MATLAB: Problem graphing a piece-wise function in R2016a?

MATLABpiecewisepiecewise function

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!

Best Answer

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;

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MATLAB: Matlab won’t plot the function

t=0:0.1:1;
y=(2*t+1) ./ (t.^2+1);
plot(t,y)

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You were close. Here is one way to do what you are trying to do, keeping with the FOR loop and IF statement use.

cnt = 0;
t = -9:.5:9
for ii = t
cnt = cnt + 1;
    if ii<0
        y(cnt)=(3*ii.^2)+5;
    else if ii>=0
       y(cnt)=(-3*ii.^2)+5;
        end
    end
end
plot(y,t)

Here is another way, without the cnt variable.

t = -9:.5:9;
for ii = 1:length(t)
      if t(ii)<0
          y(ii)=(3*t(ii).^2)+5;
      else if t(ii)>=0
         y(ii)=(-3*t(ii).^2)+5;
          end
      end
  end
  plot(y,t)

Also, here is a more MATLABish way of doing this, for future reference:

t2 = -9:.5:9
idx = t2>=0;
y2(idx) =   (-3*t2(idx).^2)+5;
y2(~idx) = (3*t2(~idx).^2)+5;
plot(y2,t2)

Best Answer

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MATLAB: Matlab won’t plot the function

MATLAB: “for” loops and branching help

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