Independent Task

You’ve now practiced opening help files, reading their sections, and using them to figure out argument order and default values, with some guidance along the way. This page works the same muscle, but without the step-by-step scaffolding. You’re on your own this time, the same way you will be when you encounter an unfamiliar function in your own work.

Practice

The strsplit() function is part of Base R, so it’s available without loading any additional packages. You haven’t used it anywhere in this module. Using only its help file, answer the following questions.

  1. Provide a one-sentence description of what strsplit() does.
  2. What are the argument names; in what order are the arguments specified?
  3. What arguments have default values, and what are those values?
?strsplit
strsplit R Documentation

Split the Elements of a Character Vector

Description

Split the elements of a character vector x into substrings according to the matches to substring split within them.

Usage

strsplit(x, split, fixed = FALSE, perl = FALSE, useBytes = FALSE)

Arguments

Details

Argument split will be coerced to character, so you will see uses with split = NULL to mean split = character(0), including in the examples below.

Note that splitting into single characters can be done via split = character(0) or split = ""; the two are equivalent. The definition of ‘character’ here depends on the locale: in a single-byte locale it is a byte, and in a multi-byte locale it is the unit represented by a ‘wide character’ (almost always a Unicode code point).

A missing value of split does not split the corresponding element(s) of x at all.

The algorithm applied to each input string is 

    repeat {
        if the string is empty
            break.
        if there is a match
            add the string to the left of the match to the output.
            remove the match and all to the left of it.
        else
            add the string to the output.
            break.
    }

Note that this means that if there is a match at the beginning of a (non-empty) string, the first element of the output is "", but if there is a match at the end of the string, the output is the same as with the match removed.

Note also that if there is an empty match at the beginning of a non-empty string, the first character is returned and the algorithm continues with the rest of the string. This needs to be kept in mind when designing the regular expressions. For example, when looking for a word boundary followed by a letter ("[[:<:]]" with perl = TRUE), one can disallow a match at the beginning of a string (via "(?!^)[[:<:]]").

Invalid inputs in the current locale are warned about up to 5 times.

Value

A list of the same length as x, the i-th element of which contains the vector of splits of x[i].

If any element of x or split is declared to be in UTF-8 (see Encoding), all non-ASCII character strings in the result will be in UTF-8 and have their encoding declared as UTF-8. (This also holds if any element is declared to be Latin-1 except in a Latin-1 locale.) For perl = TRUE, useBytes = FALSE all non-ASCII strings in a multibyte locale are translated to UTF-8.

If any element of x or split is marked as "bytes" (see Encoding), all non-ASCII character strings created by the splitting in the result will be marked as "bytes", but encoding of the resulting character strings not split is unspecified (may be "bytes" or the original). If no element of x or split is marked as "bytes", but useBytes = TRUE, even the encoding of the resulting character strings created by splitting is unspecified (may be "bytes" or "unknown", possibly invalid in the current encoding). Mixed use of "bytes" and other marked encodings is discouraged, but if still desired one may use iconv to re-encode the result e.g. to UTF-8 with suitably substituted invalid bytes.

Warning

An all too common mis-usage is to pass unnamed arguments which are then matched to one or more of fixed, perl and useBytes. So it is good practice to name all the arguments.

See Also

paste for the reverse, grep and sub for string search and manipulation; also nchar, substr, startsWith, and endsWith.

‘regular expression’ for the details of the pattern specification.

Option PCRE_use_JIT controls the details when perl = TRUE.

Examples

noquote(strsplit("A text I want to display with spaces", NULL)[[1]])

x <- c(as = "asfef", qu = "qwerty", "yuiop[", "b", "stuff.blah.yech")
# split x on the letter e
strsplit(x, "e")

unlist(strsplit("a.b.c", "."))
## [1] "" "" "" "" ""
## Note that 'split' is a regexp!
## If you really want to split on '.', use
unlist(strsplit("a.b.c", "[.]"))
## [1] "a" "b" "c"
## or
unlist(strsplit("a.b.c", ".", fixed = TRUE))

## a useful function: rev() for strings
strReverse <- function(x)
        sapply(lapply(strsplit(x, NULL), rev), paste, collapse = "")
strReverse(c("abc", "Statistics"))

## get the first names of the members of R-core
a <- readLines(file.path(R.home("doc"),"AUTHORS"))[-(1:8)]
a <- a[(0:2)-length(a)]
(a <- sub(" .*","", a))
# and reverse them
strReverse(a)

## Note that final empty strings are not produced:
strsplit(paste(c("", "a", ""), collapse="#"), split="#")[[1]]
# [1] ""  "a"
## and also an empty string is only produced before a definite match:
strsplit("", " ")[[1]]    # character(0)
strsplit(" ", " ")[[1]]   # [1] ""
  1. strsplit() uses a pattern matching to split a character vector into substrings.
  2. The arguments are x, split, fixed, perl, and useBytes, specified in that order.
  3. fixed, perl, and useBytes all default to FALSE.
Practice

Write a function call that splits the text "apple,banana,cherry" into the three individual words: "apple", "banana", "cherry".

strsplit("apple,banana,cherry", split = ",")
[[1]]
[1] "apple"  "banana" "cherry"
Ready to Continue?

You’re ready to progress if you can:

  • Explain why documentation is useful, even for functions you already know how to call.
  • Open a function’s help file, whether or not the providing package is already loaded.
  • Identify the Description, Usage, Arguments, Details, Value, and Examples sections of a help file, and explain what each contains.
  • Use a help file to find the information that you need to correctly call a function you’ve never used before.

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