CODEWITHSUNDEEP
rdata.table
Naumz
Naumz

Reputation: 481

Expanding data.table by operating on a column

I want to perform an operation on a subset of rows in a data.table that result in a greater number of rows than what I started out with. Is there an easy way to expand the original data.table to accommodate this? If not, how could I accomplish this?

Here's a sample of my original data.

DT <- data.table(my.id=c(1,2,3), unmodified=c("a","b","c"), vals=c("apple",NA,"cat"))
DT
   my.id unmodified  vals
1:     1          a apple
2:     2          b    NA
3:     3          c   cat

And this is my desired output.

DT
   my.id unmodified  vals
1:     1          a apple
2:     2          b   boy
3:     2          b   bat
4:     2          b   bag
5:     3          c   cat

The new rows can appear at the end as well, I don't care about the order. I tried DT[my.id == 2, vals := c("boy","bat","bag")], but it ignores the last 2 entries with a warning.

TIA!

EDIT: My original dataset has about 10 million rows, although the entry with a missing value occurs just once. I'd prefer not to create copies of the data.table, if possible.

Upvotes: 0

Views: 40

Answers (3)

Ajay Ohri
Ajay Ohri

Reputation: 3492

> DT <- data.table(my.id=c(1,2,3), unmodified=c("a","b","c"), vals=c("apple",NA,"cat"))
> DT
   my.id unmodified  vals
1:     1          a apple
2:     2          b    NA
3:     3          c   cat
> DT2 <- data.table(my.id=rep(2,3), unmodified=rep("b",3), vals=c("boy","bat","bag"))
> DT2
   my.id unmodified vals
1:     2          b  boy
2:     2          b  bat
3:     2          b  bag



> rbind(DT,DT2)
   my.id unmodified  vals
1:     1          a apple
2:     2          b    NA
3:     3          c   cat
4:     2          b   boy
5:     2          b   bat
6:     2          b   bag
> rbind(DT,DT2)[order(my.id),]
   my.id unmodified  vals
1:     1          a apple
2:     2          b    NA
3:     2          b   boy
4:     2          b   bat
5:     2          b   bag
6:     3          c   cat
> na.omit(rbind(DT,DT2)[order(my.id),])
   my.id unmodified  vals
1:     1          a apple
2:     2          b   boy
3:     2          b   bat
4:     2          b   bag
5:     3          c   cat

Upvotes: 0

akrun
akrun

Reputation: 886938

Another option is to subset the datasets that have 'my.id' as 2 and not 2, then rbind

rbind(DT[my.id == 2][, .(my.id, unmodified, vals = c('boy', 'bat', 
                 'bag'))], DT[my.id != 2])[order(my.id)]
#    my.id unmodified  vals
#1:     1          a apple
#2:     2          b   boy
#3:     2          b   bat
#4:     2          b   bag
#5:     3          c   cat

Upvotes: 0

akuiper
akuiper

Reputation: 214927

You can use the summarize pattern of data.table by setting the group variables to be my.id and unmodified here; this broadcasts values within each group if the length doesn't match:

DT[, .(vals = if(my.id == 2) c("boy","bat","bag") else vals), .(my.id, unmodified)]

#   my.id unmodified  vals
#1:     1          a apple
#2:     2          b   boy
#3:     2          b   bat
#4:     2          b   bag
#5:     3          c   cat

Upvotes: 1

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