pmatch
Partial String Matching
pmatch seeks matches for the elements of its first argument
among those of its second.
- Keywords
- character
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
tablebe 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.,
startsWith for particular checking of initial matches;
grep etc for more general (regexp) matching of strings.
Examples
library(base)
# NOT RUN {
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"))
# }