al=100; nipam=20; water=300; j=1; for i=1:15

rho(j:j+al)=2.07;
k=j+al;
rho(k:k+nipam)=0.81;
l=k+nipam;
rho(l:l+water)=-0.56;
m=l+water;
rho(m:m+nipam)=0.81;
j=m+nipam; end del_x=1; xmax=6600; x=(0:del_x:xmax); % plot(x,rho) A=abs(fft(rho)); I=A.^2; % del_q=2*pi./xmax;   I want to how should i