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I have been trying to create a function which will permanently change a value in specific cells of my data frame. I insert the data frame name, row index I wish to change, and the new name as a string. However, the function seems to change the value name within the local environment but not global.

The function is as follows:

#change name function
name_change <- function(df, row, name) {
  df[row, 1] = name
  return(df[row, 1])
}

E.g. if data frame was:

Name Column B
Mark 2
Beth 4

The function name_change(df, 2, 'Jess') would change Beth to Jess.

When inserted as raw code it does permanently change the value. But then does not work when used as a function.

df[2, 1] = 'Jess'

Thanks in advance for your time

Luke Jenner
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3 Answers3

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If you change your function like this:

name_change <- function(df, row, name) {
  df[row, 1] = name
  return(df)
}

and then assign the result of the function back to the original df, you will get the change you are looking for:

df = name_change(df,2,'Jess')
langtang
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    why? because R passed by value https://stackoverflow.com/questions/29831261/r-pass-by-value-inside-a-function#29831414 https://stackoverflow.com/questions/2603184/can-you-pass-by-reference-in-r#2603318 – qwr Mar 25 '22 at 17:15
  • I would rather say bcs R is a mess and it will take a variable from a global scope and instead of updating it (as it usually does in these cases) it will create a copy of the variable in a current scope. It IS indeed confusing. – Max Yari Aug 29 '23 at 14:40
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An alternative to the solutions already provided is to use the superassignment operator <<-. The ordinary assignment <- (or '=' you used) operate in your function's environment only. The superassignment reaches beyond your function's closure and can thus modify the dataframe residing in the global environment. Note, though, this is a quick'n'dirty fix only.

That said, the code would read like this:

#change name function
dirty_name_change <- function(df, row, name) {
  df[row, 1] <<- name ## note the double arrow
}
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You are returning the value of the cell, not the mutated df. R passes by arguments by value so you can imagine the function modifying a copy of df passed in. The solution is to return the mutated df and reassign it.

Can you pass-by-reference in R?

qwr
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