Does String Start or End With Another String?

Description

Determines if entries of x start or end with string (entries of) prefix or suffix respectively, where strings are recycled to common lengths.

Usage

startsWith(x, prefix)
  endsWith(x, suffix)

Arguments

Details

startsWith() is equivalent to but much faster than

  substring(x, 1, nchar(prefix)) == prefix  

or also

  grepl("^<prefix>", x)  

where prefix is not to contain special regular expression characters (and for grepl, x does not contain missing values, see below).

The code has an optimized branch for the most common usage in which prefix or suffix is of length one, and is further optimized in a UTF-8 or 8-byte locale if that is an ASCII string.

Value

A logical vector, of “common length” of x and prefix (or suffix), i.e., of the longer of the two lengths unless one of them is zero when the result is also of zero length. A shorter input is recycled to the output length.

See Also

grepl, substring; the partial string matching functions charmatch and pmatch solve a different task.

Examples

startsWith(search(), "package:") # typically at least two FALSE, nowadays often three

x1 <- c("Foobar", "bla bla", "something", "another", "blu", "brown",
        "blau blüht der Enzian")# non-ASCII
x2 <- cbind(
      startsWith(x1, "b"),
      startsWith(x1, "bl"),
      startsWith(x1, "bla"),
        endsWith(x1, "n"),
        endsWith(x1, "an"))
rownames(x2) <- x1; colnames(x2) <- c("b", "b1", "bla", "n", "an")
x2

## Non-equivalence in case of missing values in 'x', see Details:
x <- c("all", "but", NA_character_)
cbind(startsWith(x, "a"),
      substring(x, 1L, 1L) == "a",
      grepl("^a", x))