I want to select a column from a R dataframe through a function, for example :
my_new_df <- function(input_1, input_2) {
df <- input_1
col <- noquote(input_2)
df_new <- df$col
return(df_new)
}
my_new_df(mtcars, "mpg")
-> NULL
Could you explain me please why "$" is not working in a function? Thanks
Sorry I was sloppy in my comment so let's put it all together:
(1) If you want one column as a vector in return, use Soeren D.'s solution
(2) If you want a versatile solution that gives you the choice of getting a vector, or a data.frame with one or more columns back, use gpier's solution
(3) If you want one column as a data.frame in return, use
my_new_df <- function(input_1, input_2)
{
df <- input_1
df_new <- df[ input_2 ]
return(df_new)
}
You need the line return(df_new) only if you want to display the result on screen. If your goal is just to assign it to another variable, you could omit it.
return does is incorrect (in reality it does nothing here). In proper R style, the function would be written simply as {input_1[input_2]} (optionally omitting the redundant braces, and, of course, choosing appropriate parameter names).
return() does nothing as you say, but try it out - define two functions, one with the line return(df_new) and one without. At least on my machine, the first one prints the column on screen, the other doesn't. That is true only when the return value is not assigned to another variable. Arguably such a function is not needed in the first place, but that's how the OP got started.return. It’s purely due to the fact that assignment returns its result invisibly so if the last expression in your function is an assignment, then the function also returns an invisible result — same as writing invisible(df_new). But there’s no difference between return(df_new) and plain df_new. Or, while we’re at it, without the assignment (i.e. just having df[input_2] as the last expression).You can use the [[ operator to select a column based on the name.
my_new_df <- function(input_1, input_2) {
df <- input_1
df_new <- df[[input_2]]
return(df_new)
}
Edit: df[["input_2"]] is equivalent to df$input_2 , both return a vector,
not a data.frame. If returning a data.frame is required please refer to the other answer by gpier.
new_df sounds as if he wants a data.frame
$. I've edited the answer accordingly.You could do this:
my_new_df <- function(input_1, input_2) {
col <- which(colnames(input_1)%in%input_2)
df_new <- input_1[,col]
return(df_new)
}
my_new_df(data.frame(mtcars), "mpg")
[1] 21.0 21.0 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 17.8 16.4 17.3 15.2 10.4 10.4
[17] 14.7 32.4 30.4 33.9 21.5 15.5 15.2 13.3 19.2 27.3 26.0 30.4 15.8 19.7 15.0 21.4
EDIT: if you'd like to keep the data.frame format you could use drop=FALSE
my_new_df <- function(input_1, input_2) {
col <- which(colnames(input_1)%in%input_2)
df_new <- input_1[,col, drop=FALSE]
return(df_new)
}
my_new_df(data.frame(mtcars), "mpg")
mpg
Mazda RX4 21.0
Mazda RX4 Wag 21.0
Datsun 710 22.8
Hornet 4 Drive 21.4
Hornet Sportabout 18.7
Valiant 18.1
Duster 360 14.3
Merc 240D 24.4
Merc 230 22.8
Merc 280 19.2
Merc 280C 17.8
Merc 450SE 16.4
Merc 450SL 17.3
Merc 450SLC 15.2
Cadillac Fleetwood 10.4
Lincoln Continental 10.4
Chrysler Imperial 14.7
Fiat 128 32.4
Honda Civic 30.4
Toyota Corolla 33.9
Toyota Corona 21.5
Dodge Challenger 15.5
AMC Javelin 15.2
Camaro Z28 13.3
Pontiac Firebird 19.2
Fiat X1-9 27.3
Porsche 914-2 26.0
Lotus Europa 30.4
Ford Pantera L 15.8
Ferrari Dino 19.7
Maserati Bora 15.0
Volvo 142E 21.4
This would work with several column names as well.
my_new_df <- function(input_1, input_2) {
col <- which(colnames(input_1)%in%input_2)
df_new <- input_1[,col, drop=FALSE]
return(df_new)
}
my_new_df(data.frame(mtcars), c("mpg", "cyl"))
mpg cyl
Mazda RX4 21.0 6
Mazda RX4 Wag 21.0 6
Datsun 710 22.8 4
Hornet 4 Drive 21.4 6
Hornet Sportabout 18.7 8
Valiant 18.1 6
Duster 360 14.3 8
Merc 240D 24.4 4
Merc 230 22.8 4
Merc 280 19.2 6
Merc 280C 17.8 6
Merc 450SE 16.4 8
Merc 450SL 17.3 8
Merc 450SLC 15.2 8
Cadillac Fleetwood 10.4 8
Lincoln Continental 10.4 8
Chrysler Imperial 14.7 8
Fiat 128 32.4 4
Honda Civic 30.4 4
Toyota Corolla 33.9 4
Toyota Corona 21.5 4
Dodge Challenger 15.5 8
AMC Javelin 15.2 8
Camaro Z28 13.3 8
Pontiac Firebird 19.2 8
Fiat X1-9 27.3 4
Porsche 914-2 26.0 4
Lotus Europa 30.4 4
Ford Pantera L 15.8 8
Ferrari Dino 19.7 6
Maserati Bora 15.0 8
Volvo 142E 21.4 4
T and F, use the literal constants TRUE and FALSE. The former can be overridden by user code.
df_new <- df[ input2 ][.data.frame(df, input2) : object 'input2' not foundnoquoteonly controls how a string is printed on the console, not how it’s treated in code. You may be confusing it with the concept of quasi-quotations, which deal with the computation of unevaluated expressions.