Matlab demonstration codes - DFT

I made a demonstration in MATLAB today. Here is My command History and also the m file which has a simple low pass filter:

%-- 1/3/16 11:18 AM --%
t=linspace(0,1,100);
plot(cos(2*pi*t))
stem(cos(2*pi*t))
f = cos(2*pi*t);
F = fft(f);
stem(F)
stem(abs(F))
F(0)
F(1)
stem(abs(F))
stem(abs(fftshift(F)))
c = cos(2*pi*t);
c = cos(2*pi*t*30);
c30 = cos(2*pi*t*30);
c = cos(2*pi*t);
stem(c)
stem(c30)
c10 = cos(2*pi*t*10);
stem(c10)
f = c + 0.5*c10 + 0.1*c30;
stem(f)
F = fft(f);
stem(f)
stem(abs(fftshift(F)))
Fdn = zeros(F);
Fdn = zeros(size(F));
Fdn(50:52) = F(50:52);
fdn = ifft(Fdn);
stem(fdn)
stem(abs(fftshift(F)))
stem(abs(F))
stem(abs(fftshif(fftshift(F))))
stem(abs(fftshift(fftshift(F))))
stem(abs((fftshift(F))))
stem(abs(Fdn))
stem(abs(fftshift(Fdn)))
fdn = fftshift(Fdn);
fdn = ifft(fftshift(Fdn));
stem(fdn)
stem(f)
stem(fdn)
figure,stem(f)
stem(f)
stem(lowpass(f))

%%end of history

%%begining of lowpass.m file

function fl = lowpass(f)

    F = fft(f);

    F(6:end-5) = 0;

    fl = ifft(F);

end