# [R] create new matrices with specific patterns

Jim Lemon drjimlemon at gmail.com
Tue Jan 27 09:28:40 CET 2015

```Hi Kathryn,
Selecting the elements of matrix A as you describe is not too difficult:

select_A<-function(A,B) {
thisA<-A[A[,1] == B\$p[1],]
newmat<-matrix(c(rep(p[1],B\$nq[1]),rep(thisA[,2],length.out=B\$nq[1])),ncol=2)
for(i in 2:dim(B)[1]) {
thisA<-A[A[,1] == B\$p[i],]
newmat<-
rbind(newmat,
matrix(c(rep(B\$p[i],B\$nq[i]),rep(thisA[,2],length.out=B\$nq[i])),ncol=2))
}
return(newmat)
}

B<-data.frame(p=c(1,2,3,4),nq=c(4,4,4,2))
select_A(A,B)
E<-data.frame(p=c(1,2,3,4),nq=c(0,0,0,4))
select_A(A,E)

However, I suspect that you want to use the notation in your initial
post rather than specify data frames as above. If this is the case,
could you let us know how the selection of elements is to be specified
and perhaps there will be a solution.

Jim

On Tue, Jan 27, 2015 at 2:43 PM, Kathryn Lord
<kathryn.lord2000 at gmail.com> wrote:
> Sorry for inconvenience. For clarification,
>
> The matrix B looks like
>
>> B
>       P Q
>  [1,] 1 1
>  [2,] 1 2
>  [3,] 1 3
>  [4,] 1 4
>  [5,] 2 1
>  [6,] 2 2
>  [7,] 2 3
>  [8,] 2 4
> [9,] 3 1
> [10,] 3 2
> [11,] 3 3
> [12,] 3 4
> [13,] 4 1
> [14,] 4 2
>
> The matrix E is
>
>> E
>       P Q
> [1,] 4 1
> [2,] 4 2
> [3,] 4 3
> [4,] 4 4
>
>
>
> On Tue, Jan 27, 2015 at 11:47 AM, Kathryn Lord <kathryn.lord2000 at gmail.com>
> wrote:
>
>> Dear R users,
>>
>> Suppose I have a matrix A.
>>
>> > p <- 1:4
>> > q <- 1:5
>> > P<-rep(p, each=5)
>> > Q<-rep(q, 4)
>> >
>> > A <- cbind(P,Q)
>> > A
>>       P Q
>>  [1,] 1 1
>>  [2,] 1 2
>>  [3,] 1 3
>>  [4,] 1 4
>>  [5,] 1 5
>>  [6,] 2 1
>>  [7,] 2 2
>>  [8,] 2 3
>>  [9,] 2 4
>> [10,] 2 5
>> [11,] 3 1
>> [12,] 3 2
>> [13,] 3 3
>> [14,] 3 4
>> [15,] 3 5
>> [16,] 4 1
>> [17,] 4 2
>> [18,] 4 3
>> [19,] 4 4
>> [20,] 4 5
>> >
>>
>>
>> With the matrix A, I'd like to generate new matrices B, ..., E below.
>>
>> B = A[(3,4), (1,2)]
>> C = A[(2,2), (1,5), (1,1)]
>> D = A[(4,2)]
>> E = A[(3,0), (1,4)]
>>
>>
>> Matrix B means that first three 'p's (1,2,3) has four 'q's (1,2,3,4) and
>> the forth 'p' element (4) has two 'q's (1,2); in other words,
>>
>> Is there the easyiest way to create B,...,E in R?
>>
>> Actually, the example above is a toy example and the matrix A I have is
>> around 10,000 by 10,000 and the pattern is also very complicated.
>>
>> Any suggestion will be greatly appreciated.
>>
>> Best,
>>
>> Kathryn Lord
>>
>>
>
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>
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