Going up from negative in for loop counter

I'm trying to make this code

 for x=-10:10;
  y=-3*x^2;
  end
  plot(x,y)

Plot like this code

x=-10:10;
y=-3*x.^2;
plot(x,y,'--o')

I've tried

for x=-10:10
y(x)=-3*x.^2;
end
plot(x,y,'--o')

which doesn't work because I can't have the negative x numbers as y(x). I'm lost on how to fix this. Thank you.

Answer 1

[Azzi Abdelmalek]

You can avoid using negative index

x=-10:10;
n=numel(x);
y=zeros(1,n)  % Pre-allocate 
for k=1:numel(x)
  y(k)=-3*x(k)^2;
end
plot(x,y,'--o')

[Crista]

That's perfect! Thank you!

Answer 2

[Mahdi]

When you plot, you`re only plotting the last point because the variable y is inside the for loop (and is not a matrix). Other than what`s been suggested, the way that you can do it is

for x=-10:10;
y=-3*x^2;
plot(x,y)
hold on  
end

[Mahdi]

Please note that you should avoid for-loops to plot things like this because they`ll be resource-consuming.