patterns: Obtain matching indices corresponding to patterns

View source: R/fmelt.R

patternsR Documentation

Obtain matching indices corresponding to patterns


patterns returns the matching indices in the argument cols corresponding to the regular expression patterns provided. The patterns must be supported by grep.

From v1.9.6, has an enhanced functionality in which measure.vars argument can accept a list of column names and melt them into separate columns. See the Efficient reshaping using data.tables vignette linked below to learn more.


patterns(..., cols=character(0))



A set of regular expression patterns.


A character vector of names to which each pattern is matched.

See Also



DT = data.table(x1 = 1:5, x2 = 6:10, y1 = letters[1:5], y2 = letters[6:10])
# melt all columns that begin with 'x' & 'y', respectively, into separate columns
melt(DT, measure.vars = patterns("^x", "^y", cols=names(DT)))
# when used with melt, 'cols' is implicitly assumed to be names of input
# data.table, if not provided.
melt(DT, measure.vars = patterns("^x", "^y"))

Rdatatable/data.table documentation built on Nov. 15, 2023, 8:05 p.m.