[R] allocation/initialization of arrays/lists

[Martin Morgan]

Hi ZRL,

zrl wrote:
> Dear list,
> 
> I have a question regarding how to allocate or initialize the proper type of
> the arrays or lists.
> 
> I have a for loop, each iteration, I want it to assign a subset of a data
> frame to an element of an array or list.
> However, I am wondering how I should initialize or allocate the empty array
> or list so that it has the same number of the elements as the subsets I am
> going to assign and also make the data type is compatible to the subset I am
> going to assign to it.
> It should look like this:
> 
> #initialize or allocate.   ( I don't know how to)
> { some codes: initialize an array or list (which one is better?) with "n"
> elements}
> 
> #"for" loop for assigning a subset of a large data frame to the elements of
> the array or list (which on is better?)
> for (i in 1: n){
> array[i] <- df[df$V1==A[i],]
> }

You could let the software figure it out for you

   result = sapply(1:n, function(i, df, A) {
       df[df$V1==A[i],]
   }, df=df, A=A)

or better (because you don't have to ensure that n is the length of A)
if A is a vector

   result = sapply(A, function(a, df) {
       df[df$V1 == a, ]
   }, df=df)

Martin

> 
> 
> 
> 
> Thanks.
> 
> ZRL
> 
> 	[[alternative HTML version deleted]]
> 
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