Can you generate a reasonably large random number, e.g. x=rand(50000,1), then after y(y<-5)=[] and y(y>25)=[] operation, it is almost certain that y is more than 5000 in length? Then you can do y(5001:end)=[].
It is certainly possible to do it using a while-loop, but probably won't be fast.
a=10; T=3;
y=zeros(5000,1);
k=1;
while k<=5000
x= rand;
temp=a+((T/2).*(tan(pi.*(x-(0.5)))));
if -5<=temp && temp<=25
y(k)=temp;
k=k+1;
end
end
