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Trigram statistics for Icelandic

Project description

https://travis-ci.com/mideind/Icegrams.svg?branch=master

Overview

Icegrams is a Python 3.x package that encapsulates a large trigram library for Icelandic. (A trigram is a tuple of three consecutive words or tokens that appear in real-world text.)

The almost 34 million trigrams are heavily compressed using radix tries and quasi-succinct indexes employing Elias-Fano encoding. This enables the compressed trigram file to be mapped directly into memory, with no ex ante decompression, for fast queries (typically ~40 microseconds per lookup).

The Icegrams library is implemented in Python and C/C++, glued together via CFFI.

The trigram storage approach is based on a 2017 paper by Pibiri and Venturini, also referring to Ottaviano and Venturini (2014) regarding partitioned Elias-Fano indexes.

You can use Icegrams to obtain probabilities (relative frequencies) of over a million different unigrams (single words or tokens), or of bigrams (pairs of two words or tokens), or of trigrams. You can also ask it to return the N most likely successors to any unigram or bigram.

Icegrams is useful for instance in spelling correction, predictive typing, to help disabled people write text faster, and for various text generation, statistics and modelling tasks.

Icegrams is built on the database of Greynir.is, comprising over 6 million sentences parsed from Icelandic news articles.

Example

>>> from icegrams import Ngrams
>>> ng = Ngrams()
>>> # Obtain the frequency of the unigram 'Ísland'
>>> ng.freq("Ísland")
42018
>>> # Obtain the probability of the unigram 'Ísland', as a fraction
>>> # of the frequency of all unigrams in the database
>>> ng.prob("Ísland")
0.0003979926900206475
>>> # Obtain the log probability (base e) of the unigram 'Ísland'
>>> ng.logprob("Ísland")
-7.8290769196308005
>>> # Obtain the frequency of the bigram 'Katrín Jakobsdóttir'
>>> ng.freq("Katrín", "Jakobsdóttir")
3517
>>> # Obtain the probability of 'Jakobsdóttir' given 'Katrín'
>>> ng.prob("Katrín", "Jakobsdóttir")
0.23298013245033142
>>> # Obtain the probability of 'Júlíusdóttir' given 'Katrín'
>>> ng.prob("Katrín", "Júlíusdóttir")
0.013642384105960274
>>> # Obtain the frequency of 'velta fyrirtækisins er'
>>> ng.freq("velta", "fyrirtækisins", "er")
4
>>> # adj_freq returns adjusted frequencies, i.e incremented by 1
>>> ng.adj_freq("xxx", "yyy", "zzz")
1

Reference

Initializing Icegrams

After installing the icegrams package, use the following code to import it and initialize an instance of the Ngrams class:

from icegrams import Ngrams
ng = Ngrams()

Now you can use the ng instance to query for unigram, bigram and trigram frequencies and probabilities.

The Ngrams class

  • __init__(self)

    Initializes the Ngrams instance.

  • freq(self, *args) -> int

    Returns the frequency of a unigram, bigram or trigram.

    • str[] *args A parameter sequence of consecutive unigrams to query the frequency for.
    • returns An integer with the frequency of the unigram, bigram or trigram.

    To query for the frequency of a unigram in the text, call ng.freq("unigram1"). This returns the number of times that the unigram appears in the database. The unigram is queried as-is, i.e. with no string stripping or lowercasing.

    To query for the frequency of a bigram in the text, call ng.freq("unigram1", "unigram2").

    To query for the frequency of a trigram in the text, call ng.freq("unigram1", "unigram2", "unigram3").

    If you pass more than 3 arguments to ng.freq(), only the last 3 are significant, and the query will be treated as a trigram query.

    Examples:

    >>>> ng.freq("stjórnarskrá")
    2973
    >>>> ng.freq("stjórnarskrá", "lýðveldisins")
    39
    >>>> ng.freq("stjórnarskrá", "lýðveldisins", "Íslands")
    12
    >>>> ng.freq("xxx", "yyy", "zzz")
    0
    
  • adj_freq(self, *args) -> int

    Returns the adjusted frequency of a unigram, bigram or trigram.

    • str[] *args A parameter sequence of consecutive unigrams to query the frequency for.
    • returns An integer with the adjusted frequency of the unigram, bigram or trigram. The adjusted frequency is the actual frequency plus 1. The method thus never returns 0.

    To query for the frequency of a unigram in the text, call ng.adj_freq("unigram1"). This returns the number of times that the unigram appears in the database, plus 1. The unigram is queried as-is, i.e. with no string stripping or lowercasing.

    To query for the frequency of a bigram in the text, call ng.adj_freq("unigram1", "unigram2").

    To query for the frequency of a trigram in the text, call ng.adj_freq("unigram1", "unigram2", "unigram3").

    If you pass more than 3 arguments to ng.adj_freq(), only the last 3 are significant, and the query will be treated as a trigram query.

    Examples:

    >>>> ng.adj_freq("stjórnarskrá")
    2974
    >>>> ng.adj_freq("stjórnarskrá", "lýðveldisins")
    40
    >>>> ng.adj_freq("stjórnarskrá", "lýðveldisins", "Íslands")
    13
    >>>> ng.adj_freq("xxx", "yyy", "zzz")
    1
    
  • prob(self, *args) -> float

    Returns the probability of a unigram, bigram or trigram.

    • str[] *args A parameter sequence of consecutive unigrams to query the probability for.
    • returns A float with the probability of the given unigram, bigram or trigram.

    The probability of a unigram is the frequency of the unigram divided by the sum of the frequencies of all unigrams in the database.

    The probability of a bigram (u1, u2) is the frequency of the bigram divided by the frequency of the unigram u1, i.e. how likely u2 is to succeed u1.

    The probability of a trigram (u1, u2, u3) is the frequency of the trigram divided by the frequency of the bigram (u1, u2), i.e. how likely u3 is to succeed u1 u2.

    If you pass more than 3 arguments to ng.prob(), only the last 3 are significant, and the query will be treated as a trigram probability query.

    Examples:

    >>>> ng.prob("stjórnarskrá")
    2.8168929772755334e-05
    >>>> ng.prob("stjórnarskrá", "lýðveldisins")
    0.01344989912575655
    >>>> ng.prob("stjórnarskrá", "lýðveldisins", "Íslands")
    0.325
    
  • logprob(self, *args) -> float

    Returns the log probability of a unigram, bigram or trigram.

    • str[] *args A parameter sequence of consecutive unigrams to query the log probability for.
    • returns A float with the natural logarithm (base e) of the probability of the given unigram, bigram or trigram.

    The probability of a unigram is the adjusted frequency of the unigram divided by the sum of the frequencies of all unigrams in the database.

    The probability of a bigram (u1, u2) is the adjusted frequency of the bigram divided by the adjusted frequency of the unigram u1, i.e. how likely u2 is to succeed u1.

    The probability of a trigram (u1, u2, u3) is the adjusted frequency of the trigram divided by the adjusted frequency of the bigram (u1, u2), i.e. how likely u3 is to succeed u1 u2.

    If you pass more than 3 arguments to ng.logprob(), only the last 3 are significant, and the query will be treated as a trigram probability query.

    Examples:

    >>>> ng.logprob("stjórnarskrá")
    -10.477290968535172
    >>>> ng.logprob("stjórnarskrá", "lýðveldisins")
    -4.308783672906165
    >>>> ng.logprob("stjórnarskrá", "lýðveldisins", "Íslands")
    -1.1239300966523995
    
  • succ(self, n, *args) -> list[tuple]

    Returns the N most probable successors of a unigram or bigram.

    • int n A positive integer specifying how many successors, at a maximum, should be returned.
    • str[] *args One or two string parameters containing the unigram or bigram to query the successors for.
    • returns A list of tuples of (successor unigram, log probability), in descending order of probability.

    If you pass more than 2 string arguments to ng.succ(), only the last 2 are significant, and the query will be treated as a bigram successor query.

    Examples:

    >>>> ng.succ(2, "stjórnarskrá")
    [('.', -1.8259625296091855), ('landsins', -2.223111581475692)]
    >>>> ng.succ(2, "stjórnarskrá", "lýðveldisins")
    [('Íslands', -1.1239300966523995), ('og', -1.3862943611198904)]
    >>>> # The following is equivalent to ng.succ(2, "lýðveldisins", "Íslands")
    >>>> ng.succ(2, "stjórnarskrá", "lýðveldisins", "Íslands")
    [('.', -1.3862943611198908), (',', -1.6545583477145702)]
    

Notes

Icegrams is built with a sliding window over the source text. This means that a sentence such as "Maðurinn borðaði ísinn." results in the following trigrams being added to the database:

("", "", "Maðurinn")
("", "Maðurinn", "borðaði")
("Maðurinn", "borðaði", "ísinn")
("borðaði", "ísinn", ".")
("ísinn", ".", "")
(".", "", "")

The same sliding window strategy is applied for bigrams, so the following bigrams would be recorded for the same sentence:

("", "Maðurinn")
("Maðurinn", "borðaði")
("borðaði", "ísinn")
("ísinn", ".")
(".", "")

You can thus obtain the N unigrams that most often start a sentence by asking for ng.succ(N, "").

And, of course, four unigrams are also added, one for each token in the sentence.

The tokenization of the source text into unigrams is done with the Tokenizer package and uses the rules documented there.

Prerequisites

This package runs on CPython 3.4 or newer, and on PyPy 3.5 or newer. It has been tested on Linux (gcc on x86-64 and ARMhf), MacOS (clang) and Windows (MSVC).

If a binary wheel package isn’t available on PyPI for your system, you may need to have the python3-dev and/or potentially python3.6-dev packages (or their Windows equivalents) installed on your system to set up Icegrams successfully. This is because a source distribution install requires a C++ compiler and linker:

# Debian or Ubuntu:
sudo apt-get install python3-dev
sudo apt-get install python3.6-dev

Installation

To install this package:

$ pip install icegrams

If you want to be able to edit the source, do like so (assuming you have git installed):

$ git clone https://github.com/mideind/Icegrams
$ cd Icegrams
$ # [ Activate your virtualenv here if you have one ]
$ python setup.py develop

The package source code is now in ./src/icegrams.

Tests

To run the built-in tests, install pytest, cd to your Icegrams subdirectory (and optionally activate your virtualenv), then run:

$ python -m pytest

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