How to specify the operator in my function

Question

Here is my dataset

mydata<-data.frame(
  id=1:20,
  sex=sample(c(rep("M",6),rep("F",14))),
  Age=round(rnorm(20, 30,2)),
  Weight=round(rnorm(20, 65,5),2)
)

I want my function to allow me to specify on which variable I want to do the filtering but also the criterion, i.e. the operator (== or > or <=...) and the value (M or 65...)

This is the function I am trying to create. I know in advance that it won't work, it's to give an idea of what I want to create.

If I don't put the variable, value and operator of selection my function must return the original database otherwise the filtered database

    my_func<-function(select_var, select_crit){
      
      mydata<-mydata<-if(is.null(select_var)&is.null(select_crit)){mydata}else{
        mydata[ which(mydata[select_var]select_crit), ]
      }
return(mydata)
    }

For example I want to be able to select all the male with my function like this

my_func(select_var="sex",select_crit="M"),

And all the induvidual > 30 (in age) like this:

my_func(select_var="Age",select_crit=">30")

or to select with the operator %in%

my_func(select_var="Age",select_crit=%in%c(30:40))

outside a function it should be `mydata[ which(mydata["age"]%in%c(30:40)), ]` but I've just realised that even outside the function it doesn't work. The goal is to select a range of age — Seydou GORO, Oct 27 '22 at 15:29
I think it would be far simpler if you used three arguments: `function(sel_var, sel_fun, sel_val)`, where one could eventually do something like `do.call(sel_fun, list(mydata[[sel_var]], sel_val))`. FYI, `mydata[select_var]` should really be `mydata[[select_var]]` in your code. — r2evans, Oct 27 '22 at 15:41
Seems to me that you are hardcoding the data inside the function and this is not quite right. You should consider passing it as a variable. and if you do so `subset` function will be your friend. You do not need to write another function to do exactly what subset does — Onyambu, Oct 27 '22 at 16:01

Jilber Urbina · Answer 1 · 2022-10-27T15:44:54.143

1

You have to add a data argument inside your function and apply a combination of eval, parse and paste0 for building your filter (row selection) criterion. This approach will help:

my_func <- function(data, select_var=NULL, select_crit=NULL){
  
  if(is.null(select_var) & is.null(select_crit)){
    output <- data
  } else {
    output <- data[eval(parse(text=paste0("data", "$",select_var, select_crit))), select_var, drop=FALSE]
  }
  
  return(output)
}

Examples:

> my_func(mydata, select_var="Age", select_crit=">30")
   Age
1   32
5   32
7   33
8   31
9   33
13  31
16  33
18  32
19  32
> my_func(mydata, select_var="Age",select_crit="%in%c(30:40)")
   Age
1   32
2   30
5   32
7   33
8   31
9   33
11  30
13  31
14  30
16  33
17  30
18  32
19  32

Calling my_func(data) with select_var and select_crit with defult NULL will return your original dataset.

edited Oct 27 '22 at 15:44

answered Oct 27 '22 at 15:32

Jilber Urbina

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Why write a function like this yet there is `subset` ie `subset(mydata, Age > 30)` for example works great. Dont you think? – Onyambu Oct 27 '22 at 16:00
3

@onyambu, I thought the same ... I think that question should be for the OP. – r2evans Oct 27 '22 at 16:01
Your point is well taken. But filtering the database is just one part of a longer process that needs to be done in the function. – Seydou GORO Oct 27 '22 at 17:44

score 1 · Answer 2 · answered Oct 27 '22 at 16:09

Three suggestions:

Make the data an argument of the function and not accessed via scope-breach. This helped with reproducibility, troubleshooting, maintenance, etc, and as a side-effect will allow your function to operate in %>%- and |>-pipes (if so desired).
Use &&, "never" use single-& in if-conditionals unless it is wrapped in an aggregating function such as any or all. The differences between & and && are more than just vectorized-vs-nonvectorized, see Boolean operators && and ||. Further, I think you mean to use "OR" here instead of "AND", since if either one of them is null then you should not be attempting to use the operator.
Change from 2-args to 3-args, separating the operator from the second operand.

Try this:

fun <- function(mydata, sel_var, sel_op, sel_val = NULL) {
  if (is.null(sel_var) || is.null(sel_op)) return(mydata)
  if (is.character(sel_op)) sel_op <- match.fun(sel_op)
  mydata[do.call(sel_op, c(list(mydata[[sel_var]]), if (!is.null(sel_val)) list(sel_val))),]
}

fun(mtcars, "cyl", "<", 5)
fun(mtcars, "cyl", "%in%", c(4, 8))
fun(mtcars, "vs", "!")

Notes:

sel_op can be a function or a string representing one. This gives a lot more flexibility, such as the ability to do
```
fun(mtcars, "vs", Negate("!"))
fun(mtcars, "vs", function(z) !!z)
```
the c(list(..), list(if (!is.null(sel_val)) ...)) is meant to allow sel_val to be empty/NULL for unary functions.

How to specify the operator in my function

2 Answers2