R: Examples for 'data.table::setkey'

### ** Examples

# Type 'example(setkey)' to run these at the prompt and browse output

DT = data.table(A=5:1,B=letters[5:1])
DT # before

   A B
1: 5 e
2: 4 d
3: 3 c
4: 2 b
5: 1 a

setkey(DT,B)          # re-orders table and marks it sorted.
DT # after

   A B
1: 1 a
2: 2 b
3: 3 c
4: 4 d
5: 5 e

tables()              # KEY column reports the key'd columns

   NAME NROW NCOL MB COLS KEY
1:   DT    5    2  0  A,B   B
Total: 0MB

key(DT)

[1] "B"

keycols = c("A","B")
setkeyv(DT,keycols)

DT = data.table(A=5:1,B=letters[5:1])
DT2 = DT              # does not copy
setkey(DT2,B)         # does not copy-on-write to DT2
identical(DT,DT2)     # TRUE. DT and DT2 are two names for the same keyed table

[1] TRUE

DT = data.table(A=5:1,B=letters[5:1])
DT2 = copy(DT)        # explicit copy() needed to copy a data.table
setkey(DT2,B)         # now just changes DT2
identical(DT,DT2)     # FALSE. DT and DT2 are now different tables

[1] FALSE

DT = data.table(A=5:1,B=letters[5:1])
setindex(DT)          # set indices
setindex(DT, A)
setindex(DT, B)
indices(DT)           # get indices single vector

[1] "A__B" "A"    "B"

indices(DT, vectors = TRUE) # get indices list

[[1]]
[1] "A" "B"

[[2]]
[1] "A"

[[3]]
[1] "B"

Examples for 'data.table::setkey'

Create key on a data.table