function predictor_step(N,h,dt);
global rou T V dx A prou_pt pV_pt pT_pt;
%the predictor step use forward differences
 for i=2:(N-1)
     prou_pt(h,i)=-rou(h,i)*(V(h,i+1)-V(h,i))/dx-rou(h,i)*V(h,i)*(log(A(i+1))-log(A(i)))/dx-V(h,i)*(rou(h,i+1)-rou(h,i))/dx;
     pV_pt(h,i)=-V(h,i)*(V(h,i+1)-V(h,i))/dx-(1/1.4)*((T(h,i+1)-T(h,i))/dx+(T(h,i)/rou(h,i))*(rou(h,i+1)-rou(h,i))/dx);
     pT_pt(h,i)=-V(h,i)*(T(h,i+1)-T(h,i))/dx-(1.4-1)*T(h,i)*((V(h,i+1)-V(h,i))/dx+V(h,i)*((log(A(i+1))-log(A(i)))/dx));
 end
%calculate rou,V and T
 rou(h+1,:)=rou(h,:)+prou_pt(h,:)*dt;
 V(h+1,:)=V(h,:)+pV_pt(h,:)*dt;
 T(h+1,:)=T(h,:)+pT_pt(h,:)*dt;