To take parameter of a fit from one section to use in another fit in the next section.

I have a program which make a linear fit on a data. I want to use the slope of this linear fit on another data where I fit exponential (a*exp(b*x)) as b. Part of my program which do this is as below:

    %%SECTION 1:THIS PART FIT LINEAR FIT ON THE PLOT.
    [FTLx,FTLy]=ginput(2);
    indexL=find(B(:,1)>=FTLx(1) & B(:,1)<=FTLx(2));
    NewXL=B(B(:,1)>=FTLx(1) & B(:,1)<=FTLx(2));
    NewYL=B(indexL,2);
    P=polyfit(NewXL,log(NewYL),1);
    LFit=P(1)*NewXL+P(2);
    slope=P(1)       %THIS slope I USE IN THE NEXT SECTION
    hold on
    figure(3)
    subplot(1,2,1),plot(NewXL,LFit,'-b',NewX,log(NewY),':or')

    %%SECTION 2: THIS SECTION FITS EXPONENTIAL... THIS SECTION USE 'slope' FROM THE PREVIOUS SECTION
    options = fitoptions;
    options.Normal = 'on';
    ft=fittype('a*exp(slope*x)','independent','x','dependent','y');%THIS 'slope' SEEMS NOT TAKING THE VALUE FROM THE PREVIOUS SECTION!! :( :( :(
    [fitresult, gof]=fit(NewX,NewY,ft,options)
    figure(3)  
    subplot(1,2,2), plot(fitresult,'-b',NewX,NewY,':or')
    title('Exponential Fit in the selected range')

Here in the second section, I am using 'slope' from the first section. But it seems to taking the value of 'slope' from the first section! Is there anything wrong? How can I correct it?

[dpb]

I've never actually used the wrapper stuff but looks like perhaps if you were to wrap in an anonymous function --

 ft = fittype( @(a,x) a*exp(slope*x), 'problem', 'slope' );
 f1 = fit(NewX, NewY, ft, 'problem', slope);

might do the trick...

Answer 1

[Sean de Wolski]

% Sample data
x = (1:10).';
y = cumsum(rand(10,1));

% First fit
ft = fittype(@(m,b,x)m.*x+b);
fr = fit(x,y,ft)

% Second fit
ft2 = fittype(@(a,x)a*exp(fr.m*x));
fr2 = fit(x,y,ft2)

Using anonymous functions. You could do this with strings too but it would involve some kludge with sprintf / num2str etc.

[aneps]

Thanks... but i cannot plot the fit.. following is the code I used. I am not sure where is the mistake to plot

    ft1=fittype(@(m,b,x)m.*x+b);
    fr1=fit(NewXL,NewYL,ft1);
    %slope=fr1.m
    hold on
    figure(3)
    subplot(1,2,1),plot(NewXL,fr1,'-b',NewX,NewY,':or')

Could you pls check the last line where I try to plot the fit. It is showing error while I run the code.

[Sean de Wolski]

You need to pass x into the fit result object:

plot(x,y,'b*',x,fr(x),'r-')