[R] matrix element position: from length to dim

[R. Michael Weylandt]

Also take a look at the arrayInd() function which is what's used by
which() internally for the arr.ind = TRUE case.

Michael

On Thu, Feb 2, 2012 at 8:25 AM, Petr Savicky <savicky at cs.cas.cz> wrote:
> On Thu, Feb 02, 2012 at 02:08:37PM +0100, Ana wrote:
>> How can I pass from position in length inside a matrix to position in dim ?
>>
>>
>> a=matrix(c(1:999),nrow=9)
>>
>> which(a==87)    #position in length 1:length(a)
>> 87
>>
>> which(a==87,arr.ind=TRUE)   #position in dim
>>      row col
>> [1,]   6  10
>
> Hi.
>
> Assume
>
>   d <- dim(a)
>   i <- 87
>
> Try the following two approaches.
>
> 1.
>
>   x <- rep(FALSE, times=prod(d))
>   x[i] <- TRUE
>   which(array(x, dim=d), arr.ind=TRUE)
>
>       row col
>  [1,]   6  10
>
> 2.
>
>  c((i - 1) %% d[1], (i - 1) %/% d[1]) + 1
>
>  [1]  6 10
>
> Hope this helps.
>
> Petr Savicky.
>
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