data.table (version 1.6.3)

setkey: Create key on a data table

Description

Sorts a data.table and marks it as sorted. The sorted columns are the key. The key can be any columns in any order. The columns are sorted in ascending order always.

Usage

setkey(x, ..., loc=parent.frame(),verbose=getOption("datatable.verbose",FALSE))
key(x)
key(x) <- value
haskey(x)

Arguments

x

An unquoted name of a data.table.

...

The columns to sort by. Do not quote the column names. If ... is missing all the columns are used.

value

A character vector of column names.

loc

The data.table must already exist in this frame and is sorted by reference in this frame. loc=.GlobalEnv is often useful within functions.

verbose

Output status and information.

Value

• No value is returned. The data.table is modified by reference. If you require a copy, take a copy first (using DT2=data.table(DT).

Details

The sort is attempted with the very fast "radix" method in sort.list. If that fails, the sort reverts to the default method in order. That logic is repeated column by column. The sort is stable; i.e., the order of ties (if any) is preserved. If value=NULL, the key is removed.

References

http://en.wikipedia.org/wiki/Radix_sort http://en.wikipedia.org/wiki/Counting_sort

See Also

data.table, tables, J, sort.list

Examples

DT = data.table(A=5:1,B=letters[5:1])
    DT # before
    setkey(DT,B)  # re-orders table and marks it sorted.
    DT # after
    tables()      # KEY column reports the key'd columns
    key(DT)
    key(DT) = "A"
    
    DT = data.table(A=5:1,B=letters[5:1])
    DT2 = DT              # not enough to copy
    setkey(DT2,B)         # so does not copy on write to DT2
    identical(DT,DT2)     # TRUE. DT and DT2 are now the same keyed table
    
    DT = data.table(A=5:1,B=letters[5:1])
    DT2 = data.table(DT)  # does make a copy
    setkey(DT2,B)         # just changes DT2
    identical(DT,DT2)     # FALSE

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