for loop for fft

Hi, how can I write a for loop for a range

for i=1:68
    U1(1,i) = u_gdc(1,i);      
    gg = fft(U1);       
    P2 = (abs((gg/L)));
    P1 = (P2(1:L/2+1));
    P1(2:end-1) = 2*P1(2:end-1);    
end

and P1 should has finally 1 row and 68 column.

thx