清空筛选条件
清空筛选条件

How can I make curve form data?

4 次查看（过去 30 天）
Riyadh Muttaleb
Riyadh Muttaleb 2020-2-6
评论： Riyadh Muttaleb 2020-2-9
Hi All,
I would like to make a convex curve from this (x,y) data that I have. The curve must connect the first and last points of y.
x= 7000,7050,7100,7150,7200
y= -92.9,-125.6,-158.5,-190.9,-223.5
The help would be apprecited,
Thanks in advance,
Riyadh
  7 个评论
显示 5更早的评论
Walter Roberson
Walter Roberson 2020-2-7
No you didn't -- you get a curved line that was sampled over a small enough region that it looked flat.
If you look at diff(y) you will see that the differences between adjacent y values is approximately constant, which is what you would expect for a straight line.
If you fit as a polynomial of degree 2, then the minimum of that polynomial is at 23425 and the zero crossings are at 6859 and 39990

请先登录，再进行评论。

请先登录，再回答此问题。

回答（1 个）

David Goodmanson
David Goodmanson 2020-2-8
编辑：David Goodmanson 2020-2-8
Riyadh;
The differences from a straight line are small, so it make sense to look at that difference.
x = [7000,7050,7100,7150,7200]'
y = [-92.9,-125.6,-158.5,-190.9,-223.5]'
ylin = linspace(y(1),y(end),5)'
ydiff = y - ylin;
figure(1)
plot(x,ydiff,'o-');
grid on
The three differences are all negative, so absent any other information it can just be fit with a parabola that is zero at the end points. This will lead to a curve that is convex downwards.
p = (x-x(1)).*(x-x(end)); % parabola, zero at end points
c = p\ydiff; % least squares fit
xnew = linspace(7000,7200,500); % grid with more points
ydiffnew = c*(xnew-x(1)).*(xnew-x(end)); % fitted parabola, zero at end points
figure(2)
plot(x,ydiff,'o-',xnew,ydiffnew)
grid on
Then the final result ynew is this plus the linear line
slope = (y(end)-y(1))/(x(end)-x(1));
ylinnew = y(1) + slope*(xnew-x(1));
ynew = ylinnew + ydiffnew;
figure(3)
plot(x,y,'o-',xnew,ynew)
grid on
  1 个评论
Riyadh Muttaleb
Riyadh Muttaleb 2020-2-9
Thank you so much for your great work, However, the seconed part the curve is very good while in the third part it looks like a straight line!

请先登录，再进行评论。

类别

在 Help Center 和 File Exchange 中查找有关 Interpolation 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by