Date: 2022-03-31T04:46:29+00:00

Actual source code: ex3.c

      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>

     19: int main(int argc,char **argv)
     20: {
     21:   PetscMPIInt    rank;
     22:   PetscInt       i,istart,iend,n = 6,nlocal;
     23:   PetscScalar    v,*array;
     24:   Vec            x;
     25:   PetscViewer    viewer;

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

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

     32:   /*
     33:      Create a vector, specifying only its global dimension.
     34:      When using VecCreate(), VecSetSizes() and VecSetFromOptions(),
     35:      the vector format (currently parallel or sequential) is
     36:      determined at runtime.  Also, the parallel partitioning of
     37:      the vector is determined by PETSc at runtime.
     38:   */
     39:   VecCreate(PETSC_COMM_WORLD,&x);
     40:   VecSetSizes(x,PETSC_DECIDE,n);
     41:   VecSetFromOptions(x);

     43:   /*
     44:      PETSc parallel vectors are partitioned by
     45:      contiguous chunks of rows across the processors.  Determine
     46:      which vector are locally owned.
     47:   */
     48:   VecGetOwnershipRange(x,&istart,&iend);

     50:   /* --------------------------------------------------------------------
     51:      Set the vector elements.
     52:       - Always specify global locations of vector entries.
     53:       - Each processor can insert into any location, even ones it does not own
     54:       - In this case each processor adds values to all the entries,
     55:          this is not practical, but is merely done as an example
     56:    */
     57:   for (i=0; i<n; i++) {
     58:     v    = (PetscReal)(rank*i);
     59:     VecSetValues(x,1,&i,&v,ADD_VALUES);
     60:   }

     62:   /*
     63:      Assemble vector, using the 2-step process:
     64:        VecAssemblyBegin(), VecAssemblyEnd()
     65:      Computations can be done while messages are in transition
     66:      by placing code between these two statements.
     67:   */
     68:   VecAssemblyBegin(x);
     69:   VecAssemblyEnd(x);

     71:   /*
     72:      Open an X-window viewer.  Note that we specify the same communicator
     73:      for the viewer as we used for the distributed vector (PETSC_COMM_WORLD).
     74:        - Helpful runtime option:
     75:             -draw_pause <pause> : sets time (in seconds) that the
     76:                   program pauses after PetscDrawPause() has been called
     77:                   (0 is default, -1 implies until user input).

     79:   */
     80:   PetscViewerDrawOpen(PETSC_COMM_WORLD,NULL,NULL,0,0,300,300,&viewer);
     81:   PetscObjectSetName((PetscObject)viewer,"Line graph Plot");
     82:   PetscViewerPushFormat(viewer,PETSC_VIEWER_DRAW_LG);
     83:   /*
     84:      View the vector
     85:   */
     86:   VecView(x,viewer);

     88:   /* --------------------------------------------------------------------
     89:        Access the vector values directly. Each processor has access only
     90:     to its portion of the vector. For default PETSc vectors VecGetArray()
     91:     does NOT involve a copy
     92:   */
     93:   VecGetLocalSize(x,&nlocal);
     94:   VecGetArray(x,&array);
     95:   for (i=0; i<nlocal; i++) array[i] = rank + 1;
     96:   VecRestoreArray(x,&array);

     98:   /*
     99:      View the vector
    100:   */
    101:   VecView(x,viewer);

    103:   /*
    104:      Free work space.  All PETSc objects should be destroyed when they
    105:      are no longer needed.
    106:   */
    107:   PetscViewerPopFormat(viewer);
    108:   PetscViewerDestroy(&viewer);
    109:   VecDestroy(&x);

    111:   PetscFinalize();
    112:   return 0;
    113: }

    115: /*TEST

    117:      test:
    118:        nsize: 2

    120: TEST*/