average waveform over many cycles

Thanks Jim for the help.

I forgot to say that one of my main snags is figuring how how to chop up
the one long (44100 samples) vector into pieces (400 of them).

This is my C-inspired way of thinking but there must be a better way in
R.

cyclelength=44100/400
for i= 1 to 400
 for j= 1 to cyclelength
 y[i,j]=x[i*j]
next
next

Once that's done I can average over rows

Thanks for any suggestions
Bill