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Deparse substitute assign with list elements

4 messages · soeren.vogel at posteo.ch, William Dunlap

Original
soeren.vogel at posteo.ch
# 
Hello,

When I use function `foo` with list elements (example 2), it defines a new object named `b[[1]]`, which is not what I want.  How can I change the function code to show the desired behaviour for all data structures passed to the function?  Or is there a more appropriate way to sort of "pass by references" in a function?

Thanks
S?ren

<src>
bar <- function(x) {
	return( x + 3 )
}

foo <- function(x, value) {
	nm <- deparse(substitute(x))
	tmp <- bar( value )
	assign(nm, tmp, parent.frame())
}

# 1)
a <- NA
foo(a, 4)
a # 7, fine :-)

# 2)
b <- list(NA, NA)
foo(b[[1]], 4) # the first list item should be 7
b # wanted 7 but still list with two NAs :-(
</src>
William Dunlap
# 
You could use a 'replacement function' named 'bar<-', whose last argument
is called 'value', and use bar(variable) <- newValue where you currently
use foo(variable, newValue).

bar <- function(x) {
    x + 3
}
`bar<-` <- function(x, value) {
    bar(value)
}

a <- NA
bar(a) <- 4
a
# [1] 7
b <- list(NA, NA)
bar(b[[1]]) <- 4
b
#[[1]]
#[1] 7
#
#[[2]]
#[1] NA


Bill Dunlap
TIBCO Software
wdunlap tibco.com
On Thu, May 14, 2015 at 11:28 AM, <soeren.vogel at posteo.ch> wrote:

            

      
      
      
    

        

      
      
    

        

  
  


  


      

    

    

      
      

        


  

        

      soeren.vogel at posteo.ch
    

    

      
      #
    

  


  

      
Thanks, Bill, I should have googled more carefully:

http://stackoverflow.com/questions/9561053/assign-values-to-a-list-element-in-r

So, remove

	assign(nm, tmp, parent.frame())

and add

	txt <- paste( nm, '<-', tmp, sep='' )
	eval( parse(text=txt), parent.frame() )

in `foo()` will do the trick.

Bests
S?ren

            

      
      
      
    

        

      
      
    

  
  


  


      

    

    

      
      

        


  

        

      William Dunlap
    

    

      
      #
    

  


  

      
txt <- paste( nm, '<-', tmp, sep='' )
        eval( parse(text=txt), parent.frame() )

        in `foo()` will do the trick.

Yuck.

If you use that sort of syntax your code becomes hard to understand and
you risk changing variables that users do not want changed.  When the
use uses something like bar(x)<-newValue she knows that 'bar' is going to
change.

Your new code will also fail if you try something like foo(b+10).


Bill Dunlap
TIBCO Software
wdunlap tibco.com

        
On Fri, May 15, 2015 at 1:50 AM, <soeren.vogel at posteo.ch> wrote: