[R] Vectorizing a "for" loop

[Petr Pikal]

Hi

outer(rows, columns, function(x,y) abs(x-y))
shall do it.

HTH
Petr


On 3 Aug 2006 at 10:10, Daniel Gerlanc wrote:

Date sent:      	Thu, 3 Aug 2006 10:10:46 -0400
From:           	"Daniel Gerlanc" <dgerlanc at gmail.com>
To:             	r-help at stat.math.ethz.ch
Subject:        	[R] Vectorizing a "for" loop
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> Hello all,
> 
> Consider the following problem:
> 
> There are two vectors:
> 
> rows <- c(1, 2, 3, 4, 5)
> columns <- c(10, 11, 12, 13, 14)
> 
> I want to create a matrix with dimensions length(rows) x
> length(columns):
> 
> res <- matrix(nrow = length(rows), ncol = length(columns))
> 
> If "i" and "j" are the row and column indexes respectively, the values
> of the cells are abs(rows[i] - columns[j]).  The resultant matrix
> follows:
> 
>      [,1] [,2] [,3] [,4] [,5]
> [1,]    9   10   11   12   13
> [2,]    8    9    10   11   12
> [3,]    7    8      9   10   11
> [4,]    6    7      8     9   10
> [5,]    5    6      7     8    9
> 
> This matrix may be generated by using a simple "for" loop:
> 
> for(i in 1:length(rows)){
>   for(j in 1:length(columns)){
>     res[i,j] <- abs(rows[i] - columns[j])
>   }
> }
> 
> Is there a quicker, vector-based approach for doing this or a function
> included in the recommended packages that does this?
> 
> Thanks!
> 
> -- Dan Gerlanc
> Williams College
> 
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Petr Pikal
petr.pikal at precheza.cz