Identify row indices corresponding to each distinct row of a matrix
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
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