On Jul 5, 2009, at 2:43 PM, Martin Morgan wrote:
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)
Or even:
result <- subset(df, V1 %in% A)
... possibly needing some ordering.
David Winsemius, MD
Heritage Laboratories
West Hartford, CT
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