New to R

It is not transposing (it just looks that way).  The first result is a vector which is 1 dimensional, but is neither a row or a column.  The printed version of it looks like a row, because that is a more compact representation.  If you sample enough points you will see it wrap around and be represented as several rows.  If it printed as a single column, then the first values would scroll off the screen with only a moderate number of values.

The replicate function then takes these vectors and combines them into a matrix and just happens to use each vector as a column of the new matrix, this is standard, matrices by default are filled by column, look at the output of as.matrix( sample( 6, 4, replace=TRUE ) ) and you will see your vector converted to a matrix of 1 column.  It could have been done the other way, but way back the decision was made to do it this way and there are probably a lot of things that would break if it were changed now, so we get to live with it.  A single call to 't' is not too much effort to get what we expect.

So in short, a vector is neither a column or a row, but prints as a row for practical reasons, and is converted to a column by default if made into a matrix.

Hope this helps,


--
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
project.org] On Behalf Of Joe Hughes
Sent: Monday, February 02, 2009 1:09 PM
To: R help
Subject: Re: [R] New to R

All,

      Thanks for taking the time to reply.  I understand a bit more
about R
and the R way then I did before.    The final function looks like this:

#######################################################################
#######
#
# Input:
#   die_size - 4, 6, 8, 10, 20
#   number_of_dice - How many dice to roll
#   number_of_rolls - How many times to roll the dice
#
# Output:
#    The array holding the results of the rolls
#
#######################################################################
#######
#
function(die_size, number_of_dice, number_of_rolls=1)
{
      return(t(replicate(number_of_rolls, sample(die_size,
number_of_dice,
replace=TRUE))))
}

Before I take a look at the teaching demos, I have one question left.

Here is a sequence of commands and the output

 > sample(6, 4, replace=TRUE)

[1] 3  4  5  4

 > replicate(7, sample(6, 4, replace=TRUE))

      [,1] [,2] [,3] [,4] [,5] [,6] [,7]
[1,]    3    3    6    4    5    6    6
[2,]    4    4    6    5    5    1    6
[3,]    5    1    4    5    6    5    6
[4,]    4    6    3    1    1    2    2

Why does replicate transpose the vector before assigning it to the
array?  The way I would output it would be this

      [,1] [,2] [,3] [,4]
[1,]    3    4    5    4
[2,]    3    4    1    6
[3,]    6    6    4    3
[4,]    4    5    5    1
[5,]    5    5    6    1
[6,]    6    1    5    2
[7,]    6    6    6    2

Thanks,
Joe