Changing multiple columns with set in data.table

Question

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!

akrun · Accepted Answer · 2020-04-17 18:02:33Z

1

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

edited Apr 17, 2020 at 18:02

answered Apr 17, 2020 at 18:01

akrun

891k38 gold badges590 silver badges700 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Ricardo Fernandes Campos Over a year ago

If I need to loop, there goes my efficiency!

akrun Over a year ago

@RicardoFernandesCampos no, it is not the case. Here, we are doing the loop by columns

Ricardo Fernandes Campos Over a year ago

I didn't know that. Thanks!

Collectives™ on Stack Overflow

Changing multiple columns with set in data.table

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related