[R] Is there an variant of apply() that does not return anything?

Marc Schwartz marc_schwartz at me.com
Thu Nov 19 23:51:31 CET 2009


On Nov 19, 2009, at 4:31 PM, Peng Yu wrote:

> On Thu, Nov 19, 2009 at 4:27 PM, Marc Schwartz  
> <marc_schwartz at me.com> wrote:
>> On Nov 20, 2009, at 10:21 AM, Peng Yu wrote:
>>
>>> There are a few version of apply() (e.g., lapply(), sapply()). I'm
>>> wondering if there is one that does not return anything but just
>>> silently apply a function to the list argument.
>>>
>>> For example, the plot function is applied to each element in  
>>> 'alist'.
>>> It is redundant to return anything from apply.
>>>
>>> apply(alist,function(x){ plot each element of alist})
>>
>>
>>
>> Just use a for() loop. If you are plotting things, the performance
>> bottleneck is not going to be in the loop.
>>
>> Sometimes, we get too anal about avoiding for() loops.
>
> Is there a way to get the name of the list in the loop body?
>
>> List=list(a='c',b='x',e='q')
>> for(x in List) { print(x) }
> [1] "c"
> [1] "x"
> [1] "q"


Here is one approach to give you some insight into how to get the name  
(as opposed to the object itself) of an argument passed to a function:

MyList <- list(a = 1, b = 2, e = 3)

PlotFn <- function(x)
{
   # Get the name of the object passed as 'x' (eg. "MyList")
   Main <- deparse(substitute(x))

   for (i in seq(along = x))
   {
     plot(x[[i]], main = Main, ylab = names(x[i]))
   }
}

# Set to pause between each graphic
par(ask = TRUE)

PlotFn(MyList)

Take note that the main title for each graphic will be "MyList" and  
the y axis label for each, will be the name of each of the three  
components in MyList (eg. "a", "b" and "e")

HTH,

Marc Schwartz



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