Actual source code: ex3.c

petsc-master 2017-09-21
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  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: {
 22:   PetscMPIInt    rank;
 23:   PetscInt       i,istart,iend,n = 6,nlocal;
 24:   PetscScalar    v,*array;
 25:   Vec            x;
 26:   PetscViewer    viewer;

 28:   PetscInitialize(&argc,&argv,(char*)0,help);if (ierr) return ierr;
 29:   MPI_Comm_rank(PETSC_COMM_WORLD,&rank);

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

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

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

 51:   /* --------------------------------------------------------------------
 52:      Set the vector elements.
 53:       - Always specify global locations of vector entries.
 54:       - Each processor can insert into any location, even ones it does not own
 55:       - In this case each processor adds values to all the entries,
 56:          this is not practical, but is merely done as an example
 57:    */
 58:   for (i=0; i<n; i++) {
 59:     v    = (PetscReal)(rank*i);
 60:     VecSetValues(x,1,&i,&v,ADD_VALUES);
 61:   }

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

 72:   /*
 73:      Open an X-window viewer.  Note that we specify the same communicator
 74:      for the viewer as we used for the distributed vector (PETSC_COMM_WORLD).
 75:        - Helpful runtime option:
 76:             -draw_pause <pause> : sets time (in seconds) that the
 77:                   program pauses after PetscDrawPause() has been called
 78:                   (0 is default, -1 implies until user input).

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

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

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

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

112:   PetscFinalize();
113:   return ierr;
114: }