[R] Count number of rows in a matrix with a character pattern

[Rui Barradas]

Hello,


katarv wrote
> 
> Hi,
> 
> I have a large data set that I input as a matrix, where I have  1:x rows
> with names AX, then  x+1: y rows named AY, etc.  The idea is that I have
> to count how many rows exactly I have with name AX and how many I have
> with name AY (or find which row numbers have names AX). Is there any way
> in R to count a number of rows with a name matching a required pattern?
> 
> 
> Thanks in advance,
> 
> Katie
> 

Yes, there are ways of doing what you need.
In the example below I'm not assuming the names matching a pattern are
consecutive,
it's just simpler to define the names like that.

# Make an example matrix.
ax <- paste("AX", 1:4, sep="")
ay <- paste("AY", 1:6, sep="")
az <- paste("AZ", 1:2, sep="")
mat <- matrix(1:24, ncol=2, dimnames=list(c(ax, ay, az), NULL))
mat

# Now get the names and search (grep) the pattern
rnames <- rownames(mat)
grep("AY", rnames)


Two notes:
One, see the help pages for
?grep
?regex

Second, next time please post an example dataset using 'dput'

dput(mat)
structure(1:24, .Dim = c(12L, 2L), .Dimnames = list(c("AX1", 
"AX2", "AX3", "AX4", "AY1", "AY2", "AY3", "AY4", "AY5", "AY6", 
"AZ1", "AZ2"), NULL))

Like this it's just a matter of copy&paste to our R session.

Hope this helps,

Rui Barradas

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