Subsetting based on multiple function arguments

Question

I'm new to function writing so hopefully the below makes some sense.

I want to create a function which takes some arguments, which will be used to subset a data.frame. I have searched across the forums and found these Q&As interesting, but haven't been able to answer my question from the discussions:

The function I want to create will take a df, a column name, and a value to match in the rows of the column name. Here's my attempt which I can see to be wrong:

x <- data.frame("col1"=c("email","search","direct"),
            "col2"=c("direct","email","direct"),
            "col3"=c(10,15,27))

fun <- function(df,col,val) {
  result <- subset(df, col==val)
  return(result)
}

I want to pass in the df, x. A column name, let's say "col2". A value, let's say "email". My attempt to do so returns a 0-length df.

fun(x,"col2","email")

Clearly I'm doing something wrong... can anyone help?

you should have a read at this post to learn a bit more about issues with using subset inside a function. — Arun
– Arun, Commented Jun 10, 2013 at 11:32
I notice you do not use a lot of spaces in your code, e.g. function(df,col,etc) -> function(df, col, etc) or col==val -> col == val. Adding spaces makes your code easier to read, less intimidating. — Paul Hiemstra
– Paul Hiemstra, Commented Jun 10, 2013 at 11:34

Paul Hiemstra · Accepted Answer · 2013-06-10 12:05:53Z

3

You would want to do somehting like :

df[df[[col_name]] == value,]

the function then becomes:

fun <- function(df, col_name, value) {
  df[df[[col_name]] == value,]
}
fun(x, 'col2', 'email')
    col1  col2 col3
2 search email   15

and if you want to take into account NA values in the logical vector:

fun <- function(df, col_name, value) {
  logical_vector = df[[col_name]] == value
  logical_vector[is.na(logical_vector)] = FALSE
  df[logical_vector, drop = FALSE]
}

Why your example not works is because subset does not look inside the value of col. In stead, it will look for a column called col. I suspect the val parameter is also not correctly parsed. This is one of the reasons not to use subset in non-interactive mode, i.e. in anything else than an interactive R console.

edited Jun 10, 2013 at 12:05

answered Jun 10, 2013 at 11:30

Paul Hiemstra

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4 Comments

Jonathan Mulligan Over a year ago

Great - that makes total sense. Thanks for such a quick response Paul.

Arun Over a year ago

you might want to take care of NAs if you want to mimic subset using [.data.frame. subset drops those row indices for which the logical expression evaluates to NA.

hadley Over a year ago

And you should add drop = FALSE in case the user passes in a 1 col df.

Paul Hiemstra Over a year ago

@hadley I also added that improvement.

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