Identify row indices corresponding to each distinct row of a matrix

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

T <- unique(C)
T

i <- 1
which(C[,1]==T[i,1]& C[,2]==T[i,2])

Yes -- much better than mine. I didn't know about the MARGIN argument of
duplicated().

-- Bert


On Wed, Nov 7, 2018 at 10:32 PM Jeff Newmiller <jdnewmil at dcn.davis.ca.us>
wrote:

Perhaps

which( ! duplicated( m, MARGIN=1 ) )

? (untested)

On November 7, 2018 9:20:57 PM PST, Bert Gunter <bgunter.4567 at gmail.com>
wrote:

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 at 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

        [[alternative HTML version deleted]]

      [[alternative HTML version deleted]]

--
Sent from my phone. Please excuse my brevity.