# gramEvol: Grammatical Evolution for R

This package includes source code and documentation of gramEvol: Grammatical Evolution for R.

gramEvol implements grammatical evolution (GE) in native R syntax. It allows discovering programs that can achieve a desired goal, by performing an evolutionary optimization over a population of R expressions generated via a user-defined grammar. Functions are provided for creating and manipulating context-free grammars (CFGs), random search, exhaustive search, and evolutionary optimization. Users are only required to define their program structure via a grammar, and a cost function to evaluate the fitness of each program.

### Installation

You can install this package from CRAN:

``install.packages("gramEvol")``

``````if (!require("devtools")) install.packages("devtools")
devtools::install_github("fnoorian/gramEvol")``````

## Usage

A tutorial on implementing GE programs is included in the package’s vignette (PDF version).

More information regarding GE and its application in parameter optimization can be found in this paper in the Journal of Statistical Software.

## Example

This example implements the Kepler law rediscovery problem, as discussed in section 3.1 of the vignette.

``````library("gramEvol")

# grammar definition for generic symbolic regression
grammarDef <- CreateGrammar(list(
expr  = grule(op(expr, expr), func(expr), var),
func  = grule(sin, cos, log, sqrt),
op    = grule(`+`, `-`, `*`), # define unary operators
var   = grule(distance, distance^n, n),
n     = gvrule(1:4) # this is shorthand for grule(1,2,3,4)
))

# cost function and data
planets <- c("Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus")
distance <- c(0.72, 1.00, 1.52, 5.20, 9.53, 19.10)
period <- c(0.61, 1.00, 1.84, 11.90, 29.40, 83.50)

SymRegCostFunc <- function(expr) {
result <- eval(expr)

if (any(is.nan(result)))
return(Inf)

return (mean(log(1 + abs(period - result))))
}

# run GE
ge <- GrammaticalEvolution(grammarDef, SymRegCostFunc, iterations = 50)
print(ge)

# use the best expression
best.expression <- ge\$best\$expression
print(ge\$best\$expressions)
print(data.frame(distance, period, Kepler = sqrt(distance^3), GE = eval(best.expression)))``````

## Release and Development

The latest release and developmental versions of this package are available on: https://github.com/fnoorian/gramEvol