subroutine ic(ax,ay,xmax,ymax,jdim,jmax,ldim,lmax,x,y,io,u)

c General information:

c     Subroutine ic sets the initial condition for the convection
c     equation
c        u_t + ax u_x + ay u_y = 0
c     on a rectangular domain.

c     The present implementation of ic requires an exact solution
c           function exa(x,y,t,ax,ay).
c     The initial data for u are set equal to this exact solution at
c     time zero.

c     Copyright 1997 Leon van Dommelen
c     Version 1.0 Leon van Dommelen 1/10/97

c Arguments:

c     Avoid typos:
      implicit none

c     Input: components of the convection velocity in the x and y directions:
      double precision ax,ay

c     Input: x- and y-sizes of the rectangular domain:
      double precision xmax,ymax

c     Input: declared maximum indices in the x- and y-directions:
      integer jdim,ldim

c     Input: actual maximum indices in the x- and y-directions:
      integer jmax,lmax

c     Input: the x- and y-values of the mesh points:
      double precision x(0:jdim),y(0:ldim)

c     Input: Fortran I/O unit to do output on:
      integer io

c     Output: the initial values of the dependent variable u:
      double precision u(0:jdim,0:ldim)

c External variables and info for compiling or changing subroutine ic:

c     Subroutine ic requires a separate function
c           exa(x,y,t,ax,ay)
c     to return the exact solution at position (x,y) and time t.
      double precision exa
      external exa

c Local variables:

c     Indices in the x- and y-directions:
      integer j,l

c     Constants, defined to simplify changing precision:
      double precision zero
      parameter (zero=0.d0)

c Executable statements:

c     Set the initial values according to the exact solution:
      do 200 j=0,jmax
         do 100 l=0,lmax
            u(j,l)=exa(x(j),y(l),zero,ax,ay)
 100     continue
 200  continue

c     All done:
      goto 900

c Exit:
 900  return
      end