CODEWITHSUNDEEP
rdata.tablechaining
yuskam
yuskam

Reputation: 400

data.table chaining doesn't create the new variable

I can't make chaining work in this example. Can someone explain what I was missing?

library(data.table)
dt <- data.table(a=c(rep("komm", 5), rep("by", 5)), paste0("nr.",1:10))

dt[a=="komm", v3:=sub("nr.", "", V2)]
dt[, v4:=sub("\\D*(\\d)", "\\1", V2)]

# doesn't work
dt[a=="by"][
 , v5:=sub("nr.", "no.", V2)][
 , v6:=sub("\\D*(\\d)", "\\1", V2)]

I was expecting to get this output

      a    V2   v3 v4    v5   v6
1: komm  nr.1    1  1  <NA> <NA> 
2: komm  nr.2    2  2  <NA> <NA> 
3: komm  nr.3    3  3  <NA> <NA> 
4: komm  nr.4    4  4  <NA> <NA>
5: komm  nr.5    5  5  <NA> <NA> 
6:   by  nr.6 <NA>  6  no.6   6
7:   by  nr.7 <NA>  7  no.7   7
8:   by  nr.8 <NA>  8  no.8   8
9:   by  nr.9 <NA>  9  no.9   9
10:  by nr.10 <NA> 10 no.10  10

Upvotes: 2

Views: 170

Answers (1)

r2evans
r2evans

Reputation: 160687

Once you filter and close the bracket, the in-place assignment is broken. That is, DT[cond,newvar:=1] assigns based on a condition, and is equivalent to DT[,newvar:=fifelse(cond,1,newvar)] or similar. However, DT[cond,] is returning a new frame, and any work on it now is completely separate from the original DT.

Either do the conditional assignment twice

dt <- data.table(a=c(rep("komm", 5), rep("by", 5)), paste0("nr.",1:10))
dt[a=="by", v3 := sub("nr.", "", V2)][a=="by", v4:=sub("\\D*(\\d)", "\\1", V2)]
#          a     V2     v3     v4
#     <char> <char> <char> <char>
#  1:   komm   nr.1   <NA>   <NA>
#  2:   komm   nr.2   <NA>   <NA>
#  3:   komm   nr.3   <NA>   <NA>
#  4:   komm   nr.4   <NA>   <NA>
#  5:   komm   nr.5   <NA>   <NA>
#  6:     by   nr.6      6      6
#  7:     by   nr.7      7      7
#  8:     by   nr.8      8      8
#  9:     by   nr.9      9      9
# 10:     by  nr.10     10     10

... or once with multi-assignment:

dt <- data.table(a=c(rep("komm", 5), rep("by", 5)), paste0("nr.",1:10))
dt[a=="by", c("v3", "v4") := .(sub("nr.", "", V2), sub("\\D*(\\d)", "\\1", V2))]
#          a     V2     v3     v4
#     <char> <char> <char> <char>
#  1:   komm   nr.1   <NA>   <NA>
#  2:   komm   nr.2   <NA>   <NA>
#  3:   komm   nr.3   <NA>   <NA>
#  4:   komm   nr.4   <NA>   <NA>
#  5:   komm   nr.5   <NA>   <NA>
#  6:     by   nr.6      6      6
#  7:     by   nr.7      7      7
#  8:     by   nr.8      8      8
#  9:     by   nr.9      9      9
# 10:     by  nr.10     10     10

Upvotes: 2

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