Using a for loop to iterate a function over multiple variables

Question

I have a data table that looks something like this, and I want to apply to it function f

library(data.table)

dt <- data.table(id= c(1,1,1,2,2,2,3,3,3), year=c(1,2,3,1,2,3,1,2,3),y = rnorm(9), x1 = rnorm(9), x2 = c(0,0,0,0,1,0,1,1,1),c2 = rnorm(9))

f <- function(data, var1, var2){
  data[,(paste("times",var1,var2, sep = "_")) := get(var1)*get(var2)]
}

I would like to apply f over multiple variables obtaining results similar to the one below:

vars<-c("x1","x2","c2")
dt2<-lapply(vars, function(x) f(dt,"y", x))
dt2<-do.call(rbind,dt2)

I would like to obtain the same results using a for loop instead of lapply, unfortunately, I am not good with loops. Could anyone help me? so far I tried the following but it does not work properly.

for(i in vars) {
  dt<-f(dt,"y",i)
}

Thanks a lot in advance for your help

It's difficult to help when you do not have a reproducible example for the error — Marc in the box
– Marc in the box, Commented May 1, 2020 at 9:29
yes, I completely understand, unfortunately, I am not sure what is going wrong with the lapply approach as it works with other data. This is why I am looking for a different approach that uses a for loop. Any answer that would reproduce the results in dt2 of the example using a for loop would work. I will update the question to make it clearer — Alex
– Alex, Commented May 1, 2020 at 9:31

Ronak Shah · Accepted Answer · 2020-05-01 09:39:07Z

2

There could be better ways to solve this but since we don't know much about the nature of the exact problem here is a for loop attempt which reproduces the same result as lapply.

library(data.table)   

list_df <- vector('list', length(vars))   
for(i in seq_along(vars)) list_df[[i]] <- f(dt,'y',vars[i])

rbindlist(list_df)

#    id year       y     x1 x2     c2 times_y_x1 times_y_x2 times_y_c2
# 1:  1    1 -0.5605 -0.446  0  0.701     0.2498      0.000    -0.3931
# 2:  1    2 -0.2302  1.224  0 -0.473    -0.2818      0.000     0.1088
# 3:  1    3  1.5587  0.360  0 -1.068     0.5608      0.000    -1.6644
# 4:  2    1  0.0705  0.401  0 -0.218     0.0283      0.000    -0.0154
# 5:  2    2  0.1293  0.111  1 -1.026     0.0143      0.129    -0.1326
# 6:  2    3  1.7151 -0.556  0 -0.729    -0.9533      0.000    -1.2501
#...
#...

answered May 1, 2020 at 9:39

Ronak Shah

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2 Comments

Alex Over a year ago

Great thank you very much. it works well!. Can I ask why did you have to use the seq_along() command?

Ronak Shah Over a year ago

seq_along creates a sequence from 1 to length(vars). We use it here to store the list index number so that we can save each dataframe as list_df[[1]], list_df[[2]] etc.

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