Yes, you can avoid that warning by pre-allocating sweptsin to the size it needs to be, which is the same size as t:
T=5; %size of window
fs=44100; %sampling frequency
df=1/T; %frequency res
dt=1/fs; %time resolution
t=(0:+dt:T-dt); %time vector
df_t=500; %swept rate (Hz/seconds)
% pre-allocate:
sweptsin = zeros(size(t));
for i=1:+1:length(t)
%i=i+1;
if(i==1) %initialise f and t.
f=20; ti=0;
else
ti=ti+dt; %time increment
f=f+df_t*dt; %freq increment
end
w=2*pi*f; %omega
sweptsin(i)=sin(w*ti); %swept sine wave
end
plot(t, sweptsin)
xlim([0 0.1])
However, you may want to consider doing the calculation without a for loop at all:
sweptsin = sin(2*pi*(20+df_t*dt*(0:numel(t)-1)).*t);
plot(t,sweptsin)
xlim([0 0.1])
