N-grams

N-grams is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

An N-gram is a sequence of N contiguous elements of a given text. Although N-grams refer sometimes to words or syllables, in this task we will consider only sequences of characters. The task consists in, given a text and an integer size of the desired N-grams, find all the different contiguous sequences of N characters, together with the number of times they appear in the text. For example, the 2-grams of the text "Live and let live" are:

 "LI" - 2
 "IV" - 2
 "VE" - 2
 " L" - 2
 "E " - 1
 " A" - 1
 "AN" - 1
 "ND" - 1
 "D " - 1
 "LE" - 1
 "ET" - 1
 "T " - 1

Note that space and other non-alphanumeric characters are taken into account.

See also

• Related task: Sorensen–Dice coefficient

Arturo

ngrams: function [s :string n :integer][
    0..sub size s n | map 'i -> slice upper s i i+n-1
                    | tally
]

loop [2 3 4] 'n [
    print ~"|n|-grams:"
    loop ngrams "Live and let live" n [k v] -> print [~{"|k|"} v]
    print ""
]

Output:

2-grams:
"LI" 2 
"IV" 2 
"VE" 2 
"E " 1 
" A" 1 
"AN" 1 
"ND" 1 
"D " 1 
" L" 2 
"LE" 1 
"ET" 1 
"T " 1 

3-grams:
"LIV" 2 
"IVE" 2 
"VE " 1 
"E A" 1 
" AN" 1 
"AND" 1 
"ND " 1 
"D L" 1 
" LE" 1 
"LET" 1 
"ET " 1 
"T L" 1 
" LI" 1 

4-grams:
"LIVE" 2 
"IVE " 1 
"VE A" 1 
"E AN" 1 
" AND" 1 
"AND " 1 
"ND L" 1 
"D LE" 1 
" LET" 1 
"LET " 1 
"ET L" 1 
"T LI" 1 
" LIV" 1

Common Lisp

A hash table is used to store and retrieve the n-grams fast.

(defun n-grams (text n)
"Return a list of all the N-grams of length n in the text, together with their frequency"
	(let* (res (*ht-n-grams* (make-hash-table :test 'equal)) )
		(loop for i from 0 to (- (length text) n) do
			(let* ((n-gram (string-upcase (subseq text i (+ i n))))
			       (freq (gethash n-gram *ht-n-grams*)))
		  		(setf (gethash n-gram *ht-n-grams*) (if (null freq) 1 (1+ freq))) ))
			(maphash #'(lambda (key val)
									(push (cons key val) res) )
				*ht-n-grams* )
			(sort res #'> :key #'cdr) ))

Output:

> (n-grams "Live and let live" 2)

(("LI" . 2) ("IV" . 2) ("VE" . 2) (" L" . 2) ("E " . 1) (" A" . 1) ("AN" . 1)
 ("ND" . 1) ("D " . 1) ("LE" . 1) ("ET" . 1) ("T " . 1))

Factor

Works with: Factor version 0.99 2022-04-03

USING: ascii grouping kernel math.statistics prettyprint ;

: n-grams ( str n -- assoc ) [ >upper ] dip clump histogram ;

"Live and let live" 2 n-grams .

Output:

H{
    { "ET" 1 }
    { "IV" 2 }
    { "T " 1 }
    { " A" 1 }
    { "VE" 2 }
    { "LI" 2 }
    { "E " 1 }
    { "D " 1 }
    { " L" 2 }
    { "ND" 1 }
    { "LE" 1 }
    { "AN" 1 }
}

jq

Works with jq and gojq, that is, the C and Go implementations of jq.

# Generic "bag of words" utility:
def bow(stream): 
  reduce stream as $word ({}; .[($word|tostring)] += 1);

# The ngrams as a bow
def ngrams($n):
  ascii_upcase as $text
  | bow( range(0;$text|1+ length - $n) as $i | $text[$i:$i+$n]);

# The task
# Sort by increasing frequency, then by lexicographical order
def ngrams($text; $n):
  ($text|ngrams($n)) as $ngrams
  | "\nAll \($n)-grams of '\($text)' and their frequencies:",
    ($ngrams|to_entries|sort_by(.value,.key)[] | "\(.key): \(.value)" ) ;

ngrams("Live and let live"; 2,3,4)

Output:

All 2-grams of 'Live and let live' and their frequencies:
 A: 1
AN: 1
D : 1
E : 1
ET: 1
LE: 1
ND: 1
T : 1
 L: 2
IV: 2
LI: 2
VE: 2

All 3-grams of 'Live and let live' and their frequencies:
 AN: 1
 LE: 1
 LI: 1
AND: 1
D L: 1
E A: 1
ET : 1
LET: 1
ND : 1
T L: 1
VE : 1
IVE: 2
LIV: 2

All 4-grams of 'Live and let live' and their frequencies:
 AND: 1
 LET: 1
 LIV: 1
AND : 1
D LE: 1
E AN: 1
ET L: 1
IVE : 1
LET : 1
ND L: 1
T LI: 1
VE A: 1
LIVE: 2

Python

This example generates n-grams lazily, much like the sliding_window recipe from the Python itertools docs.

from collections import Counter
from collections import deque
from itertools import islice


def n_grams(text, n):
    """Generate contiguous sequences of _n_ characters from _text_."""
    it = iter(text.upper())
    ngram = deque(islice(it, n), maxlen=n)
    if len(ngram) == n:
        yield "".join(ngram)
    for ch in it:
        ngram.append(ch)
        yield "".join(ngram)


if __name__ == "__main__":
    import pprint

    example = "Live and let live"

    for n in range(2, 5):
        result = Counter(n_grams(example, n)).most_common()
        print(
            f"{len(result)} {n}-grams of {example!r}:\n",
            pprint.pformat(result, compact=True),
            end="\n\n",
        )

Output:

12 2-grams of 'Live and let live':
 [('LI', 2), ('IV', 2), ('VE', 2), (' L', 2), ('E ', 1), (' A', 1), ('AN', 1),
 ('ND', 1), ('D ', 1), ('LE', 1), ('ET', 1), ('T ', 1)]

13 3-grams of 'Live and let live':
 [('LIV', 2), ('IVE', 2), ('VE ', 1), ('E A', 1), (' AN', 1), ('AND', 1),
 ('ND ', 1), ('D L', 1), (' LE', 1), ('LET', 1), ('ET ', 1), ('T L', 1),
 (' LI', 1)]

13 4-grams of 'Live and let live':
 [('LIVE', 2), ('IVE ', 1), ('VE A', 1), ('E AN', 1), (' AND', 1), ('AND ', 1),
 ('ND L', 1), ('D LE', 1), (' LET', 1), ('LET ', 1), ('ET L', 1), ('T LI', 1),
 (' LIV', 1)]

Raku

sub n-gram ($this, $N=2) { Bag.new( flat $this.uc.map: { .comb.rotor($N => -($N-1))».join } ) }
dd 'Live and let live'.&n-gram; # bi-gram
dd 'Live and let live'.&n-gram(3); # tri-gram

Output:

("IV"=>2,"T "=>1,"VE"=>2,"E "=>1,"LE"=>1,"AN"=>1,"LI"=>2,"ND"=>1,"ET"=>1," L"=>2," A"=>1,"D "=>1).Bag
("ET "=>1,"AND"=>1,"LIV"=>2," LI"=>1,"ND "=>1," LE"=>1,"IVE"=>2,"E A"=>1,"VE "=>1,"T L"=>1,"D L"=>1,"LET"=>1," AN"=>1).Bag

Wren

Library: Wren-str

Library: Wren-maputil

Library: Wren-fmt

import "./str" for Str
import "./maputil" for MapUtil
import "./fmt" for Fmt

var findNgrams = Fn.new { |n, text|
    text = Str.upper(text)
    var ngrams = {}
    for (i in 0..text.count-n) {
        var ngram = text[i...i+n]
        MapUtil.increase(ngrams, ngram)
    }
    return ngrams
}

// sort by decreasing frequency, then by lexicographical order
var comparer = Fn.new { |i, j|
    if (i.value != j.value) return j.value < i.value
    return Str.lt(i.key, j.key)
}

var text = "Live and let live"
for (n in [2, 3, 4]) {
    var ngrams = findNgrams.call(n, text)
    System.print("All %(n)-grams of '%(text)' and their frequencies:")
    var ng = ngrams.toList.sort(comparer).map { |me| "(\"%(me.key)\" : %(me.value))"}
    Fmt.tprint("$s  ", ng, 5)
    System.print()
}

Output:

All 2-grams of 'Live and let live' and their frequencies:
(" L" : 2)   ("IV" : 2)   ("LI" : 2)   ("VE" : 2)   (" A" : 1)   
("AN" : 1)   ("D " : 1)   ("E " : 1)   ("ET" : 1)   ("LE" : 1)   
("ND" : 1)   ("T " : 1)   

All 3-grams of 'Live and let live' and their frequencies:
("IVE" : 2)   ("LIV" : 2)   (" AN" : 1)   (" LE" : 1)   (" LI" : 1)   
("AND" : 1)   ("D L" : 1)   ("E A" : 1)   ("ET " : 1)   ("LET" : 1)   
("ND " : 1)   ("T L" : 1)   ("VE " : 1)   

All 4-grams of 'Live and let live' and their frequencies:
("LIVE" : 2)   (" AND" : 1)   (" LET" : 1)   (" LIV" : 1)   ("AND " : 1)   
("D LE" : 1)   ("E AN" : 1)   ("ET L" : 1)   ("IVE " : 1)   ("LET " : 1)   
("ND L" : 1)   ("T LI" : 1)   ("VE A" : 1)