R help - Oct 2007 - Dice simulation: Getting rep to re-evaluate sample()?

I'm trying to get R to simulate the sum of the values on 10 fair dice
(yes, it's related to a homework problem, but is not the problem
itself). I tried to do this:
> rep(sum(sample(1:6,100,replace=T)), times=10)
 [1] 341 341 341 341 341 341 341 341 341 341
 
and noticed that sum(sample()) seems to be only evaluated once. How can
I overcome this, so that I get a vector of values that correspond to
independent throws of 10 dice each time?

Thanks for your advice and suggestions.

-Kevin

Kevin Zembower
Internet Services Group manager
Center for Communication Programs
Bloomberg School of Public Health
Johns Hopkins University
111 Market Place, Suite 310
Baltimore, Maryland  21202
410-659-6139

See

 	?replicate

which I think is what you are after.

Chuck


On Mon, 8 Oct 2007, Zembower, Kevin wrote:
> I'm trying to get R to simulate the sum of the values on 10 fair dice
> (yes, it's related to a homework problem, but is not the problem
> itself). I tried to do this:
>> rep(sum(sample(1:6,100,replace=T)), times=10)
> [1] 341 341 341 341 341 341 341 341 341 341
>
> and noticed that sum(sample()) seems to be only evaluated once. How can
> I overcome this, so that I get a vector of values that correspond to
> independent throws of 10 dice each time?
>
> Thanks for your advice and suggestions.
>
> -Kevin
>
> Kevin Zembower
> Internet Services Group manager
> Center for Communication Programs
> Bloomberg School of Public Health
> Johns Hopkins University
> 111 Market Place, Suite 310
> Baltimore, Maryland  21202
> 410-659-6139
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901

See ?replicate, e.g.

replicate(sum(sample(1:6,100,replace=TRUE)), n=10)

Function arguments are (in gneral) evaluated as if they were evaluated 
when passed to the function.

On Mon, 8 Oct 2007, Zembower, Kevin wrote:
> I'm trying to get R to simulate the sum of the values on 10 fair dice
> (yes, it's related to a homework problem, but is not the problem
> itself). I tried to do this:
>> rep(sum(sample(1:6,100,replace=T)), times=10)
> [1] 341 341 341 341 341 341 341 341 341 341
>
> and noticed that sum(sample()) seems to be only evaluated once. How can
> I overcome this, so that I get a vector of values that correspond to
> independent throws of 10 dice each time?
> Kevin Zembower
> Internet Services Group manager
> Center for Communication Programs
> Bloomberg School of Public Health
> Johns Hopkins University
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

rep(
    sum(sample(1:6,100,replace=T)),
    times=10)

Read carefully what you wrote.  You asked R to sum a single sample, then
make
ten copies of the sum.

You need to take ten samples, then sum each.   apply() would be helpful.

-----Original Message-----
I'm trying to get R to simulate the sum of the values on 10 fair dice
(yes, it's related to a homework problem, but is not the problem
itself). I tried to do this:
> rep(sum(sample(1:6,100,replace=T)), times=10)
 [1] 341 341 341 341 341 341 341 341 341 341

Kevin Zembower wrote:
>
> I'm trying to get R to simulate the sum of the values on 10 fair dice
> (yes, it's related to a homework problem, but is not the problem
> itself). I tried to do this:
> > rep(sum(sample(1:6,100,replace=T)), times=10)
>  [1] 341 341 341 341 341 341 341 341 341 341
> 
rep(stuff, times=10) will just take stuff and repeat 10 times.
stuff is constant (but every time you repeat that line you
get a different stuff to be repeated), so you get a vector
of 10 equal numbers.
> I overcome this, so that I get a vector of values that correspond to
> independent throws of 10 dice each time?
> 
# get 10*100 random d6s
x <- sample(1:6, 10*100, replace=T) 

# transform into a 10 x 100 matrix
y <- matrix(x, 10, 100)

# sum the cols
z <- colSums(y)

Of course, you can combine these three lines into one.

Alberto Monteiro

Well, if the dice are fair, then all rolls are independent.  If you want to roll
each dice 100 times, you can do something like

rolls=matrix(sample(1:6,1000,replace=T),ncol=10)

apply(rolls,2,sum)

I'm sure that there must be a more elegant way to do it, though.

Julian


On Mon, 8 Oct 2007, Zembower, Kevin wrote:
> I'm trying to get R to simulate the sum of the values on 10 fair dice
> (yes, it's related to a homework problem, but is not the problem
> itself). I tried to do this:
>> rep(sum(sample(1:6,100,replace=T)), times=10)
> [1] 341 341 341 341 341 341 341 341 341 341
>
> and noticed that sum(sample()) seems to be only evaluated once. How can
> I overcome this, so that I get a vector of values that correspond to
> independent throws of 10 dice each time?
>
> Thanks for your advice and suggestions.
>
> -Kevin
>
> Kevin Zembower
> Internet Services Group manager
> Center for Communication Programs
> Bloomberg School of Public Health
> Johns Hopkins University
> 111 Market Place, Suite 310
> Baltimore, Maryland  21202
> 410-659-6139
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>