[R] algorithm to merge same entries in a matrix

[Paul Hiemstra]

 Hi,

Take a look at the rle() function. This finds runs of equal values in
vectors. Using apply this can quite easily be run for all the
rows/columns of a matrix.

cheers,
Paul

On 08/30/2011 04:58 PM, Martin Batholdy wrote:
> Hi,
>
> I have the following, rather unorthodox problem:
>
>
> I have a matrix that looks like this:
>
> m1 <- matrix(c('a','b','d',NA,'c','c'), 2,3, byrow=TRUE)
>
>
>         [,1] [,2] [,3]
> [1,]  "a"  "b"  "d" 
> [2,] NA   "c"  "c" 
>
>
> now I would like to transform this matrix into this matrix:
>
>         [,1] [,2] [,3] [,4]
> [1,]  "a"  "b"  "c"  "d" 
> [2,] NA NA NA NA
>
>
> so the rule is:
>
> if I find the same value (above: 'c') in neighbored cells (here: row wise) put this value in row 1 in a new column between
> column of neighbor a and column of neighbor b.
>
>
>
>
>
> too make it even more complicated:
> the definition of neighbored cells is not exclusively row wise.
>
>
>
>
> So for example I could encounter this kind of matrix:
>
> m2 <- matrix(c('a','b','d',NA,NA,'c',NA, 'c', NA), 3,3, byrow=TRUE)
>
>
>        [,1] [,2] [,3]
> [1,] "a"  "b"  "d" 
> [2,] NA NA  "c" 
> [3,] NA "c"  NA  
>
> which should also be transformed into:
>
>         [,1] [,2] [,3] [,4]
> [1,]  "a"  "b"  "c"  "d" 
> [2,] NA NA NA NA
>
>
> (or m3 <- matrix(c('a','b','d',NA,'c',NA,NA, NA, 'c'), 3,3, byrow=TRUE) ).
>
>
>
> Can perhaps someone give me some hints how to solve this efficiently?
>
> I think I could solve that with some loops … but I also need to make sure that it is efficient / not taking too much time.
>
>
> thanks!
>
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-- 
Paul Hiemstra, Ph.D.
Global Climate Division
Royal Netherlands Meteorological Institute (KNMI)
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