[R] Identify row indices corresponding to each distinct row of a matrix

[Bert Gunter]

A mess -- due to your continued use of html formatting.

But something like this may do what you want (hard to tell with the mess):

> m <- matrix(1:16,nrow=8)[rep(1:8,2),]
> m
      [,1] [,2]
 [1,]    1    9
 [2,]    2   10
 [3,]    3   11
 [4,]    4   12
 [5,]    5   13
 [6,]    6   14
 [7,]    7   15
 [8,]    8   16
 [9,]    1    9
[10,]    2   10
[11,]    3   11
[12,]    4   12
[13,]    5   13
[14,]    6   14
[15,]    7   15
[16,]    8   16
> vec <- apply(m,1,paste,collapse="-") ## converts rows into character
vector
> vec
 [1] "1-9"  "2-10" "3-11" "4-12" "5-13" "6-14" "7-15" "8-16" "1-9"  "2-10"
"3-11" "4-12" "5-13" "6-14"
[15] "7-15" "8-16"
> ## Then maybe:
> tapply(seq_along(vec),vec, I)
$`1-9`
[1] 1 9

$`2-10`
[1]  2 10

$`3-11`
[1]  3 11

$`4-12`
[1]  4 12

$`5-13`
[1]  5 13

$`6-14`
[1]  6 14

$`7-15`
[1]  7 15

$`8-16`
[1]  8 16

> ## gives the row numbers for each unique row

There may well be slicker ways to do this -- if this is actually what you
want to do.

-- Bert



On Wed, Nov 7, 2018 at 7:56 PM li li <hannah.hlx using gmail.com> wrote:

> Hi all,
>    I use the following example to illustrate my question. As you can see,
> in matrix C some rows are repeated and I would like to find the indices of
> the rows corresponding to each of the distinct rows.
>   For example, for the row c(1,9), I have used the "which" function to
> identify the row indices corresponding to c(1,9). Using this approach, in
> order to cover all distinct rows, I need to use a for loop.
>    I am wondering whether there is an easier way where a for loop can be
> avoided?
>    Thanks very much!
>       Hanna
>
>
>
> > A <- matrix(c(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16),8,2)> B <-
> rbind(A,A,A)> C <- as.data.frame(B[sample(nrow(B)),])> C   V1 V2
> 1   1  9
> 2   2 10
> 3   3 11
> 4   5 13
> 5   7 15
> 6   6 14
> 7   4 12
> 8   3 11
> 9   8 16
> 10  5 13
> 11  7 15
> 12  2 10
> 13  1  9
> 14  8 16
> 15  1  9
> 16  3 11
> 17  7 15
> 18  4 12
> 19  2 10
> 20  6 14
> 21  4 12
> 22  8 16
> 23  5 13
> 24  6 14> T <- unique(C)> T  V1 V2
> 1  1  9
> 2  2 10
> 3  3 11
> 4  5 13
> 5  7 15
> 6  6 14
> 7  4 12
> 9  8 16> > i <- 1                    > which(C[,1]==T[i,1]&
> C[,2]==T[i,2])[1]  1 13 15
>
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>
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