Somewhat out of my depth here (I have only 2 arms, but am swimming in waters which require 3): My interpretation is that a and M are basically vectors, with dimension attributes, accessed down the columns.
The array 'a' consists of 30 elements 1:30 in that order, accessed by each of 3 rows for each of 5 columns in each of two "layers", in that order of precedence. The matrix M consusts of 15 elements, accessed by each of 35 rows for each of 5 columns. Thus a(M) treats M as an selection vector, ignoring dimension, and reads along a and at the same time along M, selecting according to TRUE of FALSE. Then, when it gets to the end of the first "layer" in 'a' it re-cycles M. When I tried a[M,] I got a[M,] # Error in a[M, ] : incorrect number of dimensions I infer that it is treating the M as a vector, so there are only 2 dimensions in a[M,] instead of 3. So try: a[M,,] # Error: (subscript) logical subscript too long which is not surprising since one is applying a 15-long selector to the first dimension of 'a', which has only length 3, just as in a[rep(TRUE,15),,] # Error: (subscript) logical subscript too long which, interestingly, differs from a[rep(1,15),,] # (Output, which is what you'd expect, omitted) (Hmm, out of my depth again ... ). Well, maybe two arms is not enough when you need three to swim in these waters; but it seems that one long swishy tail will do nicely. That being said, I still find the water quite murky! Ted. On 27-Sep-11 22:12:26, robin hankin wrote: > thank you Simon. > > I find a[M] working to be unexpected, but consistent with (a close > reading of) Extract.Rd > > Can we reproduce a[,M]? > > [I would expect this to extract a[,j,k] where M[j,k] is TRUE] > > try this: > > >> a <- array(1:30,c(3,5,2)) >> M <- matrix(1:10,5,2) %% 3==1 >> a[M] > [1] 1 4 7 10 11 14 17 20 21 24 27 30 > > This is not doing what I would want a[,M] to do. > > > > > I'll checkout afill() right now.... > > best wishes > > > Robin > > > On Wed, Sep 28, 2011 at 10:39 AM, Simon Knapp <sleepingw...@gmail.com> > wrote: >> a[M] gives the same as your `cobbled together' code. >> >> On Wed, Sep 28, 2011 at 6:35 AM, robin hankin <hankin.ro...@gmail.com> >> wrote: >>> >>> hello everyone. >>> >>> Look at the following R idiom: >>> >>> _a <- array(1:30,c(3,5,2)) >>> _M <- (matrix(1:15,c(3,5)) %% 4) < 2 >>> _a[M,] <- 0 >>> >>> Now, I think that "a[M,]" has an unambiguous meaning (to a human). >>> However, the last line doesn't work as desired, but I expected it >>> to...and it recently took me an indecent amount of time to debug an >>> analogous case. _Just to be explicit, I would expect a[M,] to extract >>> a[i,j,] where M[i,j] is TRUE. _(Extract.Rd is perfectly clear here, >>> and R >>> is >>> behaving as documented). >>> >>> The best I could cobble together was the following: >>> >>> _ind <- which(M,arr.ind=TRUE) >>> _n <- 3 >>> _ind <- >>> cbind(kronecker(ind,rep(1,dim(a)[n])),rep(seq_len(dim(a)[n]),nrow(ind) >>> )) >>> _a[ind] <- 0 >>> >>> >>> but the intent is hardly clear, certainly compared to "a[M,]" >>> >>> I've been pondering how to implement such indexing, and its >>> generalization. >>> >>> Suppose 'a' is a seven-dimensional array, and M1 a matrix and M2 a >>> three-dimensional array (both Boolean). _Then "a[,M1,,M2]" is a >>> natural generalization of the above. _I would want a[,M1,,M2] to >>> extract a[i1,i2,i3,i4,i5,i6,i7] where M1[i2,i3] and M[i5,i6,i7] are >>> TRUE. >>> >>> One would need all(dim(a)[2:3] == dim(M1)) and all(dim(a)[5:7] == >>> dim(M2)) for consistency. >>> >>> Can any R-devel subscribers advise? >>> >>> >>> >>> >>> -- >>> Robin Hankin >>> Uncertainty Analyst >>> hankin.ro...@gmail.com >>> >>> ______________________________________________ >>> R-devel@r-project.org mailing list >>> https://stat.ethz.ch/mailman/listinfo/r-devel >> >> > > > > -- > Robin Hankin > Uncertainty Analyst > hankin.ro...@gmail.com > > ______________________________________________ > R-devel@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel -------------------------------------------------------------------- E-Mail: (Ted Harding) <ted.hard...@wlandres.net> Fax-to-email: +44 (0)870 094 0861 Date: 28-Sep-11 Time: 00:28:37 ------------------------------ XFMail ------------------------------ ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
