n-gram

n-gram is a series of n adjacent letters (including punctuation marks and blanks), syllables, or rarely whole words found in a language dataset; or adjacent phonemes extracted from a speech-recording dataset, or adjacent base pairs extracted from a genome. They are collected from a text or speech corpus. If Latin numerical prefixes are used, then n-gram of size 1 is called a "unigram", size 2 a "bigram" (or, less commonly, a "digram") etc. If, instead of the Latin ones, the English cardinal numbers are furtherly used, then they are called "four-gram", "five-gram", etc. Similarly, using Greek numerical prefixes such as "monomer", "dimer", "trimer", "tetramer", "pentamer", etc., or English cardinal numbers, "one-mer", "two-mer", "three-mer", etc. are used in computational biology, for polymers or oligomers of a known size, called k-mers. When the items are words, n-grams may also be called shingles.[1]

For other uses, see N-gram (disambiguation).
Not to be confused with word n-gram language model.
Six n-grams frequently found in titles of publications about Coronavirus disease 2019 (COVID-19), as of 7 May 2020

Examples
Figure 1 n-gram examples from various disciplines
FieldUnitSample sequence1-gram sequence2-gram sequence3-gram sequence
Vernacular nameunigrambigramtrigram
Order of resulting Markov model012
Protein sequencingamino acid... Cys-Gly-Leu-Ser-Trp ......, Cys, Gly, Leu, Ser, Trp, ......, Cys-Gly, Gly-Leu, Leu-Ser, Ser-Trp, ......, Cys-Gly-Leu, Gly-Leu-Ser, Leu-Ser-Trp, ...
DNA sequencingbase pair...AGCTTCGA......, A, G, C, T, T, C, G, A, ......, AG, GC, CT, TT, TC, CG, GA, ......, AGC, GCT, CTT, TTC, TCG, CGA, ...
Language modelcharacter...to_be_or_not_to_be......, t, o, _, b, e, _, o, r, _, n, o, t, _, t, o, _, b, e, ......, to, o_, _b, be, e_, _o, or, r_, _n, no, ot, t_, _t, to, o_, _b, be, ......, to_, o_b, _be, be_, e_o, _or, or_, r_n, _no, not, ot_, t_t, _to, to_, o_b, _be, ...
Word n-gram language modelword... to be or not to be ......, to, be, or, not, to, be, ......, to be, be or, or not, not to, to be, ......, to be or, be or not, or not to, not to be, ...

Figure 1 shows several example sequences and the corresponding 1-gram, 2-gram and 3-gram sequences.

Here are further examples; these are word-level 3-grams and 4-grams (and counts of the number of times they appeared) from the Google n-gram corpus.[2]

3-grams

  • ceramics collectables collectibles (55)
  • ceramics collectables fine (130)
  • ceramics collected by (52)
  • ceramics collectible pottery (50)
  • ceramics collectibles cooking (45)

4-grams

  • serve as the incoming (92)
  • serve as the incubator (99)
  • serve as the independent (794)
  • serve as the index (223)
  • serve as the indication (72)
  • serve as the indicator (120)

References
  1. Broder, Andrei Z.; Glassman, Steven C.; Manasse, Mark S.; Zweig, Geoffrey (1997). "Syntactic clustering of the web". Computer Networks and ISDN Systems. 29 (8): 1157–1166. doi:10.1016/s0169-7552(97)00031-7. S2CID 9022773.
  2. Alex Franz and Thorsten Brants (2006). "All Our N-gram are Belong to You". Google Research Blog. Archived from the original on 17 October 2006. Retrieved 16 December 2011.

Further reading
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