R: Partial String Matching

pmatch {base} | R Documentation

## Partial String Matching

### Description

`pmatch` seeks matches for the elements of its first argument among those of its second.

### Usage

    pmatch(x, table, nomatch = NA_integer_, duplicates.ok = FALSE)

### Arguments

`x` | the values to be matched: converted to a character vector by `as.character`. Long vectors are supported.
`table` | the values to be matched against: converted to a character vector. Long vectors are not supported.
`nomatch` | the value to be returned at non-matching or multiply partially matching positions. Note that it is coerced to `integer`.
`duplicates.ok` | should elements be in `table` be used more than once?

### Details

The behaviour differs by the value of `duplicates.ok`. Consider first the case if this is true. First exact matches are considered, and the positions of the first exact matches are recorded. Then unique partial matches are considered, and if found recorded. (A partial match occurs if the whole of the element of `x` matches the beginning of the element of `table`.) Finally, all remaining elements of `x` are regarded as unmatched. In addition, an empty string can match nothing, not even an exact match to an empty string. This is the appropriate behaviour for partial matching of character indices, for example.

If `duplicates.ok` is `FALSE`, values of `table` once matched are excluded from the search for subsequent matches. This behaviour is equivalent to the R algorithm for argument matching, except for the consideration of empty strings (which in argument matching are matched after exact and partial matching to any remaining arguments).

`charmatch` is similar to `pmatch` with `duplicates.ok` true, the differences being that it differentiates between no match and an ambiguous partial match, it does match empty strings, and it does not allow multiple exact matches.

`NA` values are treated as if they were the string constant `"NA"`.

### Value

An integer vector (possibly including `NA` if `nomatch = NA`) of the same length as `x`, giving the indices of the elements in `table` which matched, or `nomatch`.

### References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

Chambers, J. M. (1998) Programming with Data. A Guide to the S Language. Springer.

### See Also

`match`, `charmatch` and `match.arg`, `match.fun`, `match.call`, for function argument matching etc., `grep` etc for more general (regexp) matching of strings.

### Examples

    pmatch("", "")                             # returns NA
    pmatch("m",   c("mean", "median", "mode")) # returns NA
    pmatch("med", c("mean", "median", "mode")) # returns 2

    pmatch(c("", "ab", "ab"), c("abc", "ab"), dup = FALSE)
    pmatch(c("", "ab", "ab"), c("abc", "ab"), dup = TRUE)
    ## compare
    charmatch(c("", "ab", "ab"), c("abc", "ab"))

[Package base version 3.1.2 Index]