MATLAB: “for” loops and branching help

for loopif statementMATLABplot

I'm having trouble solving this question: Write the MATLAB statements required to calculate y(t) from the equation y(t) = -3*t^2 + 5 when t>=0
y(t) = 3*t^2 + 5 when t<0
for the values of t between -9 and 9 in steps of .5. Use loops and branches to solve
Heres what i have;
for t = -9:.5:9
    if t<0
        y=(3*t.^2)+5;
    else if t>=0
       y=(-3*t.^2)+5;
        end
    end
end
plot(y,t)
I know Im missing something, like maybe count=0; and count = count + 1 but I've tried it still was unsuccessful

Best Answer

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)
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