How do I transform a string of expression into a quotable expression?
Example: This is the result I want:
mutate(mtcars,answer=wt+wt)
# mpg cyl disp hp drat wt qsec vs am gear carb answer
# 1 21.0 6 160.0 110 3.90 2.620 16.46 0 1 4 4 5.240
# 2 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4 4 5.750
# 3 22.8 4 108.0 93 3.85 2.320 18.61 1 1 4 1 4.640
# 4 21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1 6.430
# 5 18.7 8 360.0 175 3.15 3.440 17.02 0 0 3 2 6.880
# 6 18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1 6.920
...
Here's the function I am writing:
f<-function(df,string_expression){
se<-enexpr(string_expression)
mutate(df,answer=!!se)
}
It will work if I use the following functional call:
f(mtcars,wt+wt)
# mpg cyl disp hp drat wt qsec vs am gear carb answer
# 1 21.0 6 160.0 110 3.90 2.620 16.46 0 1 4 4 5.240
# 2 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4 4 5.750
# 3 22.8 4 108.0 93 3.85 2.320 18.61 1 1 4 1 4.640
# 4 21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1 6.430
# 5 18.7 8 360.0 175 3.15 3.440 17.02 0 0 3 2 6.880
# 6 18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1 6.920
...
However, I would like to provide the expression as a string, so I must use the following function call:
f(mtcars,'wt+wt')
# mpg cyl disp hp drat wt qsec vs am gear carb answer
# 1 21.0 6 160.0 110 3.90 2.620 16.46 0 1 4 4 wt+wt
# 2 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4 4 wt+wt
# 3 22.8 4 108.0 93 3.85 2.320 18.61 1 1 4 1 wt+wt
# 4 21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1 wt+wt
# 5 18.7 8 360.0 175 3.15 3.440 17.02 0 0 3 2 wt+wt
...
How do I make it (either change the function definition or function call) to get the result I want?
What I have tried:
I have tried to sym(string_expression) -- didn't work.
I have tried to quo(string_expression) -- didn't work.
Thank you!
You could change your f function to something this:
f<-function(df,string_expression){
mutate(df, answer = eval(parse(text = string_expression)))
}
head(f(mtcars,'wt+wt'))
mpg cyl disp hp drat wt qsec vs am gear carb answer
1 21.0 6 160 110 3.90 2.620 16.46 0 1 4 4 5.24
2 21.0 6 160 110 3.90 2.875 17.02 0 1 4 4 5.75
3 22.8 4 108 93 3.85 2.320 18.61 1 1 4 1 4.64
4 21.4 6 258 110 3.08 3.215 19.44 1 0 3 1 6.43
5 18.7 8 360 175 3.15 3.440 17.02 0 0 3 2 6.88
6 18.1 6 225 105 2.76 3.460 20.22 1 0 3 1 6.92