Applying a function to each element of an array

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Well, since Sum(i=1 to i-n) =n*(n+1)/2, your loop simply gives
1/4*count*(count-1).
So if your matrix is A, A*(A-1)/4 is about the quickest way to get your
answer I think.

-- Bert Gunter
Genentech Non-Clinical Statistics
South San Francisco, CA

"The business of the statistician is to catalyze the scientific learning
process."  - George E. P. Box
-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch 
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Tim Smith
Sent: Friday, October 07, 2005 11:48 AM
To: r-help at stat.math.ethz.ch
Subject: [R] Applying a function to each element of an array

Hi,

I have a 7000x7000 matrix, and each element is an integer. 
For each element, I want to apply the function :

wt <- 0
 for(q in 1:count){
 wt <- wt + 0.5^(q-1)
}

I get the value of 'count' from the elements in the matrix , 
and want to store the corresponding 'wt' value for that element.

I suppose I could loop through the matrix, and apply the 
function to each element but this would take a really really 
long time. Are there any quicker ways to get the same result?

many thanks,

Tim

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Hi,

I have a 7000x7000 matrix, and each element is an integer. For each element, I want to apply the function :

wt <- 0
 for(q in 1:count){
 wt <- wt + 0.5^(q-1)
}

This is not a function!  Maybe you want

helper <- function(count) sum(0.5^((1:count)-1))
I get the value of 'count' from the elements in the matrix , and want to store the corresponding 'wt' value for that element.

I suppose I could loop through the matrix, and apply the function to each element but this would take a really really long time. Are there any quicker ways to get the same result?

many thanks,
mapply(helper, yourmat)

gives you the elements of the matrix, as a vector. So you only must 
reassemble as a matrix:

n <- nrow(yourmat)
p <- ncol(yourmat)
matrix( mapply(helper, yourmat), n, p)

Kjetil

Tim

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--
If A is the matrix the answer is 2*(1 - 2^(-A)), which took about 10secs 
for an example of your size.
From \sum_{i=1}^n x^{1-i} = (1-x^{-n})/(1-x), E & OE.

I have a 7000x7000 matrix, and each element is an integer. For each 
element, I want to apply the function :

wt <- 0
for(q in 1:count){
wt <- wt + 0.5^(q-1)
}

I get the value of 'count' from the elements in the matrix , and want to 
store the corresponding 'wt' value for that element.

I suppose I could loop through the matrix, and apply the function to 
each element but this would take a really really long time. Are there 
any quicker ways to get the same result?

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

If A is the matrix the answer is 2*(1 - 2^(-A)), which took about 10secs
for an example of your size.

From \sum_{i=1}^n x^{1-i} = (1-x^{-n})/(1-x), E & OE.
NB: I have assumed n >= 1 here, as people nornally do when using 1:count.
On Fri, 7 Oct 2005, Tim Smith wrote:

I have a 7000x7000 matrix, and each element is an integer. For each
element, I want to apply the function :

wt <- 0
for(q in 1:count){
wt <- wt + 0.5^(q-1)
}

I get the value of 'count' from the elements in the matrix , and want to
store the corresponding 'wt' value for that element.

I suppose I could loop through the matrix, and apply the function to
each element but this would take a really really long time. Are there
any quicker ways to get the same result?
-- 
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

______________________________________________
R-help at stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html

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
If there weren't an analytic solution to your problem,
then you could build a vector of the answers from 1
to the maximum in the matrix.  Call that 'wtvec'.  Then:

ans <- array(NA, dim(A), dimnames(A))
ans[] <- wtvec[as.vector(A)]

should get you what you want.

Patrick Burns
patrick at burns-stat.com
+44 (0)20 8525 0696
http://www.burns-stat.com
(home of S Poetry and "A Guide for the Unwilling S User")

Hi,

I have a 7000x7000 matrix, and each element is an integer. For each element, I want to apply the function :

wt <- 0
for(q in 1:count){
wt <- wt + 0.5^(q-1)
}

I get the value of 'count' from the elements in the matrix , and want to store the corresponding 'wt' value for that element.

I suppose I could loop through the matrix, and apply the function to each element but this would take a really really long time. Are there any quicker ways to get the same result?

many thanks,

Tim

---------------------------------

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______________________________________________
R-help at stat.math.ethz.ch mailing list
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If there weren't an analytic solution to your problem,
then you could build a vector of the answers from 1
to the maximum in the matrix.  Call that 'wtvec'.  Then:

ans <- array(NA, dim(A), dimnames(A))
ans[] <- wtvec[as.vector(A)]

should get you what you want.
Well, we don't actually know 'count' is >= 1.  I almost suggested this, 
but suppose the counts were in billions?

It's a good solution for a small set of continguous values.  If there is a 
small set of values, do a two-step lookup using e.g. match to find the 
enumeration of the values.
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
The sum in the loop is simply:  2 - (0.5)^count.  

So you don't need this loop.  As "count" gets large, the sum approaches 2.

Ravi.
-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch [mailto:r-help-
bounces at stat.math.ethz.ch] On Behalf Of Patrick Burns
Sent: Friday, October 07, 2005 4:29 PM
To: Tim Smith
Cc: r-help at stat.math.ethz.ch
Subject: Re: [R] Applying a function to each element of an array

If there weren't an analytic solution to your problem,
then you could build a vector of the answers from 1
to the maximum in the matrix.  Call that 'wtvec'.  Then:

ans <- array(NA, dim(A), dimnames(A))
ans[] <- wtvec[as.vector(A)]

should get you what you want.

Patrick Burns
patrick at burns-stat.com
+44 (0)20 8525 0696
http://www.burns-stat.com
(home of S Poetry and "A Guide for the Unwilling S User")

Tim Smith wrote:

Hi,

I have a 7000x7000 matrix, and each element is an integer. For each
element, I want to apply the function :
wt <- 0
for(q in 1:count){
wt <- wt + 0.5^(q-1)
}

I get the value of 'count' from the elements in the matrix , and want to
store the corresponding 'wt' value for that element.
I suppose I could loop through the matrix, and apply the function to each
element but this would take a really really long time. Are there any
quicker ways to get the same result?
many thanks,

Tim

---------------------------------

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______________________________________________
R-help at stat.math.ethz.ch mailing list
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PLEASE do read the posting guide! http://www.R-project.org/posting-
guide.html