Why add column data to data frame not working with a function in r

Question

I am curious that why the following code doesn't work for adding column data to a data frame.

a <- c(1:3)
b <- c(4:6)
df <- data.frame(a,b)  # create a data frame example

add <- function(df, vector){
    df[[3]] <- vector
}                     # create a function to add column data to a data frame

d <- c(7:9)           # a new vector to be added to the data frame
add(df,d)             # execute the function

If you run the code in R, the new vector doesn't add to the data frame and no error also.

You need to return df on the line following df[[3]] <- vector then assign df <- add(df, d). It's best to never modify objects in different environments from inside a function. — Rich Scriven
– Rich Scriven, Commented Jul 1, 2017 at 1:56
Why does the df needs to be returned? And the function also doesn't modify the data frame object. — Jian
– Jian, Commented Jul 1, 2017 at 1:58

R Yoda · Accepted Answer · 2018-02-12 20:21:41Z

1

R passes parameters to functions by value - not by reference - that means inside the function you work on a copy of the data.frame df and when returning from the function the modified data.frame "dies" and the original data.frame outside the function is still unchanged.

This is why @RichScriven proposed to store the return value of your function in the data.frame df again.

Credits go to @RichScriven please...

PS: You should use cbind ("column bind") to extend your data.frame independently of how many columns already exist and ensure unique column names:

add <- function(df, vector){
    res <- cbind(df, vector)
    names(res) <- make.names(names(res), unique = T)
    res   # return value
}

PS2: You could use a data.table instead of a data.frame which is passed by reference (not by value).

edited Feb 12, 2018 at 20:21

answered Jul 1, 2017 at 8:21

R Yoda

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1 Comment

Jian Over a year ago

Thanks for the detailed explanation!

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