TPGenerator: Generates transition probabilities for n-grams

In wordpredictor: Develop Text Prediction Models Based on N-Grams

Description

It provides a method for generating transition probabilities for the given n-gram size. It also provides a method for generating the combined transition probabilities data for n-gram sizes from 1 to the given size. The combined transition probabilities data can be used to implement back-off.

Details

It provides a method for generating n-gram transition probabilities. It reads n-gram frequencies from an input text file that is generated by the TokenGenerator class.

It parses each n-gram into a prefix, a next word, the next word frequency and the next word probability. Maximum Likelihood count is used to generate the next word probabilities.

Each n-gram prefix is converted to a numeric hash using the digest2int function. The next word is replaced with the position of the next word in the list of all words. The transition probabilities data is stored as a dataframe in a file.

Another method is provided that combines the transition probabilities for n-grams of size 1 to the given size. The combined transition probabilities can be saved to a file as a data frame. This file may be regarded as a completed self contained n-gram model. By combining the transition probabilities of n-grams, back-off may be used to evaluate word probabilities or predict the next word.

Super class

wordpredictor::Base -> TPGenerator

Methods

Public methods

• TPGenerator$new()

• TPGenerator$generate_tp()

• TPGenerator$generate_tp_for_n()

• TPGenerator$clone()

Inherited methods

---

Method new()

It initializes the current obj. It is used to set the transition probabilities options and verbose option.

Usage

TPGenerator$new(opts = list(), ve = 0)

Arguments

opts

The options for generating the transition probabilities.

• save_tp. If the data should be saved.

• n. The n-gram size.

• dir. The directory containing the input and output files.

• format. The format for the output. There are two options.

  • plain. The data is stored in plain text.

  • obj. The data is stored as a R obj.

ve

The level of detail in the information messages.

---

Method generate_tp()

It first generates the transition probabilities for each n-gram of size from 1 to the given size. The transition probabilities are then combined into a single data frame and saved to the output folder that is given as parameter to the current object.

By combining the transition probabilities for all n-gram sizes from 1 to n, back-off can be used to calculate next word probabilities or predict the next word.

Usage

TPGenerator$generate_tp()

Examples

# Start of environment setup code
# The level of detail in the information messages
ve <- 0
# The name of the folder that will contain all the files. It will be
# created in the current directory. NULL implies tempdir will be used
fn <- NULL
# The required files. They are default files that are part of the
# package
rf <- c("n1.RDS", "n2.RDS", "n3.RDS", "n4.RDS")
# An object of class EnvManager is created
em <- EnvManager$new(ve = ve, rp = "./")
# The required files are downloaded
ed <- em$setup_env(rf, fn)
# End of environment setup code

# The list of output files
fns <- c("words", "model-4", "tp2", "tp3", "tp4")

# The TPGenerator object is created
tp <- TPGenerator$new(opts = list(n = 4, dir = ed), ve = ve)
# The combined transition probabilities are generated
tp$generate_tp()

# The test environment is removed. Comment the below line, so the
# files generated by the function can be viewed
em$td_env()

---

Method generate_tp_for_n()

It generates the transition probabilities table for the given n-gram size. It first reads n-gram token frequencies from an input text file.

It then generates a data frame whose columns are the n-gram prefix, next word and next word frequency. The data frame may be saved to a file as plain text or as a R obj. If n = 1, then the list of words is saved.

Usage

TPGenerator$generate_tp_for_n(n)

Arguments

n

The n-gram size for which the tp data is generated.

---

Method clone()

The objects of this class are cloneable with this method.

Usage

TPGenerator$clone(deep = FALSE)

Arguments

deep

Whether to make a deep clone.

Examples

## ------------------------------------------------
## Method `TPGenerator$generate_tp`
## ------------------------------------------------

# Start of environment setup code
# The level of detail in the information messages
ve <- 0
# The name of the folder that will contain all the files. It will be
# created in the current directory. NULL implies tempdir will be used
fn <- NULL
# The required files. They are default files that are part of the
# package
rf <- c("n1.RDS", "n2.RDS", "n3.RDS", "n4.RDS")
# An object of class EnvManager is created
em <- EnvManager$new(ve = ve, rp = "./")
# The required files are downloaded
ed <- em$setup_env(rf, fn)
# End of environment setup code

# The list of output files
fns <- c("words", "model-4", "tp2", "tp3", "tp4")

# The TPGenerator object is created
tp <- TPGenerator$new(opts = list(n = 4, dir = ed), ve = ve)
# The combined transition probabilities are generated
tp$generate_tp()

# The test environment is removed. Comment the below line, so the
# files generated by the function can be viewed
em$td_env()

wordpredictor documentation built on Jan. 4, 2022, 5:07 p.m.