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confusion regarding interp1 command?

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ABTJ
ABTJ on 27 Jun 2020
Commented: Star Strider on 28 Jun 2020
I am trying to watch difference between cubic interpolation and spline interpolation using matlab plot but i am getting same plots in both cases using interp1 command
My code is below. In below code ,if i use spline in place of cubic, i still exactly get same plot,why?
clc
clear all
close all
x = 0:pi/4:2*pi;
v = sin(x);
xq = 0:pi/16:2*pi;
vq2 = interp1(x,v,xq,'cubic');
plot(x,v,'o',xq,vq2,':.');

Accepted Answer

Star Strider
Star Strider on 27 Jun 2020
The two methods do not produce exactly the same results, however with respect to the amplitude of the values in the plot, they only appear to be the same.
Do this experiment to see how they differ:
x = 0:pi/4:2*pi;
v = sin(x);
xq = 0:pi/16:2*pi;
vq1 = interp1(x,v,xq,'spline');
vq2 = interp1(x,v,xq,'cubic');
plot(x,v,'o',xq,vq2,':.');
difvct = vq1-vq2;
figure
plot(xq, difvct)
grid
.
  2 Comments
Star Strider
Star Strider on 28 Jun 2020
It likely does not matter. They are almost the same, nowever not actually the same. That was the oint of my post.
I usually use 'pchip' if I want something other than 'linear'.

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