Terminology and Definitions

Learning to use graphs can be challenging because some concepts have multiple equivalent or similar terms and definitions. For instance, the words ‘node’ and ‘vertex’ typically mean the same thing, but some industries or fields may prefer one to the other.

To help clarify what is meant throughout this project, we define the following terms and definitions. We make heavy use of familial terms, which can help with mentally visualizing the concepts.

This document does is not intended as a course in general graph theory. A graph in the context of this project is made up of nodes which are connected by edges. Edges typically link two nodes asymmetrically in all of the directed graphs within django-directed.

Node

Here, A is a node. Another equivalent name for node that you may sometimes hear is vertex. While they are interchangeable, we will use the term node (or nodes for plural) exclusively within this project for consistency.

graph TD; A((A)); style A fill:green,stroke:#333,stroke-width:4px;

Edge

Here, e is an edge in the graph between nodes A and B. Edges connect nodes, and are directed (denoted here with an arrowhead). Edges are also called lines, links, arcs, or arrows. For consistency, this project will always use the term edge (or edges for plural).

graph TD; A((A)); B((B)); A--e-->B; linkStyle 0 stroke-width:3px,fill:none,stroke:green;

Root

Here, Node A is the root of the graph. It has an in-degree (number of edges coming ‘in’) of 0.

graph TD; A((A)); B((B)); C((C)); D((D)); A-->B; A-->C; B-->D; C-->D; style A fill:green,stroke:#333,stroke-width:4px; linkStyle default fill:none,stroke:gray

Roots

Some types of graphs may have multiple roots. Here, Nodes A and B are roots of the graph. Again, if the in-degree is 0, the node is a root.

graph TD; A((A)); B((B)); C((C)); D((D)); A-->C; B-->C; B-->D; C-->D; style A fill:green,stroke:#333,stroke-width:4px; style B fill:green,stroke:#333,stroke-width:4px; linkStyle default fill:none,stroke:gray

Leaf / Leaves

Here, Nodes D and e are leaves in the graph. They both have an out-degree (number of edges ‘out’ of the node) of 0.

graph TD; A((A)); B((B)); C((C)); D((D)); E((E)); A-->B; A-->C; B-->D; C-->D; A-->E; style D fill:green,stroke:#333,stroke-width:4px; style E fill:green,stroke:#333,stroke-width:4px; linkStyle default fill:none,stroke:gray

Orphan

In a given Graph, an orphan is a node with no parents nor children. Orphans have an in-degree of 0 and and out-degree of 0. Here, node E is an orphan. There are no edges connecting it to any other node.

(Note, there is no equivalent for edges. Every edge connects two [or in special cases, more] nodes.)

graph TD; A((A)); B((B)); C((C)); D((D)); E((E)); A-->B; %% 0 A-->C; %% 1 B-->D; %% 2 C-->D; %% 3 style E fill:green,stroke:#333,stroke-width:4px; linkStyle default fill:none,stroke:gray

Parent / Parents

The parents for a given node x, if any exist, are those nodes which have a directed edge ‘in’ to node x. In graph theory, this may be refered to as a direct predecessor.

Here, node A is a parent of node B, and node B is a parent of node C. Depending on the type of graph, nodes may have zero, one, or multiple parents.

We also refer to parent edges, which are the directed edges themselves which point to the node. In this example, edge e1 is a parent edge of node B, and edge e2 is a parent edge of node C.

graph TD; A((A)); B((B)); C((C)); A--e1-->B; B--e2-->C; style A fill:#20961d,stroke:#333,stroke-width:4px; style B fill:#bdad01,stroke:#333,stroke-width:4px; style C fill:#f86f06,stroke:#333,stroke-width:4px; linkStyle default fill:none,stroke:gray

Child / Children

The children for a given node x, if any exist, are those nodes which have a directed edge ‘out’ from node x. In graph theory, this may be refered to as a direct successor.

Here, node B is a child of node A, and node C is a child of node B. Depending on the type of graph, nodes may have zero, one, or multiple children.

We also refer to children edges, which are the directed edges themselves which point from the node. In this example, edge e1 is a child edge of node A, and edge e2 is a child edge of node B.

graph TD; A((A)); B((B)); C((C)); A--e1-->B; B--e2-->C; style A fill:#f86f06,stroke:#333,stroke-width:4px; style B fill:#bdad01,stroke:#333,stroke-width:4px; style C fill:#20961d,stroke:#333,stroke-width:4px; linkStyle default fill:none,stroke:gray

Ancestors

All nodes in connected paths in a rootward direction. In graph theory, this may be refered to as predecessors.

In this example, the ancestors for node I are nodes A, C, E, and F.

graph TD; A((A)); B((B)); C((C)); D((D)); E((E)); F((F)); G((G)); H((H)); I((I)); J((J)); K((K)); A-->B; %% 0 A-->C; %% 1 B-->D; %% 2 C-->D; %% 3 A-->E; %% 4 C-->F; %% 5 E-->F; %% 6 D-->G; %% 7 G-->H; %% 8 F-->H; %% 9 F-->I; %% 10 E-->J; %% 11 J-->K; %% 12 style I fill:green,stroke:#333,stroke-width:4px; style A fill:#f86f06,stroke:#333,stroke-width:4px; style C fill:#f86f06,stroke:#333,stroke-width:4px; style F fill:#f86f06,stroke:#333,stroke-width:4px; style E fill:#f86f06,stroke:#333,stroke-width:4px; linkStyle default fill:none,stroke:gray linkStyle 1 stroke-width:3px,fill:none,stroke:green; linkStyle 4 stroke-width:3px,fill:none,stroke:green; linkStyle 5 stroke-width:3px,fill:none,stroke:green; linkStyle 6 stroke-width:3px,fill:none,stroke:green; linkStyle 10 stroke-width:3px,fill:none,stroke:green;

Descendants

All nodes in connected paths in a leafward direction. In graph theory, this may be refered to as successors.

In this example, the descendants for node C are nodes D, F, G, H, and I.

graph TD; A((A)); B((B)); C((C)); D((D)); E((E)); F((F)); G((G)); H((H)); I((I)); J((J)); K((K)); A-->B; %% 0 A-->C; %% 1 B-->D; %% 2 C-->D; %% 3 A-->E; %% 4 C-->F; %% 5 E-->F; %% 6 D-->G; %% 7 G-->H; %% 8 F-->H; %% 9 F-->I; %% 10 E-->J; %% 11 J-->K; %% 12 style C fill:green,stroke:#333,stroke-width:4px; style D fill:#f86f06,stroke:#333,stroke-width:4px; style F fill:#f86f06,stroke:#333,stroke-width:4px; style G fill:#f86f06,stroke:#333,stroke-width:4px; style H fill:#f86f06,stroke:#333,stroke-width:4px; style I fill:#f86f06,stroke:#333,stroke-width:4px; linkStyle default fill:none,stroke:gray linkStyle 3 stroke-width:3px,fill:none,stroke:green; linkStyle 5 stroke-width:3px,fill:none,stroke:green; linkStyle 7 stroke-width:3px,fill:none,stroke:green; linkStyle 8 stroke-width:3px,fill:none,stroke:green; linkStyle 9 stroke-width:3px,fill:none,stroke:green; linkStyle 10 stroke-width:3px,fill:none,stroke:green;

Clan

The clan of a node includes all ancestor nodes, the node itself, and all descendant nodes. In graph theory, this can be refered to as the maximal paths through a given node.

In this example, the clan for node F includes nodes A, C, E, H, and I.

graph TD; A((A)); B((B)); C((C)); D((D)); E((E)); F((F)); G((G)); H((H)); I((I)); J((J)); K((K)); A-->B; %% 0 A-->C; %% 1 B-->D; %% 2 C-->D; %% 3 A-->E; %% 4 C-->F; %% 5 E-->F; %% 6 D-->G; %% 7 G-->H; %% 8 F-->H; %% 9 F-->I; %% 10 E-->J; %% 11 J-->K; %% 12 style F fill:green,stroke:#333,stroke-width:4px; style A fill:#f86f06,stroke:#333,stroke-width:4px; style C fill:#f86f06,stroke:#333,stroke-width:4px; style E fill:#f86f06,stroke:#333,stroke-width:4px; style H fill:#f86f06,stroke:#333,stroke-width:4px; style I fill:#f86f06,stroke:#333,stroke-width:4px; linkStyle default fill:none,stroke:gray linkStyle 1 stroke-width:3px,fill:none,stroke:green; linkStyle 4 stroke-width:3px,fill:none,stroke:green; linkStyle 5 stroke-width:3px,fill:none,stroke:green; linkStyle 6 stroke-width:3px,fill:none,stroke:green; linkStyle 9 stroke-width:3px,fill:none,stroke:green; linkStyle 10 stroke-width:3px,fill:none,stroke:green;

Siblings

All nodes that share a parent with this node, excluding the node itself.

In this example, the siblings of node C are nodes B, and E, because they all have node A in common as a parent.

graph TD; A((A)); B((B)); C((C)); D((D)); E((E)); F((F)); G((G)); H((H)); I((I)); J((J)); K((K)); A-->B; %% 0 A-->C; %% 1 B-->D; %% 2 C-->D; %% 3 A-->E; %% 4 C-->F; %% 5 E-->F; %% 6 D-->G; %% 7 G-->H; %% 8 F-->H; %% 9 F-->I; %% 10 E-->J; %% 11 J-->K; %% 12 style C fill:green,stroke:#333,stroke-width:4px; style B fill:#f86f06,stroke:#333,stroke-width:4px; style E fill:#f86f06,stroke:#333,stroke-width:4px; linkStyle default fill:none,stroke:gray linkStyle 1 stroke-width:3px,fill:none,stroke:green; linkStyle 0 stroke-width:3px,fill:none,stroke:green; linkStyle 4 stroke-width:3px,fill:none,stroke:green;

Partners

All nodes that share a child with this node, excluding the node itself.

In this example, the partners of node C are nodes B, and E, because nodes B and C share node D as a child, and nodes C and E share node F as a child.

graph TD; A((A)); B((B)); C((C)); D((D)); E((E)); F((F)); G((G)); H((H)); I((I)); J((J)); K((K)); A-->B; %% 0 A-->C; %% 1 B-->D; %% 2 C-->D; %% 3 A-->E; %% 4 C-->F; %% 5 E-->F; %% 6 D-->G; %% 7 G-->H; %% 8 F-->H; %% 9 F-->I; %% 10 E-->J; %% 11 J-->K; %% 12 style C fill:green,stroke:#333,stroke-width:4px; style B fill:#f86f06,stroke:#333,stroke-width:4px; style E fill:#f86f06,stroke:#333,stroke-width:4px; linkStyle default fill:none,stroke:gray linkStyle 2 stroke-width:3px,fill:none,stroke:green; linkStyle 3 stroke-width:3px,fill:none,stroke:green; linkStyle 5 stroke-width:3px,fill:none,stroke:green; linkStyle 6 stroke-width:3px,fill:none,stroke:green;

Distance

The shortest number of hops from one node to a target node. The distance between node C and node H is 2. This is because the path from C to F to H involves 2 edges.

There is another path from C to H through nodes D and G, but that path is longer (3 edges), and when we refer to distance in this project, we always mean the smallest number of hops.

graph TD; A((A)); B((B)); C((C start)); D((D)); E((E)); F((F)); G((G)); H((H end)); I((I)); J((J)); K((K)); A-->B; %% 0 A-->C; %% 1 B-->D; %% 2 C--1-->D; %% 3 A-->E; %% 4 C--1-->F; %% 5 E-->F; %% 6 D--2-->G; %% 7 G--3-->H; %% 8 F--2-->H; %% 9 F-->I; %% 10 E-->J; %% 11 J-->K; %% 12 style C fill:green,stroke:#333,stroke-width:4px; style F fill:#f86f06,stroke:#333,stroke-width:4px; style H fill:green,stroke:#333,stroke-width:4px; linkStyle default fill:none,stroke:gray linkStyle 5 stroke-width:3px,fill:none,stroke:green; linkStyle 9 stroke-width:3px,fill:none,stroke:green;

Node Depth

The distance of the node from furthest root in the graph. Because this can be a bit challenging to visualize, a few examples are provided below.

Because node A is the highest (and only) root in the following graph, its node depth is 0.

graph TD; A((A)); B((B)); C((C)); D((D)); E((E)); F((F)); G((G)); H((H)); I((I)); J((J)); K((K)); A-->B; %% 0 A-->C; %% 1 B-->D; %% 2 C-->D; %% 3 A-->E; %% 4 C-->F; %% 5 E-->F; %% 6 D-->G; %% 7 G-->H; %% 8 F-->H; %% 9 F-->I; %% 10 E-->J; %% 11 J-->K; %% 12 style A fill:green,stroke:#333,stroke-width:4px; linkStyle default fill:none,stroke:gray

Using the same graph as before, consider the depth of node H. There is only a single root (node A) in this graph, and the distance between node A and node H is 3. So the node depth of node H is 3.

graph TD; A((A)); B((B)); C((C)); D((D)); E((E)); F((F)); G((G)); H((H)); I((I)); J((J)); K((K)); A-->B; %% 0 A--1-->C; %% 1 B-->D; %% 2 C-->D; %% 3 A-->E; %% 4 C--2-->F; %% 5 E-->F; %% 6 D-->G; %% 7 G-->H; %% 8 F--3-->H; %% 9 F-->I; %% 10 E-->J; %% 11 J-->K; %% 12 style A fill:green,stroke:#333,stroke-width:4px; style C fill:#f86f06,stroke:#333,stroke-width:4px; style F fill:#f86f06,stroke:#333,stroke-width:4px; style H fill:green,stroke:#333,stroke-width:4px; linkStyle default fill:none,stroke:gray linkStyle 1 stroke-width:3px,fill:none,stroke:green; linkStyle 5 stroke-width:3px,fill:none,stroke:green; linkStyle 9 stroke-width:3px,fill:none,stroke:green;

Finally, we will look at a more complicated example with multiple roots at different levels. Here we want the node depth of node F.

While both nodes A and D are roots in this graph (they have in-degree of 0), node A has a greater distance from node F, so we determine the depth of node F from the viewpoint of node A. It takes 3 hops to reach node F from node A, so the node depth of node F is 3.

graph TD; A((A)); B((B)); C((C)); D((D)); E((E)); F((F)); A--1-->B; %% 0 B--2-->C; %% 1 D-->E; %% 2 E-->F; %% 3 C--3-->F; %% 4 style A fill:green,stroke:#333,stroke-width:4px; style B fill:#f86f06,stroke:#333,stroke-width:4px; style C fill:#f86f06,stroke:#333,stroke-width:4px; style F fill:green,stroke:#333,stroke-width:4px; linkStyle default fill:none,stroke:gray linkStyle 0 stroke-width:3px,fill:none,stroke:green; linkStyle 1 stroke-width:3px,fill:none,stroke:green; linkStyle 4 stroke-width:3px,fill:none,stroke:green;