CODEWITHSUNDEEP
rdata.table
Ricardo Fernandes Campos
Ricardo Fernandes Campos

Reputation: 911

Changing multiple columns with set in data.table

I'm trying to change lines and columns in data.table using set for efficiency.

The documentation on set states that the argument j is: "Column name(s) (character) or number(s) (integer) to be assigned value when column(s) already exist, and only column name(s) if they are to be created."

and value argument is: "A list of replacement values to assign by reference to x[i, j]."

However, I'm getting an error. This is an example code:

iris = as.data.table(iris)
set(iris,i=1L,j=as.integer(1:3),value=list(1:3))

this is the error I'm getting:

Error in set(iris, i = 1L, j = as.integer(1:3), value = list(1:3)) : Supplied 3 items to be assigned to 1 items of column 'Sepal.Length'. If you wish to 'recycle' the RHS please use rep() to make this intent clear to readers of your code.

I know I can use the other alternatives to assign it, but set is MUCH more efficient. I'd like to know if this is possible.

Thanks!

Upvotes: 1

Views: 85

Answers (1)

akrun
akrun

Reputation: 886938

If we are using set for multiple columns, then loop it

library(data.table)
for(j in 1:3) set(iris, i = 1L, j = j, value = j)
head(iris, 2)
#   Sepal.Length Sepal.Width Petal.Length Petal.Width Species
#1:          1.0         2.0          3.0         0.2  setosa
#2:          4.9         3.0          1.4         0.2  setosa

In base R, this can be done more easily

data(iris)
iris[1, 1:3] <- as.list(1:3)
head(iris, 2)
#  Sepal.Length Sepal.Width Petal.Length Petal.Width Species
#1          1.0           2          3.0         0.2  setosa
#2          4.9           3          1.4         0.2  setosa

Upvotes: 1

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