Suppose I have the following function that takes in the parameter var_name. var_name refers to the name of a variable in the data frame. Now consider the following function:
library(dplyr)
calculate_mean <- function(data, var_name) {
lapply(select(data, var_name), mean, na.rm=TRUE)
}
However, I am getting the error:
Error: All select() inputs must resolve to integer column positions.
The following do not: * var_name
df <- head(iris)
f <- function(data, var_name) {
select(data, var_name)
}
f(df, "Petal.Width")
#Error: All select() inputs must resolve to integer column positions.
#The following do not:
#* var_name
The author of that package tends to write optional versions of functions that accept character strings as arguments. Try adding an underscore to the function:
f2 <- function(data, var_name) {
select_(data, var_name)
}
f2(df, "Petal.Width")
# Petal.Width
#1 0.2
#2 0.2
#3 0.2
#4 0.2
#5 0.2
#6 0.4
Further Explanation Usually an unquoted string is considered a variable. If we try x in the console, the evaluator will search the environment for a variable with that name. When used with a function the same search will occur. With mean(x) the variable x must be defined.
This behavior can become confusing when the function is written to not search for a variable. It is called, non-standard evaluation, NSE. There is a base R function that uses NSE. subset(df, select= -Petal.Width) returns the data frame without Petal.Width. This convenience makes for easier programming. select was designed in a similar way.
When you created your function it evaluated in a standard way; unquoted arguments were considered variables. But you are using it for an NSE function select. That function will look for var_name even though you were expecting it to be replaced by the user's input. Let's demonstrate the behavior by creating a literal var_name column:
df$var_name <- 1
f(df, "Petal.Width")
var_name
1 1
2 1
3 1
4 1
5 1
6 1
The original function with select returned the column var_name, not the column we hoped for. Hadley Wickham created select_ in part, to anticipate this discrepancy.
For more information on NSE http://adv-r.had.co.nz/Computing-on-the-language.html
select()), the other standard (select_()).