Add example of associative array (#4199)

Add another example of string lookup using associative array. It allows to introduce AA type that a pure C developer would not expect as a builtin in language.

Author: schlupa

articles/ctod.dd

@@ -637,6 +637,23 @@ void dostring(string s)
        to generate a fast lookup scheme for it, eliminating the bugs
        and time required in hand-coding one.
 
+       Another possibility of associating a string to a number is to
+       use an associative array.
+
+$(RUNNABLE_EXAMPLE_COMPILE
+----------------------------
+ulong[string] map = ["hello"    : 1616515,
+                     "goodbye"  : 42,
+                     "maybe"    : ulong.max
+                    ];
+string s = "goodbye";
+
+writeln("number is ", map[s]);
+----------------------------
+)
+
+       Any type can be used as key or as value.
+
 <hr>$(COMMENT  ============================================ )
 $(H2 $(LNAME2 align, Setting struct member alignment))