transform.data.table {data.table} | R Documentation

## Data table utilities

### Description

Utilities for `data.table` transformation.

`transform` by group is particularly slow. Please use `:=` by group instead.

`within`, `transform` and other similar functions in `data.table` are not just provided for users who expect them to work, but for non-data.table-aware packages to retain keys, for example. Hopefully the (much) faster and more convenient `data.table` syntax will be used in time. See examples.

### Usage

    ## S3 method for class 'data.table'
    transform(`_data`, ...)
    ## S3 method for class 'data.table'
    within(data, expr, ...)

### Arguments

`data, _data` | data.table to be transformed.
`...` | for `transform`, Further arguments of the form `tag=value`. Ignored for `within`.
`expr` | expression to be evaluated within the data.table.

### Details

`within` is like `with`, but modifications (columns changed, added, or removed) are updated in the returned data.table.

Note that `transform` will keep the key of the `data.table` provided the targets of the transform (i.e. the columns that appear in ...) are not in the key of the data.table. `within` also retains the key provided the key columns are not touched.

### Value

The modified value of a copy of `data`.

### See Also

`transform`, `within` and `:=`

### Examples

    DT <- data.table(a=rep(1:3, each=2), b=1:6)

    DT2 <- transform(DT, c = a^2)
    DT[, c:=a^2]
    identical(DT,DT2)

    DT2 <- within(DT, {
      b <- rev(b)
      c <- a*2
      rm(a)
    })
    DT[,`:=`(b = rev(b),
             c = a*2,
             a = NULL)]
    identical(DT,DT2)

    DT$d = ave(DT$b, DT$c, FUN=max)               # copies entire DT, even if it is 10GB in RAM
    DT = DT[, transform(.SD, d=max(b)), by="c"]   # same, but even worse as .SD is copied for each group
    DT[, d:=max(b), by="c"]                       # same result, but much faster, shorter and scales

    # Multiple update by group. Convenient, fast, scales and easy to read.
    DT[, `:=`(minb = min(b),
              meanb = mean(b),
              bplusd = sum(b+d)),  by=c%/%5]
    DT

[Package data.table version 1.10.4-1 Index]