Re: [R] vectorisation suggestion

From: Wiener, Matthew <matthew_wiener_at_merck.com>
Date: Tue 21 Jun 2005 - 06:38:10 EST


Federico -

"match" will give you the (first) index of each element of its first argument in its second argument. So match(vector.1, vector.2) tells you where each element of vector.1 appears in vector.2. So if you use "table" on that vector, you'll see how many times each element of vector.2 appears.

Something like:

first.occur <- match(vector.1, vector.2) table(factor(vector.2[first.occur], levels = sort(unique(vector.2)))

(changing it into a factor means you won't lose values of vector.2 that never appear in vector.1.)

An example:

> vec1 <- sample(1:10, 500, replace = TRUE)
> table(vec1)

vec1
 1 2 3 4 5 6 7 8 9 10
61 37 43 47 49 59 51 48 53 52
> vec2 <- 0:11
> vec3 <- match(vec1, vec2)
> table(factor(vec2[vec3], levels = sort(unique(vec2))))
 0 1 2 3 4 5 6 7 8 9 10 11
 0 61 37 43 47 49 59 51 48 53 52 0
>

This also works if one of the members of vec1 is not in vec2 -- that member simply gets ignored. (As you can see if add, say, a "20" at the end of vec1.)

Hope this helps,

Matt Wiener

-----Original Message-----
From: r-help-bounces@stat.math.ethz.ch
[mailto:r-help-bounces@stat.math.ethz.ch] On Behalf Of Federico Calboli Sent: Monday, June 20, 2005 4:16 PM
To: r-help
Subject: [R] vectorisation suggestion

Hi All,

I am counting the number of occurrences of the terms listed in one vector in another vector.

My code runs:

for( i in 1:length(vector3)){

   vector3[i] = sum(1*is.element(vector2, vector1[i])) }

where

vector1 = vector containing the terms whose occurrences I want to count vector2 = made up of a number of repetitions of all the elements of vector1
vector3 = a vector of NAs that is meant to get the result of the counting

My problem is that vector1 is about 60000 terms, and vector2 is 620000... can anyone suggest a faster code than the one I wrote?

Cheers,

Federico Calboli

--
Federico C. F. Calboli
Department of Epidemiology and Public Health
Imperial College, St. Mary's Campus
Norfolk Place, London W2 1PG

Tel +44 (0)20 75941602   Fax +44 (0)20 75943193

f.calboli [.a.t] imperial.ac.uk
f.calboli [.a.t] gmail.com

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Received on Tue Jun 21 06:50:39 2005

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