On 2010-10-07 10:10, Jim Silverton wrote:
Hello,
I gave  a list of 2 x 2 matrices called matlist. I have about 5000 2 x 2
matrices. I would like to count how many of each 2 x 2 unique matrix I have.
So I am thinking that I need a list of the unique 2 x 2 matrices and their
counts. Can anyone help.


Here's one way, using the plyr package:

 require(plyr)
 ## make a list of 2X2 matrices
 L <- vector('list', 5000)
 set.seed(4321)
 for(i in 1:5000) L[[i]] <- matrix(round(runif(4), 1), 2, 2)

 ## convert each matrix to a string of 4 numbers, then
 ## form dataframe
 dL <- ldply(L, function(.x) toString(unlist(.x)))

 ## add an index vector
 dL$ind <- seq_len(5000)

 ## count unique strings; return string, frequency, indeces
 result <- ddply(dL, .(V1), summarize,
                              freq=length(V1),
                              idx=toString(ind))


   -Peter Ehlers

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