r - sum in function not working properly -

this question has answer here:

• dynamically select data frame columns using $ , vector of column names 4 answers

i want create function count values in variable in subsetted dataset, function not working supposed to.

selected_cyl_6 <- subset(mtcars, mtcars$cyl==6) selected_cyl_4 <- subset(mtcars, mtcars$cyl==4)  count <- function(group,variable) {   sum(group$variable == 4) }  count(selected_cyl_6,gear) # [1] 0  

the answer should 4. however, if use sum directly correct answer

sum(selected_cyl_6$gear==4) # [1] 4 

another example

count(selected_cyl_4,gear) # [1] 0 sum(selected_cyl_4$gear==4) # [1] 8 

what doing wrong?

it's using dollar sign shortcut in function. see fortunes::fortune(343).

some options, using bracket notation.

first, standard evaluation give variable name in quotes when use function.

count <- function(group, variable) {     sum(group[[variable]] == 4) }  count(selected_cyl_6, "gear") 

if want use non-standard evalution don't need quotes, can use deparse substitute in function.

count <- function(group, variable) {     sum(group[[deparse(substitute(variable))]] == 4) }  count(selected_cyl_6, gear)