R use string to refer to column

Question

I would like to subset a dataframe by referring to a column with a string and select the values of that column that fulfill a condition. From the following code

 employee <- c('John Doe','Peter Gynn','Jolie Hope')
 salary <- c(21000, 23400, 26800)
 startdate <- as.Date(c('2010-11-1','2008-3-25','2007-3-14'))
 employ.data <- data.frame(employee, salary, startdate)
 salary_string <- "salary"

I want to get all salaries over 23000 by using the salary_string to refer to the column name.

I tried without succes:

set <- subset(employ.data, salary_string > 23000)
set2 <- employ.data[, employ.data$salary_string > 23000)

This does not seem to work because the salary_string is of type character but what I need is some sort of "column name object". Using as.name(salary_string) does not work neither. I know I could get the subset by using

set <- subset(employ.data, salary > 23000)

But my goal is to use the column name that is of type character (salary_string) once with subset(employ.data, ... ) and once with employ.data[, ...]

Answer 1

简短的回答是：不要使用subset而是使用类似的东西

employ.data[employ.data[salary_string]>23000,]

Answer 2

Here's another idea:

dplyr::filter(employ.data, get(salary_string) > 23000)

Which gives:

#    employee salary  startdate
#1 Peter Gynn  23400 2008-03-25
#2 Jolie Hope  26800 2007-03-14

Answer 3

For the sake of showing how to achieve the result with subset() :

The issue you're having is because subset() uses non-standard evaluation. Here's one way to substitute your string into the subset() function.

## set up an unevaluated call
e <- call(">", as.name(salary_string), 23000)
## evaluate it in subset()
subset(employ.data, eval(e))
#     employee salary  startdate
# 2 Peter Gynn  23400 2008-03-25
# 3 Jolie Hope  26800 2007-03-14

Or as Steven suggests, the following would also work well.

subset(employ.data, eval(as.name(salary_string)) > 23000)