Checking row membership of a matrix

Question

I have a matrix in which each row vector has a name. I would like to check row membership in my matrix, that is I would like to turn the following into R code:

if(mat contains "rowname")
{  do appropriate task ....}
else if(mat contains "otherrowname")
{  do appropriate task ....}
else
{  do different task....}

1. How can I test for row membership in a matrix?

All help is appreciated!

Answer 1

It is fairly common to see code that looks like:

 if( sum( rowNameToFind %in% rownames(mat)) ) { TRUE }else{ FALSE }

This deals simultaneously with the rownames-missing-entirely possibility at the same time as target-not-in-rownames.

Answer 2

A matrix may or may not have rownames for you to index. You can index them with the %in% operator. Here's a quick example:

 #Sample matrix
mat <- matrix(rnorm(100), ncol = 10)
#Find the row 'b'
rowNameToFind <- "b"


if (is.null(rownames(mat))) {
  print("no rownames to index!")
} else if  (rowNameToFind %in% rownames(mat)) {
  print("hurrary")
} else {
  print("boo")
}

#Returns
[1] "no rownames to index!"

#Define the rownames
rownames(mat) <- letters[1:10]


if (is.null(rownames(mat))) {
  print("no rownames to index!")
} else if  (rowNameToFind %in% rownames(mat)) {
  print("hurrary")
} else {
  print("boo")
}

#Returns
[1] "hurrary"

Answer 3

As long as each row has a rowname you can do the following:

> if("somerowname" %in% rownames(somematrix))
+ { print("true") } else print("false")
[1] "true"

I hope this helps and that the code is clear!