[R] 3 questions regarding matrix copy/shuffle/compares

Esmail esmail.js at gmail.com
Sun Apr 26 13:48:56 CEST 2009


Hello David,

Let me try again, I don't think this was the best post ever I've made :-)
Hopefully this is clearer, or otherwise I may break this up into
three separate simple queries as this may be too long.


 > "==" is not an assignment operator in R, so the answer is that it
 > would do neither.  "<-" and "=" can do assignment. In neither case
 > would it be a "deep copy".

It was late when I posted the code, I made a mistake with regard to
the assignment operator and used the boolean compare instead -- thanks
for catching that.

It should have been:

     keep_pop[1:POP_SIZE] = pop[1:POP_SIZE]


-------- Here's an edited and clearer version I hope:


The basic idea is that I am trying to keep track of a number of bitrings.

Therefore I am creating a matrix (named 'pop') whose rows are made up
of bit vectors (ie my bitstrings).  I only initialize half of the rows
with my bitstrings of random 1s and 0s, the rest of the rows are set
to all zeros).

So I use following function call to create a matrix and fill it with
bit strings:

    pop=create_pop_2(POP_SIZE, LEN)

where

    POP_SIZE refers to the number of rows
    LEN to the columns (length of my bitstrings)



This is the code I call:

####################################################
# create a random binary vector of size "len"
#
create_bin_Chromosome <- function(len)
{
   sample(0:1, len, replace=T)
}



############## create_population ###################
# create population of chromosomes of length len
# the matrix contains twice as much space as popsize
#
create_pop_2 <- function(popsize, len)
{
   datasize=len*popsize
   print(datasize)
   npop <- matrix(0, popsize*2, len, byrow=T)

   for(i in 1:popsize)
     npop[i,] = create_bin_Chromosome(len)

   npop
}


My 3 questions:

(1) If I did

     keep_pop[1:POP_SIZE] = pop[1:POP_SIZE]

     to keep a copy of the original data structure before manipulating
     'pop' potentially, would this make a deep copy or just shallow? Ie
     if I change something in pop would keep_pop change too? I would
     like two independent copies so that 'keep_pop' stays intact while
     'pop' may change.

     > "<-" and "=" can do assignment. In neither case would it be a
     > "deep copy".

     Is there a deepcopy operator, or would I have to have two nested
     loops and iterate through them? Or is there a nice R-idiomatic way
     to do this?


(2) If I wanted to change the order of rows in my matrix 'pop', is
     there an easy way to shuffle these?  I.e., I don't want to change
     any of the bitstrings vectors/rows, just the order of the rows in the
     matrix 'pop'. (E.g., in Python I could just say something like
     suffle(pop)) - is there an equivalent for R?

     So if pop [ [0, 0, 0]
                 [1, 1, 1]
                 [1, 1, 0] ]

     after the shuffle it may look like

               [ [1, 1, 0]    (originally at index 2)
	        [1, 1, 1]    (originally at index 1)
                 [0, 0, 0] ]  (originally at index 0)

     the rows themselves remained intact, just their order changes.
     This is a tiny example, in my case I may have 100 rows (POPS_SIZE)
     and rows of LEN 200.


(3) I would like to compare the contents of 'keep_pop' (a copy of the
     original 'pop') with the current 'pop'. Though the order of rows
     may be different between the two, it should not matter as long as
     the same rows are present.  So for the example given above, the
     comparison should return True.

     For instance, in Python this would be simply

     if sorted(keep_pop) == sorted(pop):
        print 'they are equal'
     else
        print 'they are not equal'

     Is there an equivalent R code segment?


I hope this post is clearer than my original one. Thank you David for
pointing out some of the shortcomings of my earlier post.

Thanks,

Esmail



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