[R] Container/Collection Object to keep data structures and object

Rolf Turner r.turner at auckland.ac.nz
Wed Dec 2 03:18:31 CET 2009


On 2/12/2009, at 3:08 PM, Sharpie wrote:

>
>
> Amir Liu wrote:
>>
>> In l[3] <- matrix(1:4, 2, 2) :
>>   number of items to replace is not a multiple of replacement length
>>
>
> When inserting single entries into a list list, you should use the  
> '[[ ]]'
> notation.  Use '[ ]' when you want to extract multiple entries or copy
> multiple entries from one list to another:
>
>   l <- list()
>   l[[ 1 ]] <- 5
>   l[[ 2 ]] <- 'cd'
>   l[[ 3 ]] <- matrix( 1:4, 2, 2 )
>
>   l
>   [[1]]
>   [1] 5
>
>   [[2]]
>   [1] "cd"
>
>   [[3]]
>         [,1] [,2]
>   [1,]    1    3
>   [2,]    2    4
>
> Hope this helps!

The key issue is that l[3] is a ***list*** of length 1 whose single  
entry
is the object that was the third entry of ``l''.  But l[[3]] is  
***the object***
that was the third entry of ``l''.  It's like the difference a box  
containing
the third chocolate from a box of chocolates and that third chocolate.

	cheers,

		Rolf Turner

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