parse

R Base 3.6.2

parse() returns the parsed but unevaluated expressions in an expression, a “list” of calls. str2expression(s) and str2lang(s) return special versions of parse(text=s, keep.source=FALSE) and can therefore be regarded as transforming character strings s to expressions, calls, etc.

Syntax

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

Arguments

Parameter	Description
`file`	a connection, or a character string giving the name of a file or a URL to read the expressions from. If file is "" and text is missing or NULL then input is taken from the console.
`n`	integer (or coerced to integer). The maximum number of expressions to parse. If n is NULL or negative or NA the input is parsed in its entirety.
`text`	character vector. The text to parse. Elements are treated as if they were lines of a file. Other R objects will be coerced to character if possible.
`prompt`	the prompt to print when parsing from the keyboard. NULL means to use R's prompt, getOption("prompt").
`keep.source`	a logical value; if TRUE, keep source reference information.
`srcfile`	NULL, a character vector, or a srcfile object. See the ‘Details’ section.
`encoding`	encoding to be assumed for input strings. If the value is "latin1" or "UTF-8" it is used to mark character strings as known to be in Latin-1 or UTF-8: it is not used to re-encode the input. To do the latter, specify the encoding as part of the connection con or via options(encoding=): see the example under file.
`s`	a character vector of length 1, i.e., a “string”.

Return Value

parse() and str2expression() return an object of type "expression", for parse() with up to n elements if specified as a non-negative integer. str2lang(s), s a string, returns “a call or simpler”, see the ‘Details:’ section. When srcfile is non-NULL, a "srcref" attribute will be attached to the result containing a list of srcref records corresponding to each element, a "srcfile" attribute will be attached containing a copy of srcfile, and a "wholeSrcref" attribute will be attached containing a sr

Details

parse(....): If text has length greater than zero (after coercion) it is used in preference to file.All versions of R accept input from a connection with end of line marked by LF (as used on Unix), CRLF (as used on DOS/Windows) or CR (as used on classic Mac OS). The final line can be incomplete, that is missing the final EOL marker.When input is taken from the console, n = NULL is equivalent to n = 1, and n < 0 will read until an EOF character is read. (The EOF character is Ctrl-Z for the Windows front-ends.) The line-length limit is 4095 bytes when reading from the console (which may impose a lower limit: see ‘An Introduction to R’).The default for srcfile is set as follows. If keep.source is not TRUE, srcfile defaults to a character string, either "<text>" or one derived from file. When

Examples

Example

# NOT RUN {
fil <- tempfile(fileext = ".Rdmped")
cat("x <- c(1, 4)\n  x ^ 3 -10 ; outer(1:7, 5:9)\n", file = fil)
# parse 3 statements from our temp file
parse(file = fil, n = 3)
unlink(fil)

## str2lang(<string>)  || str2expression(<character>) :
stopifnot(exprs = {
  identical( str2lang("x[3] <- 1+4"), quote(x[3] <- 1+4))
  identical( str2lang("log(y)"),      quote(log(y)) )
  identical( str2lang("abc"   ),      quote(abc) -> qa)
  is.symbol(qa) & !is.call(qa)           # a symbol/name, not a call
  identical( str2lang("1.375" ), 1.375)  # just a number, not a call
})

# A partial parse with a syntax error
txt <- "
x <- 1
an error
"
sf <- srcfile("txt")
try(parse(text = txt, srcfile = sf))
getParseData(sf)
# }