How to fit an exponential curve? error: Dimensions of arrays being concatenated are not consistent

Hello,

I have an energy plot that I would like to fit an exponential curve to.

I have something like this

numSteps = 5000;
x = 0:1:100;
time = zeros(1,numSteps+1);
ex_data = zeros(length(x),numSteps+1); 
ex_data(:,1) = ex;
E_f = zeros(1, numSteps+1); 
E_f(1) = 0.5*sum(ex.^2);

for i = 1:numSteps
    %some calculations
    [ex] = func(..) % ex calculated through some function
    time(i+1) = (i)*dt; 
    E_f(i+1) = 0.5*sum(ex.^2 ) ;
    
end
%plot 
figure(1)
s = fittype('a-b*exp(-c*x)');
f = fit(time, E_f, s, 'StartPoint', [[ones(size(time)), -exp(-time)]\E_f; 1]);
xx = linspace(0,50,250);
plot(time, E_f, 'o', xx, f(xx), 'r-');
grid on
title('Electric Field Energy')

I am trying to fit an exponential to this plot from xlim([ 0 80]) but I am getting the error

Error using vertcat
Dimensions of arrays being concatenated are not consistent.

Error in WeakBeam (line 184)
f = fit(time, E_f, s, 'StartPoint', [[ones(size(time)), -exp(-time)]\E_f; 1]);

I will post the entire code with functions if needed, thank you.

Answer 1

[Matt J]

Did you mean,

[[ones(size(time)); -exp(-time)].\E_f; ones(size(time))]

[Jamie Al]

Hmm I am getting this error now:

Error using fit>iFit (line 135)
X must be a matrix with one or two columns.

Error in fit (line 116)
[fitobj, goodness, output, convmsg] = iFit( xdatain, ydatain, fittypeobj, ...

Error in WeakBeam (line 223)
f = fit(time, E_f, s, 'StartPoint',[[ones(size(time)); -exp(-time)].\E_f; ones(size(time))]);

[Matt J]

time , E_f, s must all be column vectors. It looks like you made them row vectors. Also, make sure that that complicated looking calculation of your StartPoint really results in a vector of 3 parameters.