Gnome sort

Gnome sort (nicknamed stupid sort) is a variation of the insertion sort sorting algorithm that does not use nested loops. Gnome sort was originally proposed by Iranian computer scientist Hamid Sarbazi-Azad (professor of Computer Science and Engineering at Sharif University of Technology)[1] in 2000. The sort was first called stupid sort[2] (not to be confused with bogosort), and then later described by Dick Grune and named gnome sort.[3]


Gnome sort
Visualisation of Gnome sort
ClassSorting algorithm
Data structureArray
Worst-case performance
Best-case performance
Average performance
Worst-case space complexity auxiliary

Gnome sort performs at least as many comparisons as insertion sort and has the same asymptotic runtime characteristics. Gnome sort works by building a sorted list one element at a time, getting each item to the proper place in a series of swaps. The average running time is O(n2) but tends towards O(n) if the list is initially almost sorted.[4][note 1]

Dick Grune described the sorting method with the following story:[3]

Gnome Sort is based on the technique used by the standard Dutch Garden Gnome (Du.: tuinkabouter).
Here is how a garden gnome sorts a line of flower pots.
Basically, he looks at the flower pot next to him and the previous one; if they are in the right order he steps one pot forward, otherwise, he swaps them and steps one pot backward.
Boundary conditions: if there is no previous pot, he steps forwards; if there is no pot next to him, he is done.

— "Gnome Sort - The Simplest Sort Algorithm". Dickgrune.com

Pseudocode

Here is pseudocode for the gnome sort using a zero-based array:

procedure gnomeSort(a[]):
    pos := 0
    while pos < length(a):
        if (pos == 0 or a[pos] >= a[pos-1]):
            pos := pos + 1
        else:
            swap a[pos] and a[pos-1]
            pos := pos - 1

Example

Given an unsorted array, a = [5, 3, 2, 4], the gnome sort takes the following steps during the while loop. The current position is highlighted in bold and indicated as a value of the variable pos.

Current array pos Condition in effect Action to take
[5, 3, 2, 4]0pos == 0increment pos
[5, 3, 2, 4]1a[pos] < a[pos-1]swap, decrement pos
[3, 5, 2, 4]0pos == 0increment pos
[3, 5, 2, 4]1a[pos] ≥ a[pos-1]increment pos
[3, 5, 2, 4]2a[pos] < a[pos-1]swap, decrement pos
[3, 2, 5, 4]1a[pos] < a[pos-1]swap, decrement pos
[2, 3, 5, 4]0pos == 0increment pos
[2, 3, 5, 4]1a[pos] ≥ a[pos-1]increment pos
[2, 3, 5, 4]2a[pos] ≥ a[pos-1]increment pos:
[2, 3, 5, 4]3a[pos] < a[pos-1]swap, decrement pos
[2, 3, 4, 5]2a[pos] ≥ a[pos-1]increment pos
[2, 3, 4, 5]3a[pos] ≥ a[pos-1]increment pos
[2, 3, 4, 5]4pos == length(a)finished

Notes
  1. Almost sorted means that each item in the list is not far from its proper position (not farther than some small constant distance).

References
  1. Hamid, Sarbazi-Azad. "Hamid Sarbazi-Azad profile page". Archived from the original on 2018-10-16. Retrieved October 16, 2018.
  2. Sarbazi-Azad, Hamid (2 October 2000). "Stupid Sort: A new sorting algorithm" (PDF). Newsletter. Computing Science Department, Univ. of Glasgow (599): 4. Archived (PDF) from the original on 7 March 2012. Retrieved 25 November 2014.
  3. "Gnome Sort - The Simplest Sort Algorithm". Dickgrune.com. 2000-10-02. Archived from the original on 2017-08-31. Retrieved 2017-07-20.
  4. Paul E. Black. "gnome sort". Dictionary of Algorithms and Data Structures. U.S. National Institute of Standards and Technology. Archived from the original on 2011-08-11. Retrieved 2011-08-20.
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