"for" loops and branching help

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

Answer 1

[Matt Fig]

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)

Answer 2

[Paulo Silva]

t=-9:0.5:9;
y=0*t;
y(t<0)=3*t(t<0).^2+5;
y(t>=0)=-3*t(t>=0).^2+5;

Here's another way

    t=-9:0.5:9;
    y=sign(t).*3.*t.^2+5;

PS: I know the OP wants loops but I couldn't resist :)

[Matt Tearle]

Yes. Logical indexing > loops + branching. All bow before the logical index!!!

[Paulo Silva]

Yeah, baby, yeah!