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https://github.com/skroutz/turkish_stemmer

A simple Turkish stemming library
https://github.com/skroutz/turkish_stemmer

nlp stemmer

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A simple Turkish stemming library

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README 

        
# TurkishStemmer



Stemmer algorithm for Τurkish language.



## Introduction to Turkish language morphology



> Turkish is an agglutinative language and has a very rich morphological

  stucture. In Turkish, you can form many different words from a single stem by

  appending a sequence of suffixes. For example The word "doktoruymuşsunuz"

  means "You had been the doctor of him". The stem of the word is "doktor" and

  it takes three different suffixes -sU, -ymUş, and -sUnUz.



From "Snowball Description":



> Words are usually composed of a stem and of at least two or three affixes

  appended to it.



> We can analyze noun suffixes in Turkish in two groups. Noun suffixes (eg.

  "doktor-um" meaning "my doctor") and nominal verb suffixes (eg. "doktor-dur"

  meaning ‘is a doctor’). The words ending with nominal verb suffixes can be

  used as verbs in sentences. There are over thirty different suffixes

  classified in these two general groups of suffixes.



> In Turkish, the suffixes are affixed to the stem according to definite

  ordering rules.



From "An affix stripping morphological analyzer for Turkish" paper:



> Turkish has a special place within the natural languages not only being a

  fully concatenative language but also having the suffixes as the only affix

  type. Another feature of the language is that, someone who knows Turkish can

  easily analyze a word even if he/she does not know its stem.



> The phonological rules of Turkish are significant factors that influence

  this feature.

  Ex: (any word)lerim => (any word)-ler-im

  "ler" plural suffix, "im" 1st singular person possessive.



### Rules



1. The only affix type in Turkish is the suffix.



2. A plural suffix cannot follow a possesive suffix.



3. A suffix in Turkish can have multiple allomorphs in order to provide sound

   harmony in the word to which it is affixed.



4. In Turkish each vowel indicates a distinct syllable.



5. In Turkish, single syllable words are mostly the stem itself



6. If a word has nominal __verb__ suffixes, they always appear at the end of

   the word. They follow __noun__ suffixes or the stem itself at the absence

   of noun suffixes



7. In Turkish, “-lAr” suffix can be used both as a nominal verb suffix (third

   person plural present tense) and as a noun suffix (plural inflection).



8. In Turkish, words do not end with consonants 'b', 'c', 'd', and 'ğ'.

   However, when a suffix starting with a vowel is affixed to a word ending

   with 'p', 'ç', 't' or 'k', the last consonant is transformed into 'b', 'c',

   'd', or 'ğ' respectively. The postlude routine transforms last consonants

   'b', 'c','d', or 'ğ'' back to 'p', 'ç', 't' or 'k', respectively, after

   stemming is complete.



### Suffix Classes



Class                        | Type

-----------------------------|----------------

Nominal verb suffixes        | Inflectional

Derivational suffixes        | Derivational

Noun suffixes                | Inflectional

Tense & person verb suffixes | Inflectional

Verb suffixes                | Inflectional



### Suffix allomorphs



Suffix allomorphs are used to create a good sound harmony. They do not change

the meaning of the word. If a suffix has a capital letter then it has an

allomorh. If a suffix has a letter in parentheses then it can be omitted.

Possible allomorphs are given below:



Letter | Allomorph

-------|------------

U      | ı,i,u,ü

C      | c,ç

A      | a,e

D      | d,t

I      | ı,I



### Nominal Verb Suffixes



a/a | Suffix

----|------------------

1   | –(y)Um

2   | –sUn

3   | –(y)Uz

4   | –sUnUz

5   | –lAr

6   | –md

7   | –n

8   | –k

9   | –nUz

10  | –DUr

11  | –cAsInA

12  | –(y)DU

13  | –(y)sA

14  | –(y)mUş

15  | –(y)ken



Suffix transition ordering for nominal verbs can be seen in References[5]



### Noun Suffixes



a/a | Suffixes

----|-------------

1   | –lAr

2   | –(U)m

3   | –(U)mUz

4   | –(U)n

5   | –(U)nUz

6   | –(s)U

7   | –lArI

8   | –(y)U

9   | –nU

10  | –(n)Un

11  | –(y)A

12  | –nA

13  | –DA

14  | –nDA

15  | –DAn

16  | –nDAn

17  | –(y)lA

18  | –ki

19  | –(n)cA



Suffix transition ordering for nouns can be seen in References[5]



### Derivational Suffixes



a/a | Suffixes

----|----------

1   | –lUk

2   | –CU

3   | –CUk

4   | –lAş

5   | –lA

6   | –lAn

7   | –CA

8   | –lU

9   | –sUz



Initially, we will handle only a small subset of the above suffixes which are

more common in our domain.



### Vowel Harmony



This routine checks whether __the last two__ vowels of the word obey vowel

harmony rules. A brief description of Turkish vowel harmony follows.



Turkish vowel harmony is a two dimensional vowel harmony system, where vowels

are characterised by two features named frontness and roundness. There are

vowel harmony rules for each feature.



1. Vowel harmony rule for frontness: Vowels in Turkish are grouped into two

   according to where they are produced. Front produced vowels are formed at

   the front of the mouth ('e', 'i', 'ö', 'ü') and back produced vowels are

   produced nearer to throat ('a', 'ı', 'o', 'u'). According to the vowel

   harmony rule, words cannot contain both front and back vowels. This is one

   of the reasons why suffixes containing vowels can take different forms to

   obey vowel harmony.



2. Vowel harmony rule for roundness: Vowels in Turkish are grouped into two

   according to whether lips are rounded while producing it. 'o', 'ö', 'u' and

   'ü' are rounded vowels whereas 'a', 'e', 'ı' and 'i' are unrounded.

   According to the vowel harmony rules, if the vowel of a syllable is

   unrounded, the following vowel is unrounded as well. If the vowel of a

   syllable is rounded, the following vowels are 'a', 'e', 'u' or 'ü'.



### Last consonant



Another interesting case in detecting suffixes in Turkish is that, for some

suffixes, if the word ends with a vowel, a consonant is inserted between the

rest of the word and the suffix. These merging consonants can be 'y', 'n' or

's'. When a merging consonant can be inserted before the suffix, the

representation of the suffix starts with the optional consonant surrounded by

paranthesis (eg. –(y)Um, -(n)cA). For these kinds of suffixes, if existence of

a merging consonant is considered, the candidate stem is checked whether it

ends with a vowel.



If there is no 'y' consonant before the suffix, only the real part of the

suffix (eg. -Um) is marked for stemming. If there is a 'y' consonant and it is

preceded by a vowel, 'y' is treated as a merging consonant and both 'y' and

the candidate suffix (eg. -Um) is marked for stemming. If there is a consonant

just before 'y', the decision is that the consonant 'y' and the candidate

suffix are really a part of the stem. In such a case, cursor is not advanced

to prevent over-stemming. The last case can occur especially when the stem

originates from another language like in 'lityum' (meaning the element

Lithium). If the check for vowel harmony was not made, the word would be

stemmed to 'lit', for '–(y)Um' would be treated as a suffix affixed to it. But

according to morphological rules of Turkish, the final word would be 'litim',

not 'lityum' if 'lit' were really the stem of the word and the suffix '–(y)Um'

were affixed to it. So detecting 'lit' as the stem of the word would be an over

-stemming.



### Merging Vowel



Similar to merging consonants, there are merging vowels for some suffixes

starting with consonants. They can be preceded by merging vowels like in '-(U)

mUz' suffix when they are affixed to a stem ending with a consonant. In such a

case, a U vowel ('ı', 'i', 'u' or 'ü' depending on vowel harmony) is inserted

between the stem and real suffix (e.g. '-mUz') for ease of pronunciation.



### Some examples



Word / Analysis                | Meaning / Stem

------------------------------ |--------------------------------

Kalelerimizdekilerden          | From the ones at one of our castles

Kale-lAr-UmUz-DA-ki-lAr-DAn    | Kale

Çocuğuymuşumcasına             | As if I were her child

Çocuk-(s)U-(y)mUş-(y)Um-cAsInA | Çocuk

Kedileriyle                    | With their cats

Kedi-lAr-(s)U-(y)lA            | Kedi

Çocuklarımmış                  | Someone told me that they were my children

çocuk-lAr-(U)m-(y)mUş          | Çocuk

Kitabımızdı                    | It was our book

kitap-UmUz-(y)DU               | Kitap



## Future Work



* Add more verbs suffixes.

* Add more derivational suffixes.



## References



1. [Turkish Stemmer used in Lucene](http://snowball.tartarus.org/algorithms/turkish/stemmer.html)

2. [Java Implementation](http://snowball.tartarus.org/archives/snowball-discuss/att-0875/02-TurkishStemmer.java)

3. [Snowball Implementation](http://snowball.tartarus.org/algorithms/turkish/stem_Unicode.sbl)

4. [Snowball Description](http://snowball.tartarus.org/algorithms/turkish/accompanying_paper.doc)

5. [An affix stripping morphological analyzer for Turkish](http://web.itu.edu.tr/~gulsenc/papers/iasted.pdf)

6. [Lead Generation](https://en.wikipedia.org/wiki/Lead_generation)

7. [Vowel Harmony](https://en.wikipedia.org/wiki/Vowel_harmony#Turkish)

8. [Turkish Suffixes](https://en.wiktionary.org/wiki/Appendix:Turkish_suffixes)

9. [Turkish Grammar](https://en.wikipedia.org/wiki/Turkish_grammar)

10. [Turkish Language](https://en.wikipedia.org/wiki/Turkish_language)

11. [Tartarus](http://tartarus.org/)

12. [Information Retrieval on Turkish Texts](http://www.users.muohio.edu/canf/papers/JASIST2008offPrint.pdf)



## Installation



Add this line to your application's Gemfile:



    gem 'turkish_stemmer'



And then execute:



    $ bundle



Or install it yourself as:



    $ gem install turkish_stemmer



## Usage



```ruby

  require 'turkish_stemmer'



  TurkishStemmer.stem("gözlükler") # => "gözlük"

```



## Contributing



1. Fork it ( http://github.com//turkish_stemmer/fork )

2. Create your feature branch (`git checkout -b my-new-feature`)

3. Commit your changes (`git commit -am 'Add some feature'`)

4. Push to the branch (`git push origin my-new-feature`)

5. Create new Pull Request



## License



turkish_stemmer is licensed under MIT. See [LICENSE](LICENSE.txt).