R help - Apr 2012 - Count number of rows in a matrix with a character pattern

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 

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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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I'd use a combination of rownames(), grepl() and sum(). 

Get the names with the first, test with the second and count the positives (by
coercing TRUE -> 1) with the last

Michael

On Apr 29, 2012, at 3:46 PM, katarv <katiasmirn at gmail.com> 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 
> 
> --
> View this message in context:
http://r.789695.n4.nabble.com/Count-number-of-rows-in-a-matrix-with-a-character-pattern-tp4596848p4596848.html
> Sent from the R help mailing list archive at Nabble.com.
> 
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