beginner question :subject : For loops , while loops , index variable

Write a 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
The values of t between -9 and 9 in steps of 0.5. (Use loops and branches)

??????????????is this correct ?????????????????????????

OR DO I HAVE TO PRE-ALLOCATE TIME (t) before I START THE LOOP

my program:

t = -9:0.5:9; % initialize the control expression 

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

    if true

[Walter Roberson]

The "if true" at the end is a mistake.

[Marco jacome]

Thanks walter. I forgot to delete the bottom line.

Answer 1

[Walter Roberson]

When you display "y" afterwards, is the number of values the same as the number of values in "t" ?

Answer 2

[Carlos]

This is what I would do:

for k=1:length(t)
  if t(k)<=0
  y4(k)=3*t(k)^2 + 5;
  else
  y4(k)=-3*t(k)^2 + 5;
  end
  end

You don't need the point after t here y = -3*t.^2 + 5;

Compare with the answer with no loops and branches

>> t2=0:0.5:9;
>> y2=-3*t2.^2 + 5;
>> t1=-9:0.5:-0.5;
>> y1=3*t1.^2 + 5;
>> y=[y1 y2];
>> isequal(y,y4)

ans =

     1

Answer 3

[Marco jacome]

Thanks Carlos and Walter , but it is still not clear to me... nonetheless, your advice helps alot. I just have to practice a lil bit