How to plot a piecewise defined function?

2012 12 6
1 个回答
3 次查看（30 天）

0 个投票

Hi
I want to plot a piecewise defined function. My Problem is, that the plot isn't right and I don't find the mistake... would be great, if somebody could help me!
function CubicSpline
clear all;
clc;
close all;
%CatmullRom:
B1=0;
C1=1/2;
x=-2:.01:2;
y1=zeros(size(x));
if abs(x)< 1,
y1=1/6*((12-(9*B1)-(6*C1))*abs(x).^3 +(-18+(12*B1)+(6*C1))*abs(x).^2 + (6-(2*B1)));
elseif 1 <= abs(x) < 2
y1 = 1/6*(((-B1-6*C1)*abs(x).^3 + (6*B1+30*C1)*abs(x).^2 + (-12*B1-48*C1)*abs(x)+(8*B1+24*C1)));
else y1 = 0;
end
figure(1);clf; hold on;
plot(x,y1,'r')%,x, y2, 'b', x, y3, 'g'
xlabel('X-Axis')
ylabel('Function values')
grid on;
legend({'CatmullRom: B = 0, C = 1/2'},'Location','EastOutside')

5 个评论
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Azzi Abdelmalek
Azzi Abdelmalek 2012-12-6
Your program is working, what is the expected result?
Simon
Simon 2012-12-6
编辑：Simon 2012-12-6
If B1 = 0, x = 0, abs(x) is 0 and as consequence the y1 value must be 1. When I plot it the y1-value at x=0 is 2
Jan
Jan 2012-12-6
It is a waste of time to remove all loaded function after starting this function by "clear all". This does not even clean the local workspace, because there are no variables declared before.
Simon
Simon 2012-12-6
Ah, thanks. Your're right!
Simon
Simon 2012-12-7
I still have the problem to plot this function. Could anybody help me? Would be nice :-)

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回答（1 个）

Andrei Bobrov
Andrei Bobrov 2012-12-6
编辑：Andrei Bobrov 2012-12-6

0 个投票

B1=0;
C1=1/2;
x0 = -2:.1:2;
x=abs(x0);
y1=zeros(size(x));
t = [x < 1, x >= 1 & x < 2];
y1(t(:,1)) = 1/6*((12-(9*B1)-(6*C1))*x(t(:,1)).^3 +...
(-18+(12*B1)+(6*C1))*x(t(:,1)).^2 + (6-(2*B1)));
y1(t(:,2)) = 1/6*(((-B1-6*C1)*x(t(:,2)).^3 +...
(6*B1+30*C1)*x(t(:,2)).^2 + (-12*B1-48*C1)*x(t(:,2))+(8*B1+24*C1)));
ADD (corrected)
t1 = x0 <= -1;
t2 = x0 > -1 & x0 < 1;
t3 = x0 >= 1;
plot(x0(t1),y1(t1),'r'); hold on
plot(x0(t2),y1(t2),'g');
plot(x0(t3),y1(t3),'k');

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Simon
Simon 2012-12-6
Hey, thanks! But now I tried to plot y1, but it doesn't work... how can I plot y1?
plot(x, y1(t(:,:,1,2)),'r')
Sry for the stupid question..
Simon
Simon 2012-12-6
The added Code doesn't work :(
plot(x(t(:,1)),y1(t(:,1)),'r',x(t(:,2)),y1(t(:,2)),'g',x(t(:,3)),(t(:,3)),'r');
This doesn't work, too :(
Simon
Simon 2012-12-6
Hm... there's only a line with the values x=[-2:.1:2] and y=0
Simon
Simon 2012-12-6
It still doesn'´t work. It could not be that difficult to plot this function. Would be great, if somebody could post the right code and explain it to me :)

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Simon
Simon
2012-12-6

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