<!doctype html public "-//W3C//DTD W3 HTML//EN">
<html><head><style type="text/css"><!--
blockquote, dl, ul, ol, li { padding-top: 0 ; padding-bottom: 0 }
--></style><title>list indexing</title></head><body>
<div>Hello</div>
<div><br></div>
<div>I've got a question about list indexing. If anyone can help
or point in the direction of some documentation, that'd be great. I've
checked the Introduction to R and R language definition.</div>
<div><br></div>
<div>Say you have a list of vectors:</div>
<div><br></div>
<div><tt><font color="#000000">></font><font color="#0000FF">
x_list(c("a","b","c"),c("a",<span
></span
>"b","c","d"),c("a","b<span
></span
>"),c("a","b","c","d",<span
></span>"e"))<br>
</font><font color="#000000">></font><font color="#0000FF"> x<br>
</font><font color="#000000">[[1]]<br>
[1] "a" "b" "c"<br>
<br>
[[2]]<br>
[1] "a" "b" "c" "d"<br>
<br>
[[3]]<br>
[1] "a" "b"<br>
<br>
[[4]]</font></tt></div>
<div><tt><font color="#000000">[1] "a" "b"
"c" "d" "e"</font></tt><br>
<tt><font color="#000000"></font></tt></div>
<div>Indexing single list components is fine using [[i]] and elements
within using [[i]][j] but what if you want to get some information,
for instance vector length or attributes of the components, across (or
should that be down) the top level components. You can do it by
looping across 1:length(list):</div>
<div><br></div>
<div><tt><font color="#000000">></font><font color="#0000FF"> store
<- numeric(0)<br>
</font><font color="#000000">></font><font color="#0000FF">
for(components in 1:length(x)) store <-
c(store,length(x[[components]]))<br>
</font><font color="#000000">></font><font color="#0000FF">
store</font></tt></div>
<div><tt><font color="#000000">[1] 3 4 2 5</font></tt><br>
<tt><font color="#000000"></font></tt></div>
<div>I know it's only two lines of code. I guess I'm wondering
whether there is something that does what I'd expect from
length(x[[1:4]]) if [[i]] allowed multiple indices.</div>
<div><br></div>
<div>Thanks,</div>
<div>David Orme.</div>
</body>
</html>