[R] New to R

[Greg Snow]

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
> 
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