Question

I ran into unexpected behavior with a function I wrote to compute the average point for a set of 3D points.

static inline Point3d
average_vertex_position( Vertex vertices[], int nPoints )
{
  Point3d average = { 0.0, 0.0, 0.0 }; // Must be there to avoid accumulation
  Point3d point;
  int i;

  for ( i = 0; i < nPoints; i++ )
  {
    point = vertices[i].position;
    average.x += point.x;
    average.y += point.y;
    average.z += point.z;
  }

  average.x /= (double) nPoints;
  average.y /= (double) nPoints;
  average.z /= (double) nPoints;

  return average;
}



// ...
Point3d average;
// ...
for ( j = i; j < nVertices; j++ )
{
  // ...
  average = average_vertex_position( vertices, nVertices );
  // ...
}

For each iteration the return value of average_vertex_position would accumulate unless I added explicitly the initialization Point3d average = { 0.0, 0.0, 0.0 };.

If the previous return value was Point3d( 10, 0, 20 ) and the next run should have returned Point3d( 20, 10, 0 ) it would instead return an accumulated result Point3d( 30, 10, 20 ).

Originally I had just Point3d average; assuming all the member values (double values) would be initialized to 0.0. I also assumed average would be in this initial state between each call. I don’t understand why I need to explicitly initialize it?

I trimmed out the code I didn’t think was relevant – but I could be wrong, in which case I’ll update if it doesn’t hold enough information.

Answer 1

That is normal with auto variables – they are not initialized to 0 if not explicitly stated.

The fact that the old values are reused is pure coincidence, as you don’t have any other function calls in-between. If you would have them, the memory at the given stack position would be overwritten and your values were different.