Tree Dict-Array

A tree node key is usually thought of as containing a simple value. This value however can contain a complex dict or array value.

A dict-array is a new object-type based on a hashtable. The values in a tree-array are indexed using the familiar array notation. This is generally usable anywhere key names are acceptable, eg:

  set t [tree create]
  set id [$t insert end]
  $t set  $id  Users       {Name Bill Age 19 Sex M}
  $t set  $id  Users(Name) "William"
  $t incr $id  Users(Age)

Array Support

Virtually all commands that take a key will also take an array indexed key.

The tree with sub-command also has support for arrays via the -array option.

The find sub-command makes it easy to find all node/keys that are (or are not) arrays, eg

  set isa  [$t find all -key Users -isarray]
  set nota [$t find all -key Users -isarray -invert]

Dicts

If a dict value is assigned to a key value, it will be preserved and used during array access. One advantage of using a dict is that it preserves order.

Unlike dict a tree-array does not store key names as objects. It stores only the key value as an object. In practice however, users should not have to worry about this distinction.

List-Array Conversions

Array objects used in list contexts automatical convert to the required type. However, Tcl will end up doing a split on the string representation. This generally loses any type information. One way around this is to use the values command.

The following example shows how to get array values as a list directly:

  set t [tree create]
  $t set root a(x) 1 a(y) 2
  set i [$t insert 0]
  $t set $i x [$t values 0 a True]
  $t set $i y [$t get 0 a]
  puts "TYPES: [$t type $i x], [$t type $i y]"
% TYPES: list, array

ie. the 4th arg to the values command enables efficient coercion of array values to a list values.

© 2008 Peter MacDonald