You could use something like
t = [-5:0.01:5];
y = (t.^2).*(t>0) + 5*(t<=0);
yy = 5*ones(numel(t),1);
yy(t>0) = t(t>0).^2;
plot(t,y,t,yy)
showing two different approaches. As for your code, try
t = [-5:0.01:5];
for i=1:length(t) % compute y for all t
if t(i)<0 % need to compare each component of t
y(i) = 5;
else
y(i) = t(i)^2; % assign y component-wise
end
end
plot(t,y)
