R: Apply a function over a string or strings.

strapply {gsubfn} | R Documentation

## Apply a function over a string or strings.

### Description

Similar to `"'gsubfn'"` except instead of performing substitutions it returns the output of `"'FUN'"`.

### Usage

    strapply(X, pattern, FUN = function(x, ...) x, backref, ..., empty,
    	ignore.case = FALSE, perl = FALSE, engine,
    	simplify = FALSE, USE.NAMES, combine = c)
    strapplyc(X, pattern, backref, ignore.case = FALSE, simplify = FALSE, USE.NAMES, engine)

### Arguments

`X` | list or (atomic) vector of character strings to be used.
`pattern` | character string containing a regular expression (or character string for `"'fixed = TRUE')"` to be matched in the given character vector.
`FUN` | a function, formula, character string, list or proto object to be applied to each element of `"'X'"`. See discussion in `gsubfn`.
`backref` | See `gsubfn`.
`empty` | If there is no match to a string return this value.
`ignore.case` | If `TRUE` then case is ignored in the `pattern` argument.
`perl` | If `TRUE` then `engine="R"` is used with perl regular expressions.
`engine` | This argument defaults to `getOption("gsubfn.engine")`. If that option has not been set `engine` defaults to the `"R"` engine if (1) `FUN` is a proto object or if (2) the R installation does not have `tcltk` capability. If the `"R"` default does not apply then it defaults to the `"tcl"` engine.
`...` | optional arguments to `"'gsubfn'"`.
`simplify` | logical or function. If logical, should the result be simplified to a vector or matrix, as in `"sapply"` if possible? If function, that function is applied to the result with each component of the result passed as a separate argument. Typically if the form is used it will typically be specified as rbind.
`USE.NAMES` | logical; if `"'TRUE'"` and if `"'X'"` is character, use `"'X'"` as 'names' for the result unless it had names already. Default is `FALSE`.
`combine` | combine is a function applied to the components of the result of `FUN`. The default is `"c"`. `"list"` is another common choice. The default may change to be `"list"` in the future.

### Details

If `FUN` is a function then for each character string in `"X"` the pattern is repeatedly matched, each such match along with back references, if any, are passed to the function `"FUN"` and the output of `FUN` is returned as a list. If `FUN` is a formula or proto object then it is interpreted to the way discussed in `gsubfn`.

If `FUN` is a proto object or if `perl=TRUE` is specified then `engine="R"` is used and the `engine` argument is ignored.

If `backref` is not specified and `engine="R"` is specified or implied then a heuristic is used to calculate the number of backreferences. The primary situation that can fool it is if there are parentheses in the string that are not back references. In those cases the user will have to specify backref. If `engine="tcl"` then an exact algorithm is used and the problem sentence never occurs.

`strapplyc` is like `strapply` but specialized to `FUN=c` for speed. If the `"tcl"` engine is not available then it calls `strapply` and there will be no speed advantage.

### Value

A list of character strings.

### See Also

See `gsubfn`. For regular expression syntax used in tcl see http://www.tcl.tk/man/tcl8.6/TclCmd/re_syntax.htm and for regular expression syntax used in R see the help page for `regex`.

### Examples

    strapply("12;34:56,89,,12", "[0-9]+")

    # separate leading digits from rest of string
    # creating a 2 column matrix: digits, rest
    s <- c("123abc", "12cd34", "1e23")
    t(strapply(s, "^([[:digit:]]+)(.*)", c, simplify = TRUE))

    # same but create matrix
    strapply(s, "^([[:digit:]]+)(.*)", c, simplify = rbind)

    # running window of 5 characters using 0-lookahead perl regexp
    # Note that the three ( in the regexp will fool it into thinking there
    # are three backreferences so specify backref explicitly.
    x <- "abcdefghijkl"
    strapply(x, "(.)(?=(....))",  paste0, backref = -2, perl = TRUE)[[1]]

    # Note difference.  First gives character vector.  Second is the same.
    # Third has same elements but is a list.
    # Fourth gives list of two character vectors. Fifth is the same.
    strapply("a:b c:d", "(.):(.)", c)[[1]]
    strapply("a:b c:d", "(.):(.)", list, simplify = unlist) # same

    strapply("a:b c:d", "(.):(.)", list)[[1]]

    strapply("a:b c:d", "(.):(.)", c, combine = list)[[1]]
    strapply("a:b c:d", "(.):(.)", c, combine = list, simplify = c) # same

    # find second CPU_SPEED value given lines of config file
    Lines <- c("DEVICE = 'PC'", "CPU_SPEED = '1999', '233'")
    parms <- strapply(Lines, "[^ ',=]+", c, USE.NAMES = TRUE,
    	simplify = ~ lapply(list(...), "[", -1))
    parms$CPU_SPEED[2]

    # return first two words in each string
    p <- proto(fun = function(this, x) if (count <=2) x)
    strapply(c("the brown fox", "the eager beaver"), "\\w+", p)

    ## Not run:
    # convert to chron
    library(chron)
    x <- c("01/15/2005 23:32:45", "02/27/2005 01:22:30")
    x.chron <- strapply(x, "(../../....) (..:..:..)",  chron, simplify = c)

    # time parsing of all 275,546 words from James Joyce's Ulysses
    joyce <- readLines("http://www.gutenberg.org/files/4300/4300-8.txt")
    joycec <- paste(joyce, collapse = " ")
    system.time(s <- strapplyc(joycec, "\\w+")[[1]])
    length(s) # 275546

    ## End(Not run)

[Package gsubfn version 0.6-6 Index]