[R] get element of list with default?

[Marc Schwartz]

On Apr 15, 2014, at 11:22 AM, Marc Schwartz <marc_schwartz at me.com> wrote:

> On Apr 15, 2014, at 10:53 AM, Spencer Graves <spencer.graves at structuremonitoring.com> wrote:
> 
>> Hello:
>> 
>> 
>>     Do you know of a simple function to return the value of a named element of a list if that exists, and return a default value otherwise?
>> 
>> 
>>     It's an easy function to write (e.g., below).  I plan to add this to the "Ecfun" package unless I find it in another CRAN package.
>> 
>> 
>>     Thanks,
>>     Spencer
>> 
>> 
>>   getElement <- function(element, default, list){
>> #       get element of list;  return elDefault if absent
>>       El <- list[[element]]
>>       if(is.null(El)){
>>           El <- default
>>       }
>>       El
>>   }
> 
> 
> Hi Spencer,
> 
> I don't know of a function elsewhere, but you can probably simplify the above with:
> 
>  getElement <- function(element, default, list) {
>    ifelse(is.null(list[[element]]), default, list[[element]])
>  }
> 
> 
> MyList <- list(L1 = 1, L2 = 2) 
> 
>> MyList
> $L1
> [1] 1
> 
> $L2
> [1] 2
> 
> 
> 
>> getElement("L1", 5, MyList) 
> [1] 1
> 
>> getElement("L2", 5, MyList) 
> [1] 2
> 
>> getElement("L3", 5, MyList) 
> [1] 5
> 
> 
> You might want to think about the ordering of the function arguments, given typical use, for ease of calling it. For example:
> 
>  getElement <- function(list, element, default = SomeValue)
> 
> Another consideration is that the above function will only get the element if it is a 'first level' element in the list. If it is in a sub-list of the main list, you would need to think about a recursive approach of some type, along the lines of what ?rapply does.
> 
> Regards,
> 
> Marc Schwartz


Spencer,

A quick heads up here. I forgot, that there is already a function called getElement() in base R which appears to be designed to handle S4 objects and slots, lacking the default return value however, where it returns NULL if the 'name' element is not present:

> getElement
function (object, name) 
{
    if (isS4(object)) 
        slot(object, name)
    else object[[name, exact = TRUE]]
}
<bytecode: 0x100905870>
<environment: namespace:base>


Thus, I would suggest calling your variant something else, or wrap the default function in your version, if you need/want to handle S4 objects and slots.

Regards,

Marc