parse
Parse Expressions

### Description

Returns an object of class "expression" that is the parsed version of the input.

### Usage

    parse(file = "", n = NULL, text = NULL, prompt = "?",
          keep.source = getOption("keep.source"),
          srcfile = NULL, encoding = "unknown")
    str2expression(text)
    str2lang(s)

### Arguments

file | the file or connection object from which input to the parser should be read.
n | the number of expressions to parse. If this number is negative, then parsing continues to the end of the file. By default, parse reads to the end of the file or text.
text | an object coerced to a character vector to use as input to the parser.
prompt | any object, as the prompt for standard input. In Spotfire Enterprise Runtime for R, this argument is not implemented, and a warning is generated if it is specified.
keep.source | a logical value. If TRUE, the file location information is stored with parsed expressions. This information is used to print the source code for functions with the original formatting.
srcfile | an environment of class "srcfile" or NULL (the default). If not NULL, this is typically an object created by utils::srcfilecopy or utils::srcfile. The integers in the output expression's srcref attribute refer to positions in srcfile$line, and the file name sometimes given in error messages is taken from srcfile$filename. If keep.source is TRUE, then a default srcfile environment is attached to the output of parse. If you supply your own value for this argument, the value should closely resemble the default one or error messages can be garbled.
encoding | the string encoding to use for newly-created strings. (See Encoding.) In Spotfire Enterprise Runtime for R, this argument is not implemented.
s | a single character string. str2lang uses s where parse and str2expression use text.

### Details

- If text is NULL, then parse reads and parses expressions from the file or connection object file.
- If text is not NULL, then parse is coerced to a character vector, combined into a single string (with carriage returns between the individual strings), and used as input to the parser.

Value

parse | returns an object of mode expression containing the parsed expression(s) read from its file or text argument. If keep.source is TRUE, then the input source of the returned object has the attributes srcref and srcfile, which describe the formatting (spaces, line feeds, and comments).

str2expression(text) | returns the same as parse(text=text, keep.sourc=FALSE): an expression with no attributes describing the formatting.

str2lang(s) | returns an object that is the sole element of the length-one expression object that is returned by str2expression(s). This object is usually a language object, but it can be a literal value. An error is given if the length of the return value of str2expression is not one.

References
Becker, R. A., Chambers, J. M., and Wilks, A. R. 1988. The New S Language. Pacific Grove, CA: Wadsworth & Brooks/Cole Advanced Books and Software.
See Also
eval, options, srcfile, scan, Encoding.
Examples

    my.file <- tempfile("my.file")
    cat("x <- 1\n y <- 2; max(x,y)\n", file = my.file)
    # parse everything in my.file
    parse(file = my.file)
    # parse the first expression in my.file
    parse(n=1, file = my.file)
    unlink(my.file)  # clean up
    # parse from a text string
    parse(text = "x <- 1\n y <- 2; max(x,y)")
    str2expression("x <- 1\n y <- 2; max(x,y)")
    str2lang("max(x,y)")

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