Object Relationships: Parents and Children

Without relationships, this figure could be dragged apart into six disconnected points and six disconnected segments.

When you create a sketch, the sketch includes not just the geometric objects you’ve constructed, but also the relationships between those objects. When you construct the triangle shown at right, your sketch includes more than just six points and six segments; it also includes the relationships between those 12 objects. For instance, midpoint E depends on segment BC; when you choose Midpoint from the Construct menu you create, not just a point, but also a relationship. Specifically, point E is the midpoint of segment BC. More generally, we describe that relationship by saying that point E is the child of segment BC and that segment BC is the parent of point E. Similarly, segment AB is the child of its two endpoints A and B, and those endpoints are the parents of segment AB.

These parent-child relationships define the mathematics of your sketch and are crucial to the way your sketches behave when you explore them by dragging. Object relationships keep the triangle together as a triangle, and relationships make the midpoints stay where they belong when you drag a vertex.

You can think of a sketch as a family tree, defined both by the objects in the sketch and by their parent-child relationships.

These parent and child relationships are implicit in everything you do in Sketchpad. For instance, to use a command from the Construct menu, you must first select certain objects (prerequisites) in your sketch. These prerequisites become the parents of the newly constructed child.

To explore an object’s mathematical definition—its family tree—use Select Parents and Select Children from the Edit menu, and use the Parents and Children pop-up menus in Object Properties. You can even rearrange your sketch’s family tree using the Split and Merge commands.