Recursive indexing: Difference between revisions

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{{Orphanno footnotes|date=NovemberJune 20062020}}

'''Recursive indexing''' is an [[algorithm]] used to represent large numeric values using members of a relatively small [[Set (mathematics)|set]].

When number (generally large number) is represented in a finite alphabet set, and it cannot be represented by just one member of the set, '''Recursive indexing''' is used.

Recursive indexing itself is a method to writewrites the successive differences of the number after extracting the maximum value of the alphabet set from the number, and continuing recursively till the difference falls in the range of the set.

Recursive indexing with a 2 -letter alphabet is called [[Unaryunary code]].

==Encoding==

To encode a number ''N'', keep reducing the maximum element of this set (''S''_max) from ''N'' and output ''S''max for each such difference, stopping when the number lies in the half closed half open

range [0–0 – ''S''_max).

=== Example ===

Let set ''S'' = [0 1 2 3 4 … 10], be aan 11 -element set, and we have to recursively index the value N=49.

According to this method, we need to keep removingsubtract 10 from 49, and keepiterate proceedinguntil tillthe wedifference reachis a number in the 0–10 range.

So theThe values are 10 (''N'' =49–10 49 – 10 =  39), 10 (''N'' =39–10 39 – 10 = 29), 10 (''N'' =29–10 29 – 10 = 19), 10 (''N'' =19–10 19 – 10 = 9), 9. The recursively indexed sequence for ''N'' = 49 with set ''S'', is 10, 10, 10, 10, 9.

Hence the recursively indexed sequence for ''N''=49 with set ''S'', is 10,10,10,10,9.

==Decoding==

Compute the sum of the index values.

Keep adding all the elements of the index, stopping when the index value is between (inclusive of ends) the least and penultimate elements of the set ''S''.

=== Example ===

ContinuingDecoding fromthe above example we haveinvolves  10 + 10 + 10 + 10 + 9  =  49.

==Uses==

This technique is most commonly used in [[RLE|Runrun-length Length Encodingencoding]] systems to encode longer runs than the alphabet sizes permit.

==References==

* Khalid Sayood, Introduction to Data Compression 3rd ed, [[Morgan Kaufmann]].

[[Category:Coding theory]]