 on 22 Jan 2020

The function handle definition is incorrect:

@(B)@ourfun

What you defined is an anonymous function which when called accepts one input argument named B (which is totally ignored) and returns a function handle to a function named ourfun. This is not very useful for fsolve.

Read about function handles:

https://www.mathworks.com/help/matlab/function-handles.html

Note that you will also need to parameterize your function, otherwise your code will not work:

https://www.mathworks.com/help/matlab/math/parameterizing-functions.html

Using a nested function would be easiest, as then you could simply call your (nested) function like this:

[B,fval] = fsolve(@ourfun,Bo);