Fast melt for data.table

An S3 method for melting data.tables written in C for speed and memory efficiency. Since v1.9.6, allows melting into multiple columns simultaneously.

It is not necessary to load reshape2 any more. But if you have to, then load reshape2 package before loading data.table.

## fast melt a data.table
# S3 method for data.table
melt(data, id.vars, measure.vars, = "variable", = "value",
    …, na.rm = FALSE, variable.factor = TRUE,
    value.factor = FALSE,
    verbose = getOption("datatable.verbose"))

A data.table object to melt.


vector of id variables. Can be integer (corresponding id column numbers) or character (id column names) vector. If missing, all non-measure columns will be assigned to it. If integer, must be positive; see Details.


Measure variables for melting. Can be missing, vector, list, or pattern-based.

  • When missing, measure.vars will become all columns outside id.vars.

  • Vector can be integer (implying column numbers) or character (column names).

  • list is a generalization of the vector version -- each element of the list (which should be integer or character as above) will become a melted column.

  • Pattern-based column matching can be achieved with the regular expression-based patterns syntax; multiple patterns will produce multiple columns.

For convenience/clarity in the case of multiple melted columns, resulting column names can be supplied as names to the elements measure.vars (in the list and patterns usages). See also Examples.

name for the measured variable names column. The default name is 'variable'.

name for the molten data values column(s). The default name is 'value'. Multiple names can be provided here for the case when measure.vars is a list, though note well that the names provided in measure.vars take precedence.


If TRUE, NA values will be removed from the molten data.


If TRUE, the variable column will be converted to factor, else it will be a character column.


If TRUE, the value column will be converted to factor, else the molten value type is left unchanged.


TRUE turns on status and information messages to the console. Turn this on by default using options(datatable.verbose=TRUE). The quantity and types of verbosity may be expanded in future.

any other arguments to be passed to/from other methods.


If id.vars and measure.vars are both missing, all non-numeric/integer/logical columns are assigned as id variables and the rest as measure variables. If only one of id.vars or measure.vars is supplied, the rest of the columns will be assigned to the other. Both id.vars and measure.vars can have the same column more than once and the same column can be both as id and measure variables. also accepts list columns for both id and measure variables.

When all measure.vars are not of the same type, they'll be coerced according to the hierarchy list > character > numeric > integer > logical. For example, if any of the measure variables is a list, then entire value column will be coerced to a list. Note that, if the type of value column is a list, na.rm = TRUE will have no effect.

From version 1.9.6, melt gains a feature with measure.vars accepting a list of character or integer vectors as well to melt into multiple columns in a single function call efficiently. The function patterns can be used to provide regular expression patterns. When used along with melt, if cols argument is not provided, the patterns will be matched against names(data), for convenience.

Attributes are preserved if all value columns are of the same type. By default, if any of the columns to be melted are of type factor, it'll be coerced to character type. This is to be compatible with reshape2's To get a factor column, set value.factor = TRUE. also preserves ordered factors.

Note that, as opposed to the (undocumented) behaviour of reshape2::melt, id.vars, when specified as numbers, must be between 1 and ncol(data).


An unkeyed data.table containing the molten data.

See Also


  • melt
DT <- data.table(
      i_1 = c(1:5, NA),
      i_2 = c(NA,6,7,8,9,10),
      f_1 = factor(sample(c(letters[1:3], NA), 6, TRUE)),
      f_2 = factor(c("z", "a", "x", "c", "x", "x"), ordered=TRUE),
      c_1 = sample(c(letters[1:3], NA), 6, TRUE),
      d_1 = as.Date(c(1:3,NA,4:5), origin="2013-09-01"),
      d_2 = as.Date(6:1, origin="2012-01-01"))
# add a couple of list cols
DT[, l_1 := DT[, list(c=list(rep(i_1, sample(5,1)))), by = i_1]$c]
DT[, l_2 := DT[, list(c=list(rep(c_1, sample(5,1)))), by = i_1]$c]

# id, measure as character/integer/numeric vectors
melt(DT, id=1:2, measure="f_1")
melt(DT, id=c("i_1", "i_2"), measure=3) # same as above
melt(DT, id=1:2, measure=3L, value.factor=TRUE) # same, but 'value' is factor
melt(DT, id=1:2, measure=3:4, value.factor=TRUE) # 'value' is *ordered* factor

# preserves attribute when types are identical, ex: Date
melt(DT, id=3:4, measure=c("d_1", "d_2"))
melt(DT, id=3:4, measure=c("i_1", "d_1")) # attribute not preserved

# on list
melt(DT, id=1, measure=c("l_1", "l_2")) # value is a list
melt(DT, id=1, measure=c("c_1", "l_1")) # c1 coerced to list

# on character
melt(DT, id=1, measure=c("c_1", "f_1")) # value is char
melt(DT, id=1, measure=c("c_1", "i_2")) # i2 coerced to char

# on na.rm=TRUE. NAs are removed efficiently, from within C
melt(DT, id=1, measure=c("c_1", "i_2"), na.rm=TRUE) # remove NA

# measure.vars can be also a list
# melt "f_1,f_2" and "d_1,d_2" simultaneously, retain 'factor' attribute
# convenient way using internal function patterns()
melt(DT, id=1:2, measure=patterns("^f_", "^d_"), value.factor=TRUE)
# same as above, but provide list of columns directly by column names or indices
melt(DT, id=1:2, measure=list(3:4, c("d_1", "d_2")), value.factor=TRUE)
# same as above, but provide names directly:
melt(DT, id=1:2, measure=patterns(f="^f_", d="^d_"), value.factor=TRUE)

# na.rm=TRUE removes rows with NAs in any 'value' columns
melt(DT, id=1:2, measure=patterns("f_", "d_"), value.factor=TRUE, na.rm=TRUE)

# return 'NA' for missing columns, 'na.rm=TRUE' ignored due to list column
melt(DT, id=1:2, measure=patterns("l_", "c_"), na.rm=TRUE)

# }
Documentation reproduced from package data.table, version 1.12.0, License: MPL-2.0 | file LICENSE

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