dft problem

--- Quote Start ---

You're doing:

t=0:1/fs:1

x=cos(2*pi*400*t);

The step is 1/fs, which is 1/800 = 0.00125

This means, that from range 0 to 1, every 0.00125 step You'll get a new frequency (because argument 't' in cosine function is changing), that is: 800 results. So what do You expect in frequency domain to see?

--- Quote End ---

but my signal is cosine and the sampling rate is also twice of highest frequency ,so corresponding i must gate only two frequency component.