# Fitting the Plot by fourier Series of either sine or cosine

I have list of data and its plot and I don't know how to fit the plot with fourier series of either sine or cosine.

Full Data:


{{0., -0.176091}, {0.034, -0.163291}, {0.067, -0.156391}, {0.1,
-0.152791}, {0.134, -0.149391}, {0.167, -0.144791}, {0.2, -0.141291},
{0.234, -0.135991}, {0.267, -0.126191}, {0.301, -0.123591}, {0.334,
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This is the graph of whole data.

• Can you provide all the data? Jul 16, 2014 at 17:16

I've done this from first principles:

some random data:

 data = Table[{x + RandomReal[{-.05, .05}] + 4,
Sin[x] + Sin[x/3] + RandomReal[{-.3, .3}]}, {x, 0, 12 Pi , .1}];


treat the discrete data as a function and and use trapezoidal integration:

 trule = Mean /@
Partition[
Differences@
{data[[1, 1]], Sequence @@ data[[;; , 1]], data[[-1, 1]]}, 2, 1];

len = Subtract @@ data[[{-1, 1}]][[;; , 1]]


the fourier sine coefficients:

 ck = 2 / len Table[
trule.(#[[2]] Sin[k (#[[1]] - data[[1, 1]]) Pi/len ] & /@ data ) ,
{k, 1, 20} ];

Show[{ListPlot[data],   Plot[ Total@
MapIndexed[# Sin[First@#2  Pi (t - data[[1, 1]]) / len ] & , ck ] ,
{t, data[[1, 1]], data[[-1, 1]]}]}]


Now I expect someone will show a built-in way to get this...

Using the example data, with 30 terms:

• I have to use 40 terms with your code to reproduce the last graph. Is that a typo? Jul 16, 2014 at 19:47
• could be a mistake.. you can play with different numbers to see how well it converges. Jul 16, 2014 at 21:42

I took george2079's statement "Now I expect someone will show a built-in way to get this..." as a challenge, so I did the same thing using FindFit. Also, writing it this way seems more clear to me what the actual function being fitted is (but obviously relying on the internal FindFit function)

len = (Max[#] - Min[#]) &@data[[All, 1]];
func = Sum[Subscript[a, n] Sin[( \[Pi] n)/len t], {n, 1, 40}];
params = Table[Subscript[a, n], {n, 1, 40}];
soln = FindFit[data, func, params, t];
function = Compile[{{t, _Real}},
Evaluate[func /. soln]];
Show[ListPlot[data],Plot[function[t], {t, 0, len}, PlotStyle -> {{Thick, Red}}]]


Fit is a function that finds the linear combination of some given terms that best fits the data. This can also be used since sine Fourier series are linear combinations of Sin.

len = Subtract @@ data[[{-1, 1}, 1]];
func =
Fit[
data,
Table[ Sin[(π n)/len x], {n, 1, 40}],
x
]
Show[
ListPlot[data],
Plot[func, {x, 0, len}, PlotStyle -> {{Thick, Red}}]
]