R help - Jul 2011 - calculating the mean of a random matrix (by row) and some general questions

Hi everyone!

I'm trying to teach myself R in order to do some data analysis. I'm a
mathematics student and (only) familiar with matlab and latex. I'm working
trough the "official" introduction to R at the moment, while
simultaneously
solving some exercises I found in the web. Before I post my (probably
stupid) question, I'd like to ask you for some general advice. How do you
work with R? Is it like in matlab, that you write your functions with a lot
of loops etc. in a textfile and then run it? Or do you just prepare your
data and then use the functions provided by R (plot, mean etc) to get some
analysis? I'd be very thankfull for some of your thoughts about
"approaches".

Now the question: I'm trying to build a vector with n entries, each
consisting of the mean of m random numbers (exponential distributed for
example). My approach was to construct a nxm random matrix and then to
somehow take the mean of each row. But in the mean function there is no
parameter to do this, so the intended approach of R is probably different..
any ideas? =)

Richard

--
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> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of RichardLang
> Sent: Tuesday, July 19, 2011 11:44 AM
> To: r-help at r-project.org
> Subject: [R] calculating the mean of a random matrix (by row) and some
> general questions
> 
> Hi everyone!
> 
> I'm trying to teach myself R in order to do some data analysis. I'm
a
> mathematics student and (only) familiar with matlab and latex. I'm
> working
> trough the "official" introduction to R at the moment, while
> simultaneously
> solving some exercises I found in the web. Before I post my (probably
> stupid) question, I'd like to ask you for some general advice. How do
> you
> work with R? Is it like in matlab, that you write your functions with a
> lot
> of loops etc. in a textfile and then run it? Or do you just prepare
> your
> data and then use the functions provided by R (plot, mean etc) to get
> some
> analysis? I'd be very thankfull for some of your thoughts about
> "approaches".
> 
> Now the question: I'm trying to build a vector with n entries, each
> consisting of the mean of m random numbers (exponential distributed for
> example). My approach was to construct a nxm random matrix and then to
> somehow take the mean of each row. But in the mean function there is no
> parameter to do this, so the intended approach of R is probably
> different..
> any ideas? =)
> 
> Richard
> 
Richard,

If you have a matrix, M, with n rows and m columns, you can use the apply()
function to get either row or column means
> n <- 10 
> m <-3
> M <- matrix(rnorm(m*n),n,m)
> M
            [,1]        [,2]       [,3]
 [1,]  0.6239267 -0.70546496  0.3682918
 [2,] -0.7326689 -1.86571052 -0.2899552
 [3,]  0.7778313 -1.01227191  0.7735718
 [4,]  0.8336683 -0.07755214 -0.1375798
 [5,] -1.6134414  0.12088648 -0.4064939
 [6,] -0.2578007  0.45142456 -1.0197297
 [7,]  1.0108260 -0.24933408 -0.4083304
 [8,] -0.7936603 -0.67286769 -0.8666802
 [9,]  1.0054039  2.52498995  1.0915742
[10,] -0.1610073  0.43504924  2.4288474
> rowMeans <- apply(M,1,mean)
> rowMeans
 [1]  0.09558452 -0.96277820  0.17971042  0.20617876 -0.63301628 -0.27536860
 [7]  0.11772050 -0.77773605  1.54065601  0.90096312
> colMeans <- apply(M,2,mean)
> colMeans
[1]  0.06930777 -0.10508511  0.15335160
>
I will let others describe how they use R.



Hope this is helpful,

Dan

Daniel J. Nordlund
Washington State Department of Social and Health Services
Planning, Performance, and Accountability
Research and Data Analysis Division
Olympia, WA 98504-5204

On Jul 19, 2011, at 2:43 PM, RichardLang wrote:
> Hi everyone!
>
> I'm trying to teach myself R in order to do some data analysis. I'm
a
> mathematics student and (only) familiar with matlab and latex. I'm  
> working
> trough the "official" introduction to R at the moment, while  
> simultaneously
> solving some exercises I found in the web. Before I post my (probably
> stupid) question, I'd like to ask you for some general advice. How  
> do you
> work with R? Is it like in matlab, that you write your functions  
> with a lot
> of loops etc. in a textfile and then run it? Or do you just prepare  
> your
> data and then use the functions provided by R (plot, mean etc) to  
> get some
> analysis? I'd be very thankfull for some of your thoughts about
> "approaches".
In R one generally tries to avoid loops. There are many functions that  
accept vector and matrix arguments and retrun similarly structured  
results. My impression was that that was also true in Matlab and that  
many of the loop-less (the R term is "vectorized") constructs were  
very similar. Matrix math and all that jazz.
>
> Now the question: I'm trying to build a vector with n entries, each
> consisting of the mean of m random numbers (exponential distributed  
> for
> example). My approach was to construct a nxm random matrix and then to
> somehow take the mean of each row. But in the mean function there is  
> no
> parameter to do this, so the intended approach of R is probably  
> different..
> any ideas? =)
Generally when performing a repeated task involving random simulation  
one turns to the replicate function. To generate 5 means of length 10,  
random exponentially distributed vectors:

 > replicate(5, mean( rexp(10, rate= 1.5) ) )
[1] 1.0786732 0.7196179 0.7179612 0.5024858 0.4624592

-- 
David.
>
> Richard
>
> --
> View this message in context:
http://r.789695.n4.nabble.com/calculating-the-mean-of-a-random-matrix-by-row-and-some-general-questions-tp3678964p3678964.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> 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.
David Winsemius, MD
West Hartford, CT

Hi Richard,

As others have said, try to use the "apply" functions rather than
loops.
There is also an apply function for lists, see ?lapply.  This is much more
efficient.  I also like writing my own functions.  For example:

f <- function(x) {
   x^2
}

Which can then be used by:
> f(2)
[1] 4

This is very useful if you're getting into maximum likelihood programming,
or want to use the "optim" function (for multivariate functions) or
"optimize" (for univariate functions).

Lastly, check out the R reference card.
http://cran.r-project.org/doc/contrib/Short-refcard.pdf

Regards,
Peter

On Tue, Jul 19, 2011 at 12:43, RichardLang <lang@zedat.fu-berlin.de>
wrote:
> Hi everyone!
>
> I'm trying to teach myself R in order to do some data analysis. I'm
a
> mathematics student and (only) familiar with matlab and latex. I'm
working
> trough the "official" introduction to R at the moment, while
simultaneously
> solving some exercises I found in the web. Before I post my (probably
> stupid) question, I'd like to ask you for some general advice. How do
you
> work with R? Is it like in matlab, that you write your functions with a lot
> of loops etc. in a textfile and then run it? Or do you just prepare your
> data and then use the functions provided by R (plot, mean etc) to get some
> analysis? I'd be very thankfull for some of your thoughts about
> "approaches".
>
> Now the question: I'm trying to build a vector with n entries, each
> consisting of the mean of m random numbers (exponential distributed for
> example). My approach was to construct a nxm random matrix and then to
> somehow take the mean of each row. But in the mean function there is no
> parameter to do this, so the intended approach of R is probably different..
> any ideas? =)
>
> Richard
>
> --
> View this message in context:
>
http://r.789695.n4.nabble.com/calculating-the-mean-of-a-random-matrix-by-row-and-some-general-questions-tp3678964p3678964.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help@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.
>
>
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