Reference to a variable without the quotation marks-CodePudding

I would like to write a function that take for input a data-frame and one of its factor variable, and that give for output a data-frame with the different levels of this factor and the number of occurence for each level.

Here is a code that do that :

df <- data.frame(ID = sample(c("a", "b", "c", "d"), 20, rep=TRUE))
df %>% group_by(ID) %>% summarise(no_rows = length(ID)) %>% arrange(desc(no_rows))

But I don't know how to put that in a function since the name of the variable (ID) is not inside quote in the second line.

f <- function(df, var){
   df %>% group_by(var) %>% summarise(no_rows = length(var)) %>% arrange(desc(no_rows))
}

f(df, ID) do not work. And I can't write f(df, "ID").

CodePudding user response：

Using dplyr::count and non-standard evaluation (see this SO post or tidyverse documentation) combined with argument sort = TRUE:

library(dplyr)
f <- function(df, var) df %>% count({{ var }}, name = "no_rows", sort = T)

set.seed(1) # using seed for reproducibility
df <- data.frame(ID = sample(c("a", "b", "c", "d"), 20, rep=TRUE))

f(df, ID)
  ID no_rows
1  b       7
2  a       6
3  c       6
4  d       1

CodePudding user response：

f <- function(df,var){
   var <- enquo(var)
   df %>% group_by(!!var) %>% summarise(no_rows = length(!!var)) %>% arrange(desc(no_rows))
}

update function in this way,

f(df, ID)

output;

  ID    no_rows
  <chr>   <int>
1 a           6
2 d           6
3 b           4
4 c           4