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    <p>Just to address the last part of your question, since that really points out the difference between a <code>list</code> and <code>vector</code> in R:</p> <blockquote> <p>Why do these two expressions not return the same result?</p> <p>x = list(1, 2, 3, 4); x2 = list(1:4)</p> </blockquote> <p>A list can contain any other class as each element. So you can have a list where the first element is a character vector, the second is a data frame, etc. In this case, you have created two different lists. <code>x</code> has four vectors, each of length 1. <code>x2</code> has 1 vector of length 4:</p> <pre><code>&gt; length(x[[1]]) [1] 1 &gt; length(x2[[1]]) [1] 4 </code></pre> <p>So these are completely different lists. </p> <p>R lists are very much like <a href="http://en.wikipedia.org/wiki/Hash_table" rel="noreferrer">a hash map</a> data structure in that each index value can be associated with any object. Here's a simple example of a list that contains 3 different classes (including a function):</p> <pre><code>&gt; complicated.list &lt;- list("a"=1:4, "b"=1:3, "c"=matrix(1:4, nrow=2), "d"=search) &gt; lapply(complicated.list, class) $a [1] "integer" $b [1] "integer" $c [1] "matrix" $d [1] "function" </code></pre> <p>Given that the last element is the search function, I can call it like so:</p> <pre><code>&gt; complicated.list[["d"]]() [1] ".GlobalEnv" ... </code></pre> <p>As a final comment on this: it should be noted that a <code>data.frame</code> is really a list (from the <code>data.frame</code> documentation):</p> <blockquote> <p>A data frame is a list of variables of the same number of rows with unique row names, given class ‘"data.frame"’</p> </blockquote> <p>That's why columns in a <code>data.frame</code> can have different data types, while columns in a matrix cannot. As an example, here I try to create a matrix with numbers and characters:</p> <pre><code>&gt; a &lt;- 1:4 &gt; class(a) [1] "integer" &gt; b &lt;- c("a","b","c","d") &gt; d &lt;- cbind(a, b) &gt; d a b [1,] "1" "a" [2,] "2" "b" [3,] "3" "c" [4,] "4" "d" &gt; class(d[,1]) [1] "character" </code></pre> <p>Note how I cannot change the data type in the first column to numeric because the second column has characters:</p> <pre><code>&gt; d[,1] &lt;- as.numeric(d[,1]) &gt; class(d[,1]) [1] "character" </code></pre>
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    1. COThis helps, thanks. (By the way, your example re 'complicated list', as you might already know, is the standard way to replicate the 'switch' statement in C++, Java, etc. in languages that don't have one; probably a good way to do this in R when i need to). +1
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    2. CORight, although there is a useful `switch` function in R that can be used for that purpose (see `help(switch)`).
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