Create columns with different rules with data.table in r

Question

I'm trying to better understant the data.table package in r. I want to do different types of calculation with some columns and assign the result to new columns with specific names. Here is an example:


set.seed(122)
df <- data.frame(rain = rep(5,10),temp=1:10, skip = sample(0:2,10,T),
                 windw_sz = sample(1:2,10,T),city =c(rep("a",5),rep("b",5)),ord=rep(sample(1:5,5),2)) 


df <- as.data.table(df)
vars <- c("rain","temp")

df[, paste0("mean.",vars) := lapply(mget(vars),mean), by="city" ]

This works just fine. But now I also want to calculate the sum of these variables, so I try:

df[, c(paste0("mean.",vars), paste("sum.",vars)) := list( lapply(mget(vars),mean),
                                                          lapply(mget(vars),sum)), by="city" ]

and I get an error.

How could I implement this last part?

Thanks a lot!

akrun · Accepted Answer · 2020-01-16 00:18:37Z

2

Instead of list wrap, we can do a c as the lapply output is a list, and when do list as wrapper, it returns a list of list. However, with c, it concats two list end to end (i.e. c(as.list(1:5), as.list(6:10)) as opposed to list(as.list(1:5), as.list(6:10))) and instead of mget, make use of .SDcols

library(data.table)
df[, paste0(rep(c("mean.", "sum."), each = 2),  vars) := 
       c(lapply(.SD, mean), lapply(.SD, sum)), by = .(city), .SDcols = vars]
df
#    rain temp skip windw_sz city ord mean.rain mean.temp sum.rain sum.temp
# 1:    5    1    0        2    a   2         5         3       25       15
# 2:    5    2    1        1    a   5         5         3       25       15
# 3:    5    3    2        2    a   3         5         3       25       15
# 4:    5    4    2        1    a   4         5         3       25       15
# 5:    5    5    2        2    a   1         5         3       25       15
# 6:    5    6    0        1    b   2         5         8       25       40
# 7:    5    7    2        2    b   5         5         8       25       40
# 8:    5    8    1        2    b   3         5         8       25       40
# 9:    5    9    2        1    b   4         5         8       25       40
#10:    5   10    2        2    b   1         5         8       25       40

edited Jan 16, 2020 at 0:18

answered Jan 16, 2020 at 0:12

akrun

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1 Comment

rt.l Over a year ago

Great, thanks a lot. Only two clarifications: (1) why did you suggest me to use .SDcols instead of mget? Is it efficiency only? mget also seems to work, right? (2) why did you wrapping the by argument with a list - .(city) - using only "city" also seems to work to me. Thanks a lot for your help.

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