Re: Question about assigning & scope

[Jordão, Felipe <Felipe.Jordao@moodys.com>]

Thanks Tony & Thierry, you have given me enough info to proceed with what I 
wanted to do.

Regards,
Felipe



-----Original Message-----
From: Tony Plate [mailto:tplate@blackmesacapital.com] 
Sent: Thursday, 18 August, 2005 11:46
To: Jordão, Felipe
Cc: s-news@lists.biostat.wustl.edu
Subject: Re: [S] Question about assigning & scope

Modifying global variables from within functions is not considered good 
S-programming style and is generally discouraged, for good reasons.

The standard S way to do something like this is to have the function 
return the modified data frame:

my.function <- function(df) {
     df$Income <- new.values.from.some.calculation
     return(df)
}

then:

 > my.data.frame <- my.function(my.data.frame)

Another way would be to use a script file and not a function

file "my.script.q" contains one line:
my.dataframe$Income <- new.values.from.some.calculation

then:

 > source("my.script.q")

Finally, if you really must have a function modify a global variable, 
you generally need to make a local copy of it first:

my.function <- function() {
     df <- my.data.frame
     df$Income <- new.values.from.some.calculation
     assign("my.data.frame", df, where=1)
}

hope this helps,

Tony Plate

Jordão wrote:
> Hi all,
> 
>  
> 
> I'm having a bit of difficulty understanding this scope business. I've 
> tried going through the help and googling to no avail. It is probably 
> very simple for you S veterans out there.
> 
>  
> 
> What I would like to do is modify a column in a global data frame from 
> within a function. For example, the current code I have looks like this 
> and works as I want it to:
> 
>  
> 
> my.dataframe <- data.frame(state.x77, row.names=state.abb)
> 
> my.dataframe$Income <- new.values.from.some.calculation
> 
>  
> 
> I want to change it so it's like this:
> 
>  
> 
> my.function <- function() {
> 
>        my.dataframe$Income <- new.values.from.some.calculation
> 
> }
> 
>  
> 
> When I call my.function, I get a message saying that I need to assign 
> my.dataframe locally before replacement. I tried using code like this
> 
>  
> 
> my.function <- function() {
> 
>        assign("my.dataframe[,\"Income\"]", new.values.from.some.calculation)
> 
> }
> 
>  
> 
> but it also didn't work. I don't get an error message, but the Income 
> column is not changed. The data I'm working with is big, so it's not 
> efficient to pass it into the function, change it, then return it.
> 
>  
> 
> thanks,
> Felipe
> 
>  
> 
> 
> 
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