[R] Extract entries from matrix

[Jorge I Velez]

Dear Jim,

Thank you very much for your quick reply.

I am sorry for the confusion it may have caused, but I messed up the
indexes in my example.  I would like, from the following matrix "m"

## input
m <- structure(c(0L, 0L, 0L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 0L,
0L, 0L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 0L, 0L, 0L, 3L, 3L,
3L, 3L, 3L, 3L, 3L, 3L, 3L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 4L,
4L, 4L, 4L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 5L, 5L), .Dim = c(12L,
5L))

to obtain

 1 2 3 1 2 3 4 5 1

Sure using m[idx] will give the desired result.  The problem is that idx is
not known and needs to be determined from "m".  I would like to use
something like

extractDiagonals(m)
## [1]  1 2 3 1 2 3 4 5 1

I look forward to your reply.  Thanks in advance.

Best regards,
Jorge Velez.-



On Tue, Oct 27, 2015 at 2:31 PM, jim holtman <jholtman at gmail.com> wrote:

> If you want to use the numbers you gave a the index into the matrix, then
> you can create a matrix with the values and then index into 'm'.  I don't
> see a '4' in the output example you gave using your index values:
>
> > m <- structure(c(0L, 0L, 0L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
> +  1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 0L, 0L, 0L, 2L, 2L, 2L, 2L,
> +  2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 0L,
> +  0L, 0L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L,
> +  3L, 3L, 3L, 3L, 3L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 4L, 4L, 4L,
> +  4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 0L, 0L, 0L, 0L, 0L,
> +  0L, 0L, 0L, 0L, 0L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L,
> +  5L), .Dim = c(22L, 5L))
> > # create index matrix
> > indx <- matrix(c(4, 1,
> +  5, 2,
> +  6, 3,
> +  7, 1,
> +  8, 2,
> +  9, 3,
> +  10, 1,
> +  11, 2,
> +  12, 3), ncol = 2, byrow = TRUE)
> >
> >
> > m
>       [,1] [,2] [,3] [,4] [,5]
>  [1,]    0    0    0    0    0
>  [2,]    0    0    0    0    0
>  [3,]    0    0    0    0    0
>  [4,]    1    2    3    0    0
>  [5,]    1    2    3    0    0
>  [6,]    1    2    3    0    0
>  [7,]    1    2    3    0    0
>  [8,]    1    2    3    0    0
>  [9,]    1    2    3    4    0
> [10,]    1    2    3    4    0
> [11,]    1    2    3    4    5
> [12,]    1    2    3    4    5
> [13,]    1    2    3    4    5
> [14,]    1    2    3    4    5
> [15,]    1    2    3    4    5
> [16,]    1    2    3    4    5
> [17,]    1    2    3    4    5
> [18,]    1    2    3    4    5
> [19,]    1    2    3    4    5
> [20,]    1    2    3    4    5
> [21,]    1    2    3    4    5
> [22,]    1    2    3    4    5
> > indx
>       [,1] [,2]
>  [1,]    4    1
>  [2,]    5    2
>  [3,]    6    3
>  [4,]    7    1
>  [5,]    8    2
>  [6,]    9    3
>  [7,]   10    1
>  [8,]   11    2
>  [9,]   12    3
> > m[indx]
> [1] 1 2 3 1 2 3 1 2 3
>
>
> Jim Holtman
> Data Munger Guru
>
> What is the problem that you are trying to solve?
> Tell me what you want to do, not how you want to do it.
>
> On Tue, Oct 27, 2015 at 2:43 PM, Jorge I Velez <jorgeivanvelez at gmail.com>
> wrote:
>
>> Dear R-help,
>>
>> I am working with a matrix "m" from which I would like to extract some
>> elements.  An toy example is as follows:
>>
>> ## input matrix
>> m <- structure(c(0L, 0L, 0L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
>> 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 0L, 0L, 0L, 2L, 2L, 2L, 2L,
>> 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 0L,
>> 0L, 0L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L,
>> 3L, 3L, 3L, 3L, 3L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 4L, 4L, 4L,
>> 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 4L, 0L, 0L, 0L, 0L, 0L,
>> 0L, 0L, 0L, 0L, 0L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L,
>> 5L), .Dim = c(22L, 5L))
>>
>> R> m
>> #         [,1]  [,2] [,3] [,4] [,5]
>> #  [1,]    0    0    0    0    0
>> #  [2,]    0    0    0    0    0
>> #  [3,]    0    0    0    0    0
>> #  [4,]    1    2    3    0    0
>> #  [5,]    1    2    3    0    0
>> #  [6,]    1    2    3    0    0
>> #  [7,]    1    2    3    0    0
>> #  [8,]    1    2    3    0    0
>> #  [9,]    1    2    3    4    0
>> # [10,]   1    2    3    4    0
>> # [11,]   1    2    3    4    5
>> # [12,]   1    2    3    4    5
>>
>> >From "m", I would like to extract the entries
>>
>> 4, 1
>> 5, 2
>> 6, 3
>> 7, 1
>> 8, 2
>> 9, 3
>> 10, 1
>> 11, 2
>> 12, 3
>>
>> so at the end of applying a function "f" to "m" I get
>>
>> 1, 2, 3, 1, 2, 3, 4, 1, 2, 3
>>
>>
>> Basically the idea is to extract the diagonal elements until a zero is
>> found.
>>
>> In the real problem the dimensions of "m" are much bigger, but this
>> smaller
>> version of "m" illustrate what needs to be done.
>>
>> I would greatly appreciate any ideas on how to do this.
>>
>> Thanks in advance,
>> Jorge Velez.-
>>
>>         [[alternative HTML version deleted]]
>>
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>>
>
>

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