# How to extrapolate the first 5 points

2 views (last 30 days)
DB on 8 Nov 2023
Edited: Torsten on 8 Nov 2023
Hello,
attached is an array of points that I have. When I use scatter plot to graph those points, I see that the first 5 points are not aligned. Does anyone know how to extrapolate those points to be aligned with the rest correctly- only by using the points that are aligned "points(6:end,:)". Thanks so much.
Dyuman Joshi on 8 Nov 2023
Why not just ignore the points?

Torsten on 8 Nov 2023
Edited: Torsten on 8 Nov 2023
px = p.points(:,1);
py = p.points(:,2);
[~,idx] = sort(px);
px = px(idx);
py = py(idx);
plot(px,py)
Another solution:
px = p.points(:,1);
py = p.points(:,2);
for i = 1:4
py(i) = interp1(px(5:end),py(5:end),px(i));
end
[~,idx] = sort(px);
px = px(idx);
py = py(idx);
plot(px,py)

John D'Errico on 8 Nov 2023
Edited: John D'Errico on 8 Nov 2023
x = points(:,1);
y = points(:,2);
i1 = 1:5;
i2 = 6:336;
plot(x(i1),y(i1),'ro',x(i2),y(i2),'g.')
I have no idea what you mean by extrapolation. The red points are essentially contained within the realm of the green ones.
Do you wish to INTERPOLATE the red points, essentially choosing new y values at each x location, so the red points now fall on the curve of the green ones? That is trivial. Just use interp1.

Les Beckham on 8 Nov 2023
Edited: Les Beckham on 8 Nov 2023
plot(points(:,1), points(:,2), '.-');
grid on
figure
plot(points(:,1), points(:,2), '.-');
grid on
xlim([100 150])
ylim([580 650])
It looks like there are 4 "bad" data points. Why do you think that you need to invent new positions for those 4 points?
I would just ignore them. Extrapolation is almost always a bad idea.
figure
idx = 5:size(points,1);
plot(points(idx,1), points(idx,2), '.-');
grid on

TENG LIU on 8 Nov 2023
I found that the first 4 are not aligned, you can consider just exclude the first 4 and interpolated them by using 5th and 6th, code is here:
x= points(5:6,1);
v = points(5:6,2);
xq = points(5,1):0.25:points(6,1);
vq1 = interp1(x,v,xq);
Then the new points display as:
points(1:6,1) = xq;
points(1:6,2) = vq1;
If you want to plot them, just:
figure,scatter(points(:,1),points(:,2))

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