R help - Nov 2010 - I need a very specific unique like function and I don't know even how to properly call this

consider this matrix:

      [,1] [,2]
 [1,]    3   7
 [2,]    6   5
 [3,]    7   5
 [4,]    3   5
 [5,]    7   5
 [6,]    5   5
 [7,]    8   4
 [8,]    2   4
 [9,]    7   4
[10,]    0   6

I need to delete all rows where column 2 above and below has the same value,
so the effect would be:

      [,1] [,2]
 [1,]    3   7
 [2,]    6   5
 [6,]    5   5
 [7,]    8   4
 [9,]    7   4
[10,]    0   6

is there a built in function for that kind of operation or I must write one
from scratch ?
Is there a name for that kind of operation ?
-- 
View this message in context:
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Sent from the R help mailing list archive at Nabble.com.

Here is a method for piecing it together using diff and indexing:

dat <- structure(c(3L, 6L, 7L, 3L, 7L, 5L, 8L, 2L, 7L, 0L, 7L, 5L, 5L,
 5L, 5L, 5L, 4L, 4L, 4L, 6L), .Dim = c(10L, 2L), .Dimnames = list(
     NULL, c("V1", "V2")))
 diffs <- abs(diff(dat[,2], 1)) # get the difference between each
value and the previous value
 new.dat <- cbind(dat, c(NA, diffs), c(diffs, NA)) # combine the diffs
with the original matrix, shifted down (is the next valued the same as
the value) and down (is the previous value the same)
 new.dat <- cbind(new.dat, rowSums(new.dat[,3:4], na.rm=TRUE)) # sum
the shifted diffs so that the value is 0 if above and below are the
same, and greater than zero if the above and below values are not the
same
 final.dat <- new.dat[new.dat[,5] !=0 ,1:2] # get rid of rows for
which the sum of the shifted diffs is not equal to zero.

HTH,
Ista
On Mon, Nov 22, 2010 at 8:53 PM, madr <madrazel at interia.pl>
wrote:
>
> consider this matrix:
>
> ? ? ?[,1] [,2]
> ?[1,] ? ?3 ? 7
> ?[2,] ? ?6 ? 5
> ?[3,] ? ?7 ? 5
> ?[4,] ? ?3 ? 5
> ?[5,] ? ?7 ? 5
> ?[6,] ? ?5 ? 5
> ?[7,] ? ?8 ? 4
> ?[8,] ? ?2 ? 4
> ?[9,] ? ?7 ? 4
> [10,] ? ?0 ? 6
>
> I need to delete all rows where column 2 above and below has the same
value,
> so the effect would be:
>
> ? ? ?[,1] [,2]
> ?[1,] ? ?3 ? 7
> ?[2,] ? ?6 ? 5
> ?[6,] ? ?5 ? 5
> ?[7,] ? ?8 ? 4
> ?[9,] ? ?7 ? 4
> [10,] ? ?0 ? 6
>
> is there a built in function for that kind of operation or I must write one
> from scratch ?
> Is there a name for that kind of operation ?
> --
> View this message in context:
http://r.789695.n4.nabble.com/I-need-a-very-specific-unique-like-function-and-I-don-t-know-even-how-to-properly-call-this-tp3054427p3054427.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.
>


-- 
Ista Zahn
Graduate student
University of Rochester
Department of Clinical and Social Psychology
http://yourpsyche.org

Given a vector, x, we can test if the value above
it is equal to itself with

    abv = c(FALSE,x[-l] == x[-1])

and if the value below is equal to itself with

    blw = c(x[-l] == x[-1],FALSE)

So, for your problem:
> abv = c(FALSE,dat[,2][-l] == dat[,2][-1])
> blw = c(dat[,2][-l] == dat[,2][-1],FALSE)
> dat[!(abv & blw),]
      [,1] [,2]
[1,]    3    7
[2,]    6    5
[3,]    5    5
[4,]    8    4
[5,]    7    4
[6,]    0    6

 					- Phil Spector
 					 Statistical Computing Facility
 					 Department of Statistics
 					 UC Berkeley
 					 spector at stat.berkeley.edu


On Mon, 22 Nov 2010, madr wrote:
>
> consider this matrix:
>
>      [,1] [,2]
> [1,]    3   7
> [2,]    6   5
> [3,]    7   5
> [4,]    3   5
> [5,]    7   5
> [6,]    5   5
> [7,]    8   4
> [8,]    2   4
> [9,]    7   4
> [10,]    0   6
>
> I need to delete all rows where column 2 above and below has the same
value,
> so the effect would be:
>
>      [,1] [,2]
> [1,]    3   7
> [2,]    6   5
> [6,]    5   5
> [7,]    8   4
> [9,]    7   4
> [10,]    0   6
>
> is there a built in function for that kind of operation or I must write one
> from scratch ?
> Is there a name for that kind of operation ?
> -- 
> View this message in context:
http://r.789695.n4.nabble.com/I-need-a-very-specific-unique-like-function-and-I-don-t-know-even-how-to-properly-call-this-tp3054427p3054427.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.
>