[R] Updating DataFrame with Function

[Ben Bolker]

BostonR wrote:
> 
> This should be very simple but I am wrestling with updating a data frame
> from within a function.  Here is a simple example:
> ### SET UP DATA FRAME
>> tFrame <- data.frame(T=c(1:5))
>> 
>> tFrame
>   T
> 1 1
> 2 2
> 3 3
> 4 4
> 5 5
>> 
> ##### Simple function that updates with 1000
>> tUpdate<- function (x){
> + tFrame$T[x] <- 1000
> + }
>> tUpdate(5)
>> tFrame
> ############# Update within function fails
>   T
> 1 1
> 2 2
> 3 3
> 4 4
> 5 5
>> 
> ######## Update dataframe outside function with 100
>> tFrame[5,1] <- 100
>> tFrame
>     T
> 1   1
> 2   2
> 3   3
> 4   4
> 5 100
>> 
> 
> 

 This is a fundamental issue with variable scope in functions.

  When you modify a variable within a function it makes a local
copy.  See section 10.5 of the introduction to R.
You either need to the function to return the changed value

 tUpdate<- function (x){
   tFrame$T[x] <- 1000
  tFrame
}

tFrame <- tUpdate(5)

  or use the <<- assignment operator.

 

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