Get Frequency using a loop in dplyr - r

Question

I am triying to get individual frequency table for each variable using a loop and dplyr package, example of my code is below using mtcars data:
library(dplyr)
var= c("vs", "am", "gear")
for (i in var){
mtcars %>%
group_by(carb) %>%
count(i)
}
Lamentably only i get:
Error: Column `i` is unknown
I also tried with
for (i in var){
mtcars %>%
group_by(carb) %>%
summarise_each(funs(n()), i)
}
But not succces,
Please any advice I will gratefull.

Answer 1

We can use !!sym() for the variable names. I would also recommend to save the results to a list as follows.
var <- c("vs", "am", "gear")
library(dplyr)
count_tables <- list()
for (i in var){
temp <- mtcars %>%
group_by(carb) %>%
count(!!sym(i))
count_tables[[i]] <- temp
}
count_tables
# $vs
# # A tibble: 8 x 3
# # Groups: carb [6]
# carb vs n
# <dbl> <dbl> <int>
# 1 1 1 7
# 2 2 0 5
# 3 2 1 5
# 4 3 0 3
# 5 4 0 8
# 6 4 1 2
# 7 6 0 1
# 8 8 0 1
#
# $am
# # A tibble: 9 x 3
# # Groups: carb [6]
# carb am n
# <dbl> <dbl> <int>
# 1 1 0 3
# 2 1 1 4
# 3 2 0 6
# 4 2 1 4
# 5 3 0 3
# 6 4 0 7
# 7 4 1 3
# 8 6 1 1
# 9 8 1 1
#
# $gear
# # A tibble: 11 x 3
# # Groups: carb [6]
# carb gear n
# <dbl> <dbl> <int>
# 1 1 3 3
# 2 1 4 4
# 3 2 3 4
# 4 2 4 4
# 5 2 5 2
# 6 3 3 3
# 7 4 3 5
# 8 4 4 4
# 9 4 5 1
# 10 6 5 1
# 11 8 5 1
It is also common to use lapply to loop through a vector or a list to apply a function and return the objects as a list. The following generates the same output as the for-loop.
count_tables <- lapply(var, function(x) {
mtcars %>%
group_by(carb) %>%
count(!!sym(i))
})
names(count_tables) <- var

Answer 2

For programmatically passing the variable as a string, you can use the version of those functions with and underscore at the end, such as count_, group_by_, etc.
In this case it would be:
for (i in var){
mtcars %>%
group_by(carb) %>%
count_(i) %>%
print()
}
You specifically asked for a for loop, but for your consideration, here goes a lapply alternative, which makes it easier to store the different results in one place for later access:
lapply(var, FUN = function(i) mtcars %>% group_by(carb) %>% count_(i))