[R] [FORGED] Re: Extract entries from matrix

[Rolf Turner]

Maybe it's just me --- I'm notoriously slow --- but I haven't a clue 
what you are trying to do.  You said:

   "... the idea is to extract the diagonal elements until a
    zero is found."

I don't see anything that resembles a "diagonal element" (*entry*, 
actually; sets have elements, arrays have entries) in your desired 
output.  What do you mean by the diagonal of a non-square matrix anyway?

cheers,

Rolf Turner

-- 
Technical Editor ANZJS
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276

8/10/15 08:43, Jorge I Velez wrote:
> 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.