Fast dcast for data.table

dcast.data.table is a much faster version of reshape2::dcast, but for data.tables. More importantly, it is capable of handling very large data quite efficiently in terms of memory usage in comparison to reshape2::dcast.

From 1.9.6, dcast is implemented as an S3 generic in data.table. To melt or cast data.tables, it is not necessary to load reshape2 any more. If you have load reshape2, do so before loading data.table to prevent unwanted masking.

NEW: dcast.data.table can now cast multiple value.var columns and also accepts multiple functions to fun.aggregate. See Examples for more.

# S3 method for data.table
dcast(data, formula, fun.aggregate = NULL, sep = "_",
    …, margins = NULL, subset = NULL, fill = NULL,
    drop = TRUE, value.var = guess(data),
    verbose = getOption("datatable.verbose"))

A data.table.


A formula of the form LHS ~ RHS to cast, see Details.


Should the data be aggregated before casting? If the formula doesn't identify a single observation for each cell, then aggregation defaults to length with a message.

NEW: it is possible to provide a list of functions to fun.aggregate. See Examples.


Character vector of length 1, indicating the separating character in variable names generated during casting. Default is _ for backwards compatibility.

Any other arguments that may be passed to the aggregating function.


Not implemented yet. Should take variable names to compute margins on. A value of TRUE would compute all margins.


Specified if casting should be done on a subset of the data. Ex: subset = .(col1 <= 5) or subset = .(variable != "January").


Value with which to fill missing cells. If fun.aggregate is present, takes the value by applying the function on a 0-length vector.


FALSE will cast by including all missing combinations.

NEW: Following #1512, c(FALSE, TRUE) will only include all missing combinations of formula LHS. And c(TRUE, FALSE) will only include all missing combinations of formula RHS. See Examples.


Name of the column whose values will be filled to cast. Function `guess()` tries to, well, guess this column automatically, if none is provided.

NEW: it is now possible to cast multiple value.var columns simultaneously. See Examples.


Not used yet. May be dropped in the future or used to provide informative messages through the console.


The cast formula takes the form LHS ~ RHS, ex: var1 + var2 ~ var3. The order of entries in the formula is essential. There are two special variables: . and . . represents no variable; represents all variables not otherwise mentioned in formula; see Examples.

dcast also allows value.var columns of type list.

When variable combinations in formula doesn't identify a unique value in a cell, fun.aggregate will have to be specified, which defaults to length if unspecified. The aggregating function should take a vector as input and return a single value (or a list of length one) as output. In cases where value.var is a list, the function should be able to handle a list input and provide a single value or list of length one as output.

If the formula's LHS contains the same column more than once, ex: dcast(DT, x+x~ y), then the answer will have duplicate names. In those cases, the duplicate names are renamed using make.unique so that key can be set without issues.

Names for columns that are being cast are generated in the same order (separated by an underscore, _) from the (unique) values in each column mentioned in the formula RHS.

From v1.9.4, dcast tries to preserve attributes wherever possible.

NEW: From v1.9.6, it is possible to cast multiple value.var columns and also cast by providing multiple fun.aggregate functions. Multiple fun.aggregate functions should be provided as a list, for e.g., list(mean, sum, function(x) paste(x, collapse=""). value.var can be either a character vector or list of length=1, or a list of length equal to length(fun.aggregate). When value.var is a character vector or a list of length 1, each function mentioned under fun.aggregate is applied to every column specified under value.var column. When value.var is a list of length equal to length(fun.aggregate) each element of fun.aggregate is applied to each element of value.var column.


A keyed data.table that has been cast. The key columns are equal to the variables in the formula LHS in the same order.

See Also

melt.data.table, rowid, https://cran.r-project.org/package=reshape

  • dcast.data.table
  • dcast
names(ChickWeight) <- tolower(names(ChickWeight))
DT <- melt(as.data.table(ChickWeight), id=2:4) # calls melt.data.table

# dcast is a S3 method in data.table from v1.9.6
dcast(DT, time ~ variable, fun=mean)
dcast(DT, diet ~ variable, fun=mean)
dcast(DT, diet+chick ~ time, drop=FALSE)
dcast(DT, diet+chick ~ time, drop=FALSE, fill=0)

# using subset
dcast(DT, chick ~ time, fun=mean, subset=.(time < 10 & chick < 20))

# drop argument, #1512
DT <- data.table(v1 = c(1.1, 1.1, 1.1, 2.2, 2.2, 2.2),
                 v2 = factor(c(1L, 1L, 1L, 3L, 3L, 3L), levels=1:3),
                 v3 = factor(c(2L, 3L, 5L, 1L, 2L, 6L), levels=1:6),
                 v4 = c(3L, 2L, 2L, 5L, 4L, 3L))
# drop=TRUE
dcast(DT, v1 + v2 ~ v3)                      # default is drop=TRUE
dcast(DT, v1 + v2 ~ v3, drop=FALSE)          # all missing combinations of both LHS and RHS
dcast(DT, v1 + v2 ~ v3, drop=c(FALSE, TRUE)) # all missing combinations of only LHS
dcast(DT, v1 + v2 ~ v3, drop=c(TRUE, FALSE)) # all missing combinations of only RHS

# using . and ...
DT <- data.table(v1 = rep(1:2, each = 6),
                 v2 = rep(rep(1:3, 2), each = 2),
                 v3 = rep(1:2, 6),
                 v4 = rnorm(6))
# }
# }
 ~ v3, value.var = "v4") #same as v1 + v2 ~ v3, value.var = "v4"
dcast(DT, v1 + v2 + v3 ~ ., value.var = "v4")

## for each combination of (v1, v2), add up all values of v4
dcast(DT, v1 + v2 ~ ., value.var = "v4", fun.aggregate = sum)

# }
# benchmark against reshape2's dcast, minimum of 3 runs
DT <- data.table(aa=sample(1e4, 1e6, TRUE),
      bb=sample(1e3, 1e6, TRUE),
      cc = sample(letters, 1e6, TRUE), dd=runif(1e6))
system.time(dcast(DT, aa ~ cc, fun=sum)) # 0.12 seconds
system.time(dcast(DT, bb ~ cc, fun=mean)) # 0.04 seconds
# reshape2::dcast takes 31 seconds
system.time(dcast(DT, aa + bb ~ cc, fun=sum)) # 1.2 seconds
# }
# NEW FEATURE - multiple value.var and multiple fun.aggregate
DT = data.table(x=sample(5,20,TRUE), y=sample(2,20,TRUE),
                z=sample(letters[1:2], 20,TRUE), d1 = runif(20), d2=1L)
# multiple value.var
dcast(DT, x + y ~ z, fun=sum, value.var=c("d1","d2"))
# multiple fun.aggregate
dcast(DT, x + y ~ z, fun=list(sum, mean), value.var="d1")
# multiple fun.agg and value.var (all combinations)
dcast(DT, x + y ~ z, fun=list(sum, mean), value.var=c("d1", "d2"))
# multiple fun.agg and value.var (one-to-one)
dcast(DT, x + y ~ z, fun=list(sum, mean), value.var=list("d1", "d2"))
# }
Documentation reproduced from package data.table, version 1.12.2, License: MPL-2.0 | file LICENSE

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