CODEWITHSUNDEEP
rfunctionstatistics
Borexino
Borexino

Reputation: 842

R function how to take the name of df

I'm really newbie with R. I've this function:

    Myfunction <- function (Amean, Asd, An, Bmean, Bsd, Bn, NameOfMYdf)
 {
    NameOfMYdf$Difference <- (Amean - Bmean)
    NameOfMYdf$SDP <- sqrt((((An - 1) * Asd^2 + (Bn - 1) * Bsd^2) / (An + Bn - 2)))

}

when i call this function I'd like to input my variables name from my dataset. with this function i wanna create in the same dataset 2 new variables:
NameOfMYdf$Difference NameOfMYdf$SDP

I think it's very easy to to but i cannot figure out

thanks folks

Is there no way to input my variables name and compute?... so i wanna input

Myfunction (meanGroupA, sdGroupA, nGroupA, meanGroupA, sdGroupB, nGroupB, NameOfMyDataset)

Basically, I wanna pass the dataset name with the function

thanks

Upvotes: 0

Views: 66

Answers (2)

Gregor Thomas
Gregor Thomas

Reputation: 146040

Your question is coming at R from a typical object-oriented perspective, where you have a function/method that modifies an object. (It looks like you want MyFunction to add columns to whatever data.frame you give it.)

R is a functional programming language which means it tends to not do this. There are ways to make it happen, but they're difficult to use well and are generally considered bad practice.

Let's do a quick example in an R-like way:

# sample data
mydata <- data.frame(a = rnorm(10), b = runif(10))

Then let's say there's a function of two columns that you want to do a lot

common_task <- function(x, y) {
    ((x - 1) * y + (y - 1) * x) / (x + y - 2)
}

The easiest/most common way to add this to your data.frame is

mydata$calc <- common_task(x = mydata$a, y = mydata$b)

If you want to use variable names, then strings work well. If your task will always be performed on a data.frame with columns named a and b, then you can right a function assuming the data.frame has those column names:

common_task2 <- function(data) {
    ((data$a - 1) * data$b + (data$b - 1) * data$a) /
        (data$a + data$b - 2)
}

A better way is to let the columns names be input as strings, but for this the $ subset shortcut won't work, we need to use [.

common_task3 <- function(data, x = "a", y = "b") {
      ((data[, x] - 1) * data[, y] + (data[, y] - 1) * data[, x]) / (data[, x] + data[, y] - 2)
}

This last function will assume the column names you want to work on are "a" and "b", unless you tell it otherwise.

However, in all three cases, the function just returns a new column. To get it in your data.frame outside of the function, you need to assign it, i.e.,

mydata$new_col3 <- common_task3(data = mydata)
mydata$new_col2 <- common_task2(data = mydata)

You could assign the columns inside the function, but you'll still need to assign the results to a data.frame, it won't just modify the data.frame outside of your function:

common_task4 <- function(data, x = "a", y = "b") {
    data$result <-((data[, x] - 1) * data[, y] + (data[, y] - 1) * data[, x]) /
        (data[, x] + data[, y] - 2)
    return(data)
}

my_modified_data <- common_task4(data = mydata)

In all of these cases, there are nice functions that can do this for you. @Jilber's answer recommends transform, which is a good one. The dplyr library is also very nice and easy to use. You can write your own versions, but the existing ones will usually be faster and more robust.

For lots more detail and examples, see Advanced R Programming: Functions.

Upvotes: 1

Jilber Urbina
Jilber Urbina

Reputation: 61214

I'd use transform to make what you want, take a look at this simple example:

> df <- head(mtcars)[, 1:3]
> transform(df,                   # a data.frame to be transformed
            Difference=mpg-cyl,   # first transformation
            DSP=disp^2)           # replace with sqrt((((An - 1) * Asd^2 + (Bn - 1) * Bsd^2) / (An + Bn - 2)))
                   mpg cyl disp Difference    DSP
Mazda RX4         21.0   6  160       15.0  25600
Mazda RX4 Wag     21.0   6  160       15.0  25600
Datsun 710        22.8   4  108       18.8  11664
Hornet 4 Drive    21.4   6  258       15.4  66564
Hornet Sportabout 18.7   8  360       10.7 129600
Valiant           18.1   6  225       12.1  50625

Upvotes: 1

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