Fitting Curve To Temperature Data

I am trying to fit a curve to temperature data taken over over a period of 48 hours. Looking at this data it seems to osculate similar to a sin function, but I am unsure how to fit a sinus to to this data. I have tried using the polyfit function, but none of those can give me an oscillating function. I have attempted to use a sine curve fitting code found here on MATLAB answers, but have ran into errors. I have attached the data sets I am using. Here is the code I am attempting to use:

yu = max(temp1);
yl = min(temp1);
yr = (yu-yl);                               % Range of ‘y’
yz = temp1-yu+(yr/2);
zx = time_hours(yz .* circshift(yz,[0 1]) <= 0);     % Find zero-crossings
per = 2*mean(diff(zx));                     % Estimate period
ym = mean(temp1);                               % Estimate offset
fit = @(b,time_hours)  b(1).*(sin(2*pi*time_hours./b(2) + 2*pi/b(3))) + b(4);    % Function to fit
fcn = @(b) sum((fit(b,time_hours) - temp1).^2);                              % Least-Squares cost function
options = optimset('MaxFunEvals',100000,'MaxIter',100000);
s = fminsearch(fcn, [yr;  per;  -1;  ym],options)                       % Minimise Least-Squares
xp = linspace(min(time_hours),max(time_hours));

figure(1)
plot(time_hours,temp1,'b',  xp,fit(s,xp), 'r')
grid

I keep getting Nan for s(3) no matter how high I increase MaxFunEvals. The only toolbox I have at my disposal is the Symbolic Math Toolbox. Any suggestions to finding a good fitting oscillating function would be greatly appreciated! Thanks.

Answer 1

[Star Strider]

You have several options, depending on what you want to do. One is: Curve fitting to a sinusoidal function (link), another is the interpft (link) function.

[Joseph Dominguez]

Thank you. While I'm glad I now know how to use the interpft function, the first link helped the most. Upon some further reading about the circshift function, I found there was a change to MATLAB version R2016b where I needed to change the code from

circshift(yz,[0 1])

to

circshift(yz,1)

This solved my NaN error and enabled your code to work as written. Thank you for your assistance Star Strider!

[Star Strider]

As always, my pleasure!

Thank you for pointing out the circshift change. (I’ve not revisited that code in a while.) I’ll add a comment mentioning the change, and citing your comment.

I later wrote a little utility function that I will also mention in my update to my previous Answer that should solve that problem:

zci = @(v) find(v(:).*circshift(v(:), [-1 0]) <= 0);                    % Returns Approximate Zero-Crossing Indices Of Argument Vector