Create new columns in data table with for loop

Question

Is there a simple way to create columns within a for loop? I know this question has been asked here multiple times and I have tried this solution adjusted to my case

for (i in 1:100) {
  eval(parse(text = paste0('a$colum', i, ' <- whatever_you_want_your_column_to_contain')))
}

from one of the posts, but it did not help. I have an existing data table data and I am trying to create columns P_1 to P_30 within a for loop and then assign them NULL (I am just trying to pre-define the columns). I have tried this:

for (i in 1:30) {
  eval(parse(text = paste0('data$P_', i, ' <- NULL')))
}

but without any success. Can you please suggest any approach that would work?

A related question - how to refer to those columns in another loop, if I have again column P_i where i is from 1 to 30, how to refer to data$P_i within a loop?

Edit:

I have this data table to make an example:

customer_id <- c("1","1","1","2","2","2","2","3","3","3")
account_id <- as.character(c(11,11,11,55,55,55,55,38,38,38))
obs_date <- c(as.Date("2017-01-01","%Y-%m-%d"), as.Date("2017-02-01","%Y-%m-%d"), as.Date("2017-03-01","%Y-%m-%d"),
              as.Date("2017-12-01","%Y-%m-%d"), as.Date("2018-01-01","%Y-%m-%d"), as.Date("2018-02-01","%Y-%m-%d"),
              as.Date("2018-03-01","%Y-%m-%d"), as.Date("2018-04-01","%Y-%m-%d"), as.Date("2018-05-01","%Y-%m-%d"),
              as.Date("2018-06-01","%Y-%m-%d"))
variable <- c(87,90,100,120,130,150,12,13,15,14)    
data <- data.table(customer_id,account_id,obs_date,variable)

and I found out that the problem is really in assigning that NULL to those columns, because when I am doing this based on the post's advice:

for (i in 1:30) {
   eval(parse(text = paste0('data$P_', i, ' <- 1')))
}

it really works, just with the NULL instead of 1 it does not. So, it is not a bad advice, it just does not work with NULL.

When asking for help, you should include a simple reproducible example with sample input and desired output that can be used to test and verify possible solutions. Data.frames really don't like NULL columns, they should be zero length vectors anyway. — MrFlick
– MrFlick, Commented Oct 10, 2018 at 16:43
Are you really dealing with a data.table? It rather seems like you have "just" a data.frame. Also, you really should study help("[["). — Roland
– Roland, Commented Oct 10, 2018 at 16:48
Please provide a link to the post you are referring to. I have a strong urge to downvote it and let others know that it contains bad advice. — Roland
– Roland, Commented Oct 10, 2018 at 16:51
Recommending eval/parse and a loop work with with data is not good advice. If you want to fill with 1, you can just do data.frame(Map(function(x) 1, paste0("P_", 1:30))) — MrFlick
– MrFlick, Commented Oct 10, 2018 at 17:06
@Roland Here's the answer making that recommendation, somehow well received so far: stackoverflow.com/a/29400819 — Frank
– Frank, Commented Oct 10, 2018 at 17:29

Paul · Accepted Answer · 2018-10-11 02:09:47Z

3

Here's a data.table answer - I think you were close, you just didn't have the right syntax to append a column to a data table:

for (i in 1:30) {
  data[, paste0("P_", i) := "whatever_you_want_your_column_to_contain"]
}

answered Oct 11, 2018 at 2:09

Paul

2,9691 gold badge15 silver badges29 bronze badges

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

Collectives™ on Stack Overflow

Create new columns in data table with for loop

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related