# struct::skiplist(3tcl) 1.4 struct "Tcl Data Structures"

## Name

struct::skiplist - Create and manipulate skiplists

## Synopsis

- package require Tcl 8.5 9
- package require struct::skiplist ?1.4?

- skiplistName option ?arg arg ...?
- skiplistName delete node ?node...?
- skiplistName destroy
- skiplistName insert key value
- skiplistName search node ?-key key?
- skiplistName size
- skiplistName walk cmd

## Description

The ::struct::skiplist command creates a new skiplist object with an associated global Tcl command whose name is skiplistName. This command may be used to invoke various operations on the skiplist. It has the following general form:

skiplistName option ?arg arg ...?

  Option and the args determine the exact behavior of the command.

Skip lists are an alternative data structure to binary trees. They can be used to maintain ordered lists over any sequence of insertions and deletions. Skip lists use randomness to achieve probabilistic balancing, and as a result the algorithms for insertion and deletion in skip lists are much simpler and faster than those for binary trees.

To read more about skip lists see Pugh, William. Skip lists: a probabilistic alternative to balanced trees In: Communications of the ACM, June 1990, 33(6) 668-676.

Currently, the key can be either a number or a string, and comparisons are performed with the built in greater than operator. The following commands are possible for skiplist objects:

skiplistName delete node ?node...?

  Remove the specified nodes from the skiplist.

skiplistName destroy

  Destroy the skiplist, including its storage space and associated command.

skiplistName insert key value

  Insert a node with the given key and value into the skiplist. If a node with that key already exists, then the that node's value is updated and its node level is returned. Otherwise a new node is created and 0 is returned.

skiplistName search node ?-key key?

  Search for a given key in a skiplist. If not found then 0 is returned. If found, then a two element list of 1 followed by the node's value is retuned.

skiplistName size

  Return a count of the number of nodes in the skiplist.

skiplistName walk cmd

  Walk the skiplist from the first node to the last. At each node, the command cmd will be evaluated with the key and value of the current node appended.

## Bugs, Ideas, Feedback

This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category struct :: skiplist of the Tcllib Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.

When proposing code changes, please provide unified diffs, i.e the output of diff -u.

Note further that attachments are strongly preferred over inlined patches. Attachments can be made by going to the Edit form of the ticket immediately after its creation, and then using the left-most button in the secondary navigation bar.

## Keywords

skiplist

## Category

Data structures

## Copyright

Copyright © 2000 Keith Vetter