# Generating a function using an array

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Brian Kettering on 26 Aug 2020
Edited: esat gulhan on 26 Aug 2020
If I have an array of data. Is there a way to generate a function from the data contained in the array? That if I evaluated the function, it would generate the data equivelent to the array. The generated function will be used to find the local min and max (critical points) of the function.

esat gulhan on 26 Aug 2020
maybe like this. if you send data it can be easier to choose the way of function. I used sin data and choose pchip.
x=[ 0 0.3142 0.6283 0.9425 1.2566 1.5708 1.8850 2.1991 2.5133 2.8274 3.1416 3.4558 3.7699 4.0841]
y=[ 0 0.3090 0.5878 0.8090 0.9511 1.0000 0.9511 0.8090 0.5878 0.3090 0.0000 -0.3090 -0.5878 -0.8090]
t=pchip(x,y) %Piecewise Cubic Hermite Interpolating Polynomial function of x y
ppval(t,2) %the value of function at 2
ppval(t,0) %the value of function at 0
plot(x,ppval(t,x))
findpeaks(ppval(t,x)) %local maksimum points
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Brian Kettering on 26 Aug 2020
Thanks for helping, but I keep getting the following error
Error using chckxy (line 30)
The X vector should have real elements.
Error in pchip (line 58)
[x,y,sizey] = chckxy(x,y);
Error in Untitled2 (line 11)
t=pchip(x,y) %Piecewise Cubic Hermite Interpolating Polynomial function of x y

esat gulhan on 26 Aug 2020
Edited: esat gulhan on 26 Aug 2020
Hi
can you try this
clear all;clc;
x=[ 0 0.3142 0.6283 0.9425 1.2566 1.5708 1.8850 2.1991 2.5133 2.8274 3.141 3.4558 3.76 4.08]
y=[ 0 0.3090 0.5878 0.8090 0.9511 1 0.9511 0.809 0.5878 0.3090 0.0 -0.309 -0.58 -0.8]
t=pchip(x,y) %Piecewise Cubic Hermite Interpolating Polynomial function of x y
ppval(t,2) %the value of function at 2
ppval(t,0) %the value of function at 0
plot(x,ppval(t,x))
findpeaks(ppval(t,x)) %local maksimum points