Problem graphing a piece-wise function in R2016a??

I need help graphing a piece wise function in Matlab, the function is

y(t) = y1=-3t^2+5 when t>=0 and y2=3t^2+5 when t<0

with t in [-9,9]

I have tried many different ways to graph this, and I honestly can't figure it out. Any help would be greatly appreciated!

Answer 1

[Adam]

It is very simple when you think about it in components. You can create y piece-wise and then plot a graph as normal.

e.g.

t = -9:9; % Or whatever granularity you want for t
y( t >= 0 ) = -3*t( t >= 0 ).^2 + 5;
y( t < 0 ) = 3*t( t < 0 ).^2 + 5;

[Leesy]

I'm confused on the "% Or whatever granularity you want for t" part... Like i'm not sure what that means. I am very new to Matlab. Also shouldn't there be a "plot" code in there somewhere? Is there anyway you could show me a complete code? Thank you so much!

[Walter Roberson]

Do you want to graph at only every 1 second, or every 1/2 second, or every 1/10 second? The "granularity" is the step size

step_size = 0.25;    %1/4 second
t = -9 : step_size : 9;

plotting:

plot(t, y)

[Leesy]

Ah, i get it. If the first line were "t=-9:0.5:9" would that work? Or does it have to be in that other format?

[Walter Roberson]

-9:0.5:9 would be fine. The two-line version was just to make it easier to how the parts went together.

[Adam]

t = linspace( -9, 9, 100 )

works fine too if you want to insist on 100 values rather than defining a step. Basically you can create your t vector any way you like. It doesn't even have to have regular steps if you don't want.