Visualizing complex structures using Graphviz

Data Structures as Hashes

In programming, data structures often come in the form of hashes, such as associative arrays [7]. Graphviz builds these structures without much fuss if you use the correct syntax. Line 2 in Listing 4 specifies that you want a directed graph – the LR determines the direction from left to right.

01 digraph G {
02 rankdir=LR;
03 node [shape=record, width=0.1, height=0.1];
04
05 node0 [label = "<f0> | <f1> | <f2>", height=1.5];
06 node [width=1.5];
07 node1 [label = "{<n> n14 | 719 | <p>}"];
08 node2 [label = "{<n> k71 | 216 | <p>}"];
09 node3 [label = "{<n> n39 | 771 | <p>}"];
10 node4 [label = "{<n> k56 | 250 | <p>}"];
11 node5 [label = "{<n> a34 | 125 | <p>}"];
12
13 node0:f0 -> node1:n;
14 node1:p -> node2:n;
15 node0:f1 -> node3:n;
16 node0:f2 -> node4:n;
17 node4:p -> node5:n;
18 }

Lines 5 through 11 define the content of the nodes as additional structures and later access the separate elements by name. The node0 in line 5 has three elements (f0 , f1 , and f2 ), whereas node1 through node5 in lines 7 through 11 each have the two elements n and p . The middle element has no identifier, only the assigned value. Linking the nodes occurs in lines 3 through 17, and Graphviz takes over from there (Figure 4).

Figure 4: If you want to see the relationship of hash tables when programming, Graphviz helps with the necessary functions.

Dependencies

The next example comes directly from the engine room of a Linux system. With the Debtree [8] program, you can explore the dependencies between packages on a Debian-based system (e.g., Ubuntu). The parameters are the names of one or more packages – for simplicity's sake, I'll use names like sqlite3 for the SQLite3 [9] database.

The program provides the result in dot format. The call in Listing 5 creates the listing on the command line and passes the results to the sqlite3.dot file (Figure 5). From this, you generate an image file with dot (Figure 6).

01 $ debtree sqlite3 > sqlite3.dot
02 $ dot -Tpng sqlite3.dot > sqlite3.png

Figure 5: The dot description of the sqlite3 package dependencies on a Debian-based system.

Figure 6: With Debtree and Graphviz, you can see in an instant what kind of a rat tail an installation drags behind it.