Use sqldf inside a function with generic column references - r

Question

I'm trying to use the sqldf package inside a user-defined function in r with generic column names. I can only get it to work if the variable names match placeholder variable names (x and y) within the function. However, I want it to work regardless of the variable name fed into the function. Here is the example I've been playing with:
Here is the form that works:
df<-data.frame(X=as.factor(c("a","a","a","b","b","b","c","c","c")), Y=c(2.5,3,4,4,5.3,6,6.555,7,8))
df
Bar_Prep1<-function(data,x,y){
library(sqldf)
require(sqldf)
dataframe<-sqldf("select a.[x] Grp, AVG(a.[y]) Mean, stdev(a.[y]) SD, Max(a.[y]) Max
from data a
group by a.[x]")
dataframe$RD<-round(dataframe$Mean,digits=0)
return(dataframe)
}
test<-Bar_Prep1(df,df$X,df$Y)
test
Which returns the following df:
Grp Mean SD Max RD
1 a 3.166667 0.7637626 4 3
2 b 5.100000 1.0148892 6 5
3 c 7.185000 0.7400507 8 7
BUT, I want to be able to use the function on various column names, so I tried this:
df1<-data.frame(a=as.factor(c("a","a","a","b","b","b","c","c","c")), b=c(2.5,3,4,4,5.3,6,6.555,7,8))
df1
test1<-Bar_Prep1(df1,df1$a,df1$b)
test1
Returns the following errors: "Error: no such column: a.x"
"object 'test1' not found
So the question is, how do I need to modify my function code to accept variable names other than "x" and "y"?

Answer 1

Pass the names rather than the columns. Change the sqldf call to fn$sqldf which will enable string interpolation using $. Then in the select statement use $x and $y.
library(sqldf)
Bar_Prep1 <- function(data, x, y) {
dataframe <- fn$sqldf("select
a.[$x] Grp,
AVG(a.[$y]) Mean,
stdev(a.[$y]) SD,
Max(a.[$y]) Max
from data a
group by a.[$x]")
dataframe$RD <- round(dataframe$Mean, digits = 0)
return(dataframe)
}
Bar_Prep1(df, "X", "Y")
## Grp Mean SD Max RD
## 1 a 3.166667 0.7637626 4 3
## 2 b 5.100000 1.0148892 6 5
## 3 c 7.185000 0.7400507 8 7
Note that it would be possible to absorb the rounding into the SQL statement:
Bar_Prep1 <- function(data, x, y) {
fn$sqldf("with tmp as (select
a.[$x] Grp,
AVG(a.[$y]) Mean,
stdev(a.[$y]) SD,
Max(a.[$y]) Max
from data a
group by a.[$x])
select *, round(Mean) RD from tmp")
}