For loop to iterate through columns in data.table [duplicate] - r

Question

This question already has answers here:
Convert *some* column classes in data.table
(2 answers)
Closed 4 years ago.
I am trying to write a "for" loop that iterates through each column in a data.table and return a frequency table. However, I keep getting an error saying:
library(datasets)
data(cars)
cars <- as.data.table(cars)
for (i in names(cars)){
print(table(cars[,i]))
}
Error in `[.data.table`(cars, , i) :
j (the 2nd argument inside [...]) is a single symbol but column name 'i' is not found. Perhaps you intended DT[, ..i]. This difference to data.frame is deliberate and explained in FAQ 1.1.
When I use each column individually like below, I do not have any problem:
> table(cars[,dist])
2 4 10 14 16 17 18 20 22 24 26 28 32 34 36 40 42 46 48 50 52 54 56 60 64 66
1 1 2 1 1 1 1 2 1 1 4 2 3 3 2 2 1 2 1 1 1 2 2 1 1 1
68 70 76 80 84 85 92 93 120
1 1 1 1 1 1 1 1 1
My data is quite large (8921483x52), that is why I want to use the "for" loop and run everything at once then look at the result.
I included the cars dataset (which is easier to run) to demonstrate my code.
If I convert the dataset to data.frame, there is no problem running the "for" loop. But I just want to know why this does not work with data.table because I am learning it, which work better with large dataset in my belief.
If by chance, someone saw a post with an answer already, please let me know because I have been trying for several hours to look for one.

Answer 1

Some solution found here
My personal preference is the apply function though
library(datasets)
data(cars)
cars <- as.data.table(cars)
apply(cars,2,table)
To make your loop work you tweak the i
library(datasets)
data(cars)
cars <- as.data.table(cars)
for (i in names(cars)){
print(table(cars[,(i) := as.character(get(i))]))
}