reverse elements in each col of a matrix without reversing col order

Question

Using R, I want to reverse all the elements in each column, for a single row of a matrix. I can get the row order to reverse (which is not what I want), but not the elements in the rows.

create example matrix

snips <- c("CCA", "ATC", "TTC")
row2 <- 1:3
my.matrix <- as.matrix(rbind(snips, row2))
my.matrix

[,1]  [,2]  [,3] 

snips "CCA" "ATC" "TTC"
row2  "1"   "2"   "3"

reverse the elements in the row = snips

my.matrix.reversed <- rev(my.matrix[my.matrix[1,], ])

Error in my.matrix[my.matrix[1, ], ] : subscript out of bounds

what I want to get is:

[,1]  [,2]  [,3] 
snips "ACC" "CTA" "CTT"
row2  "1"   "2"   "3"

score 1 · Answer 1 · answered Apr 11 '17 at 04:05

1

One approach would be to use one of the string manipulation packages, like "stringi" to reverse the strings:

library(stringi)
my.matrix[1, ] <- stri_reverse(my.matrix[1, ])
my.matrix
#       [,1]  [,2]  [,3] 
# snips "ACC" "CTA" "CTT"
# row2  "1"   "2"   "3"

answered Apr 11 '17 at 04:05

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thanks! stringi did exactly what I needed it to do. – MycoP Apr 11 '17 at 18:22

akrun · Answer 2 · 2017-04-11T04:14:40.177

We may need some string manipulation. Assuming there are only 3 characters in the original dataset, we can capture each character as a group ((.)) in sub and reverse the order of capture groups (`\3\2\1')

my.matrix[1,] <- sub("(.)(.)(.)", "\\3\\2\\1", my.matrix[1,])
my.matrix
#      [,1]  [,2]  [,3] 
#snips "ACC" "CTA" "CTT"
#row2  "1"   "2"   "3"

A more generalized approach would be to split the string into individual characters, then rev and paste it back

my.matrix[1,] <- sapply(strsplit(my.matrix[1,], ""), function(x) paste(rev(x), collapse=""))

Or using Biostrings

library(Biostrings)
my.matrix[1,] <- reverse(my.matrix[1,])

score 0 · Answer 3 · edited May 23 '17 at 12:32

You could use stringi::stri_reverse. This code assumes that the only other row is row2 and it contains only single characters, so reversal doesn't matter. If that isn't the case you need to apply the function to only the snips row.

library(stringi)
apply(my.matrix, 1:2, function(x) stri_reverse(x))

       [,1]  [,2]  [,3] 
snips "ACC" "CTA" "CTT"
row2  "1"   "2"   "3"

Lots of other solutions at this question.

reverse elements in each col of a matrix without reversing col order

3 Answers3