[求助] 一个小的程序问题（误差积累）

program main
implicit none


integer i,j,k,maxx,maxy,maxz,i0,jb,kc,pp,pp1,ct
real dx,dy,dz,ww,ww1,rrb1,rrb
real,allocatable ::dz_1(:)

        dz=0.05
        maxx=74;maxy=74;maxz=74
        i0=23;jb=52;kc=52
        ct=1.0

        allocate(dz_1(0:maxz))
        dz_1=0
        do k=1,maxz
        dz_1(k)=dz_1(k-1)+dz
        enddo

        DO k = 1,maxz-1
        DO j = 1,maxy-1
          DO i = 1,maxx



!            rrb=(i-0.5)*dx*st*cp+  &
!             j*dy*st*sp+    &
!             k*dz*ct               !(6-7-13)
            rrb= k*dz*ct   
!            rrb=(dx_1(i-1)+(dx_1(i)-dx_1(i-1))/2.)*st*cp+  &
!             dy_1(j)*st*sp+             &
!             dz_1(k)*ct     !+ dz/1.e+10
          rrb1=dz_1(k)*ct

              pp=int(rrb/dz)
          pp1=int(rrb1/dz)
          ww=abs(mod(rrb,dz))
          ww1=abs(mod(rrb1,dz))
!          ww=rrb/dz-pp
          if(i.eq.i0.and.j.eq.jb.and.k.eq.kc) then
          print*,rrb,pp,ww
          print*,rrb1,pp1,ww1
          pause
          endif

          if(abs(1-(pp+1)/rrb).lt.1.e-5)then
          pp=pp+1
          endif   
          if(ww.lt.1.e-6.or.abs(1-ww/dz).lt.1.e-5)then
          ww=0
          else
          ww=ww/dz
          endif

           ENDDO
          ENDDO
        ENDDO

end