CreateGrammar {gramEvol}

Context-free Grammar Object

Description

Creates a context-free grammar object.

Usage

  grule(...)

  gsrule(...)

  gvrule(vec)

  CreateGrammar(ruleDef, startSymb)

Arguments

...

A series of comma separated strings or expressions, for gsrule and grule respectively. Expressions can be wrapped in .() to preserve their commas or assignment operators.

vec

An iterable vector or list.

ruleDef

Grammatical rule definition. Either a list of grammar rule objects (GERule) created using grule and gsrule with a syntax similar to Backus-Naur form, or a list of character strings representing symbols and sequences in Backus-Naur form, or a filename or connection to a .bnf file.

See details.

startSymb

The symbol where the generation of a new expression should start. If not given, the first rule in ruleDef is used.

Details

The rule definition is the grammar described in Backus-Naur context-free grammatical format. The preferred way of defining a grammar is to create a list simulating BNF format, which collects several named grammar rule objects (GERule). Each name defines the non-terminal symbol, and each rule in the collection determines the production rule, i.e., possible sequences that will replace the symbol.

Defining a grammar rule object (GERule) can take three forms:

1. The first form uses grule (Grammar Rule), where R expressions are accepted. In the mapping process, variables are looked up and replaced using the production rules.

2. The second form uses gsrule (Grammar String Rule) and uses character strings. The input to gsrule are character string values, where any value surrounded by '<' or '>' is considered as non-terminal symbols and will be replaced using the rule with the same name in the mapping process. Other symbols are considered terminals. This form allows generation of sequences that are not syntactically valid in R (such as `var op var`).

3. The third form uses gvrule (Grammar Vector Rule), where objects within an iterable (vector or list) containing all of the expressions are used as individual rules.

Alternatively, CreateGrammar can read and parse .bnf text files.

Value

CreateGrammar returns a grammar object.

grule and gsrule return a GERule object.

See Also

c, GrammarMap, GrammaticalEvolution

Examples

# Define a simple grammar in BNF format
# <expr> ::= <var><op><var>
# <op>   ::= + | - | *
# <var>  ::= A | B
ruleDef <- list(expr = gsrule("<var><op><var>"),
                op   = gsrule("+", "-", "*"),
                var  = gsrule("A", "B"))

# print rules
print(ruleDef)

# create and display a vector rule
vectorRule = gvrule(1:5)
print(vectorRule)

# Create a grammar object
grammarDef <- CreateGrammar(ruleDef)

# print grammar object
print(grammarDef)

# Creating the same grammar using R expressions
ruleDef <- list(expr = grule(op(var, var)),
                op   = grule(`+`, `-`, `*`),
                var  = grule(A, B))

grammarDef <- CreateGrammar(ruleDef)

print(grammarDef)

# Two rules with commas and assignments, preserved using .()
ruleDef <- list(expr = grule(data.frame(dat)),
                dat  = grule(.(x = 1, y = 2), .(x = 5, y = 6)))
grammarDef <- CreateGrammar(ruleDef)
print(GrammarMap(c(0), grammarDef))
print(GrammarMap(c(1), grammarDef))