R help - Dec 2010 - Subarray specification problem

Hi.  I'm new to R, and I'm still learning R's system for addressing
subsets of data structures.  I'm particularly interested in the
problem of selecting subarrays based on complex criteria involving the
dimnames (as opposed to the values of the cells) of the array.  Here's
an example of such a problem.

Suppose I have an array x of unknown dimensions (it may have been
passed as the argument to a function I'm coding), but I know that one
of its dimensions is called "time", and has values that are (or can be
meaninfully coerced into) integers.  To make this specification
clearer, here's one possible example of such an array x:
> (x <- array(runif(2*5*2), dim=c(2,5,2), dimnames=list(NULL,
time=round(100*runif(5)), NULL)))
, , 1

      time
              84        69        61         16        77
  [1,] 0.4020976 0.8250189 0.3402749 0.09754860 0.2189114
  [2,] 0.5309967 0.5414850 0.9431449 0.08716723 0.5819100

, , 2

      time
              84        69        61        16        77
  [1,] 0.6238213 0.1210083 0.7823269 0.5004058 0.5474356
  [2,] 0.2491087 0.7449411 0.9561074 0.6685954 0.3871533


Now, here's the problem: I want to write an R expression that will
give me the subarray y of x consisting of the cells whose "time"
dimensions are greater than 20 but less than 80.

For the example x given above, the desired expression would evaluate
to this array:

, , 1

      time
              69        61         77
  [1,] 0.8250189 0.3402749 0.2189114
  [2,] 0.5414850 0.9431449 0.5819100

, , 2

      time
              69        61        77
  [1,] 0.1210083 0.7823269 0.5474356
  [2,] 0.7449411 0.9561074 0.3871533



How can I write such an expression in the general array x as described above?

Remember, the x shown above is just an example.  In the general case
all I know is that one of x's dimensions is called "time", and
that
its values are [or can be coerced meaningfully] into integers.  I
*don't* know where among x's dimensions it is.  Hence, the following
is *not* a solution to the problem, even though it produces the right
answer for the example above:
> t <- as.integer(dimnames(x)$time)
> y <- x[,which(t > 20 & t < 80),]
This solution does not work in general, because the expression
"x[,which(t > 20 & t < 80),]" relies on the prior knowledge
that the
"time" dimension is the second one of three.

Any ideas?

Thanks!

Roy

Use aperm() to make time the first dimension
Reshape to a matrix (all other dimensions combined)
Do your selection on X[1,]
aperm() to Permute back

On 12/16/2010 11:00 AM, Roy Shimizu wrote:
> Hi.  I'm new to R, and I'm still learning R's system for
addressing
> subsets of data structures.  I'm particularly interested in the
> problem of selecting subarrays based on complex criteria involving the
> dimnames (as opposed to the values of the cells) of the array.  Here's
> an example of such a problem.
>
> Suppose I have an array x of unknown dimensions (it may have been
> passed as the argument to a function I'm coding), but I know that one
> of its dimensions is called "time", and has values that are (or
can be
> meaninfully coerced into) integers.  To make this specification
> clearer, here's one possible example of such an array x:
>
>> (x<- array(runif(2*5*2), dim=c(2,5,2), dimnames=list(NULL,
time=round(100*runif(5)), NULL)))
> , , 1
>
>        time
>                84        69        61         16        77
>    [1,] 0.4020976 0.8250189 0.3402749 0.09754860 0.2189114
>    [2,] 0.5309967 0.5414850 0.9431449 0.08716723 0.5819100
>
> , , 2
>
>        time
>                84        69        61        16        77
>    [1,] 0.6238213 0.1210083 0.7823269 0.5004058 0.5474356
>    [2,] 0.2491087 0.7449411 0.9561074 0.6685954 0.3871533
>
>
> Now, here's the problem: I want to write an R expression that will
> give me the subarray y of x consisting of the cells whose "time"
> dimensions are greater than 20 but less than 80.
>
> For the example x given above, the desired expression would evaluate
> to this array:
>
> , , 1
>
>        time
>                69        61         77
>    [1,] 0.8250189 0.3402749 0.2189114
>    [2,] 0.5414850 0.9431449 0.5819100
>
> , , 2
>
>        time
>                69        61        77
>    [1,] 0.1210083 0.7823269 0.5474356
>    [2,] 0.7449411 0.9561074 0.3871533
>
>
>
> How can I write such an expression in the general array x as described
above?
>
> Remember, the x shown above is just an example.  In the general case
> all I know is that one of x's dimensions is called "time",
and that
> its values are [or can be coerced meaningfully] into integers.  I
> *don't* know where among x's dimensions it is.  Hence, the
following
> is *not* a solution to the problem, even though it produces the right
> answer for the example above:
>
>> t<- as.integer(dimnames(x)$time)
>> y<- x[,which(t>  20&  t<  80),]
>
> This solution does not work in general, because the expression
> "x[,which(t>  20&  t<  80),]" relies on the prior
knowledge that the
> "time" dimension is the second one of three.
>
> Any ideas?
>
> Thanks!
>
> Roy
>

-- 
Michael Friendly     Email: friendly AT yorku DOT ca
Professor, Psychology Dept.
York University      Voice: 416 736-5115 x66249 Fax: 416 736-5814
4700 Keele Street    Web:   http://www.datavis.ca
Toronto, ONT  M3J 1P3 CANADA