Date: 2013-07-02T14:40:17+00:00

Actual source code: ex3.c
petsc-3.4.2 2013-07-02

      2: static char help[] = "Parallel vector layout.\n\n";

      4: /*T
      5:    Concepts: vectors^setting values
      6:    Concepts: vectors^local access to
      7:    Concepts: vectors^drawing vectors;
      8:    Processors: n
      9: T*/

     11: /*
     12:   Include "petscvec.h" so that we can use vectors.  Note that this file
     13:   automatically includes:
     14:      petscsys.h       - base PETSc routines   petscis.h     - index sets
     15:      petscviewer.h - viewers
     16: */
     17: #include <petscvec.h>

     21: int main(int argc,char **argv)
     22: {
     24:   PetscMPIInt    rank;
     25:   PetscInt       i,istart,iend,n = 6,nlocal;
     26:   PetscScalar    v,*array;
     27:   Vec            x;
     28:   PetscViewer    viewer;

     30:   PetscInitialize(&argc,&argv,(char*)0,help);
     31:   MPI_Comm_rank(PETSC_COMM_WORLD,&rank);

     33:   PetscOptionsGetInt(NULL,"-n",&n,NULL);

     35:   /*
     36:      Create a vector, specifying only its global dimension.
     37:      When using VecCreate(), VecSetSizes() and VecSetFromOptions(),
     38:      the vector format (currently parallel or sequential) is
     39:      determined at runtime.  Also, the parallel partitioning of
     40:      the vector is determined by PETSc at runtime.
     41:   */
     42:   VecCreate(PETSC_COMM_WORLD,&x);
     43:   VecSetSizes(x,PETSC_DECIDE,n);
     44:   VecSetFromOptions(x);

     46:   /*
     47:      PETSc parallel vectors are partitioned by
     48:      contiguous chunks of rows across the processors.  Determine
     49:      which vector are locally owned.
     50:   */
     51:   VecGetOwnershipRange(x,&istart,&iend);

     53:   /* --------------------------------------------------------------------
     54:      Set the vector elements.
     55:       - Always specify global locations of vector entries.
     56:       - Each processor can insert into any location, even ones it does not own
     57:       - In this case each processor adds values to all the entries,
     58:          this is not practical, but is merely done as an example
     59:    */
     60:   for (i=0; i<n; i++) {
     61:     v    = (PetscReal)(rank*i);
     62:     VecSetValues(x,1,&i,&v,ADD_VALUES);
     63:   }

     65:   /*
     66:      Assemble vector, using the 2-step process:
     67:        VecAssemblyBegin(), VecAssemblyEnd()
     68:      Computations can be done while messages are in transition
     69:      by placing code between these two statements.
     70:   */
     71:   VecAssemblyBegin(x);
     72:   VecAssemblyEnd(x);

     74:   /*
     75:      Open an X-window viewer.  Note that we specify the same communicator
     76:      for the viewer as we used for the distributed vector (PETSC_COMM_WORLD).
     77:        - Helpful runtime option:
     78:             -draw_pause <pause> : sets time (in seconds) that the
     79:                   program pauses after PetscDrawPause() has been called
     80:                   (0 is default, -1 implies until user input).

     82:   */
     83:   PetscViewerDrawOpen(PETSC_COMM_WORLD,NULL,NULL,0,0,300,300,&viewer);
     84:   PetscObjectSetName((PetscObject)viewer,"Line graph Plot");
     85:   PetscViewerPushFormat(viewer,PETSC_VIEWER_DRAW_LG);
     86:   /*
     87:      View the vector
     88:   */
     89:   VecView(x,viewer);

     91:   /* --------------------------------------------------------------------
     92:        Access the vector values directly. Each processor has access only
     93:     to its portion of the vector. For default PETSc vectors VecGetArray()
     94:     does NOT involve a copy
     95:   */
     96:   VecGetLocalSize(x,&nlocal);
     97:   VecGetArray(x,&array);
     98:   for (i=0; i<nlocal; i++) array[i] = rank + 1;
     99:   VecRestoreArray(x,&array);

    101:   /*
    102:      View the vector
    103:   */
    104:   VecView(x,viewer);

    106:   /*
    107:      Free work space.  All PETSc objects should be destroyed when they
    108:      are no longer needed.
    109:   */
    110:   PetscViewerDestroy(&viewer);
    111:   VecDestroy(&x);

    113:   PetscFinalize();
    114:   return 0;
    115: }