plugging in a function inside fittype, error

Hello. I'm new in matlab. I'm currently trying to use the fittype function to later on use to fit.

I have a truncated exponetial whcih i define in the following matlab function.

function [yf] = Exp_t(x3,a,m)
%x and y data are arrays with size 20x1
%a and m are unknown coeficients to be fitted. 
for i=1:20
    
    if x3(i)<0 %truncation
        yf(i,1)=0;
    elseif (x3(i)>=0) && (x3(i)<=a)
        yf(i,1)=(1-exp(-m*x3(i)))/(1-exp(-m*a)); %exponential fucntion
    elseif x3(i)>a %truncation
        yf(i,1)=1;
    end
end
end

I then insaert this function in my main script

    %truncated exponential fit. 
    y3=(table1{:,4}); %data from column 4 table1
    x3=(table1{:,3}); %data from column 3 table1
    
    %inserting Exp_t(x3,a,m) into fittype function
    ft = fittype( 'Exp_t(x3,a,m)','independent', {'x3'}, 'dependent', {'yf'},'coeff',{'a','m'});
    
    %fit(x3,y3,ft,'Robust','on')

I've spent more than two days trying to fix this, any help is much apreciated.

I receive the following errors:

Error using fittype/testCustomModelEvaluation (line 12)

Expression Exp_t(x3,a,m) is not a valid MATLAB expression, has non-scalar coefficients, or cannot be evaluated:

Error in fittype expression ==> Exp_t(x3,a,m)

??? Index exceeds the number of array elements (7).

Error in fittype>iCreateFittype (line 373)

testCustomModelEvaluation( obj );

Error in fittype (line 330)

obj = iCreateFittype( obj, varargin{:} );

Error in Project_2 (line 105)

ft = fittype( 'Exp_t(x3,a,m)','independent', {'x3'}, 'dependent', {'yf'},'coeff',{'a','m'});

Caused by:

Error using fittype/evaluate>I_ERROR_FCN_ (line 179)

Error in fittype expression ==> Exp_t(x3,a,m)

??? Index exceeds the number of array elements (7).

Answer 1

[Gargi Patil]

Hi,

This error can be resolved by declaring the size of the output array yf beforehand in the function Exp_t as follows:

function [yf] = Exp_t(x3, a, m)
%x and y data are arrays with size 20x1
%a and m are unknown coeficients to be fitted. 
yf = zeros(size(x3, 1), 1);
....
end

[Alvaro Sanchez]

thanks, that worked but also had to remove the brackets.

[Matt J]

@Alvaro Sanchez If it worked, please Accept-click the answer.

Answer 2

[Walter Roberson]

    ft = fittype( 'Exp_t(x3,a,m)','independent', {'x3'}, 'dependent', {'yf'},'coeff',{'a','m'});

Instead of using a quoted character expression, use an anonymous function.