conditional colouring graph problem

Hey, I have a problem with changing the colours on my graph conditionally; my function is as follows

function [t,vx,vy] = Q3_1400000(rocket,brake)

[t,ax,ay] = getAcceleration(rocket,brake);

vx = 0; vy = 0;

for n = 1:999;

    vx(n+1) = vx(n) + ax(n+1);
    vy(n+1) = vy(n) + ay(n+1);
    k = 1:length(vx);
if vy(k) > vx(k)
    c = 'r';
else
    c = 'b';
end
end

plot(t,vx(k),c);

hold on

grid on

plot(t,vy(k),c);

xlabel('Time (s)');

ylabel('Velocity (m/s)');

title('\Velocity data for the rocket sled');

legend({'v_x','v_y'})

end

For some reason when the graph plots, the colour will always be the ' else ' colour and won't change at all. Does anybody know the solution to this please ?

Thanks

Answer 1

[Chad Greene]

It looks like you're only testing the condition of whether the very last value in vy is bigger than the very last value in vx because each time it goes through the loop it's overwriting c.

[Thomas Plank]

Thank you for the answer; how do I change it so it tests the condition for each data ?

[Chad Greene]

It's hard to understand exactly what you're trying to do. Perhaps the example below will help. It puts a red circle around all the black y1 points that are bigger than their corresponding blue y2 points:

x = 1:10; 
y1 = rand(size(x));
y2 = rand(size(x)); 

plot(x,y1,'kx',x,y2,'b+') 
hold on

% indices of y1 values that are bigger than y2: 
ind = y1>y2; 

plot(x(ind),y1(ind),'ro')
box off

[Thomas Plank]

Sorry for not being clear; what I intended to do was plot 2 lines on the same graph, vy and vx. When vy is greater than vx I wanted to change the colour of the lines to indicate that vy > vx.

Answer 2

[Chad Greene]

First create a logical array the size of vx and vy. It will have a 1 everywhere vy is bigger than vx, and a zero everywhere else:

vybigger = vy>vx;

Plot vx and vy, both in red where vy is bigger than vx:

plot(t(vybigger),vy(vybigger),'r');
hold on
plot(t(vybigger),vx(vybigger),'r');

Plot the rest of the data in blue:

plot(t(~vybigger),vy(~vybigger),'b');
plot(t(~vybigger),vx(~vybigger),'b');

[Thomas Plank]

Thanks ! This is almost exactly the effect I wanted; however for some reason it some some odd lines on it ( at the start from 0 - 10 on the x axis and the line inbetween where vy>vx shown as a thick purple colour ) and I'm not entirely sure why.

I have inserted the image below and the coding for the colour change is this if it makes a difference.

plot(t(vybigger),vy(vybigger),'g');

hold on

plot(t(vybigger),vx(vybigger),'m');

plot(t(~vybigger),vy(~vybigger),'r');

plot(t(~vybigger),vx(~vybigger),'b');

Thanks

EDIT: I have got rid of the lines from 1-10 however I still have the problem with the line inbetween vy>vx and vy<vx

Answer 3

[Kelly Kearney]

An alternative method to Chad's (which works nicely if there's only one color switch, but may need some modification if there are multiple switches and you want solid lines) is to use patches with interpolated edge color:

t = linspace(0,2*pi,100);
vx = cos(t);
vy = sin(t);

vybigger = vy > vx;
hold on;
h(1) = patch([t NaN], [vx NaN], [vybigger NaN]);
h(2) = patch([t NaN], [vy NaN], [vybigger NaN]); 

set(h, 'edgecolor', 'interp');
colormap([0 0 1; 1 0 0]);