(no subject)

Suppose X is a data.frame with n obs and k vars, all variables are
factors.

tab <- table(X)

containes a k-dim array

I would like to get a list from tab. This list is such that, each
element contain the indexes corresponding to the observations which
are in the same cell of this k-dim array. Of course, only for non
empty cell.

E.g.

set.seed(123)
X <- as.data.frame(matrix(rnorm(5000),100,5))
X$V1 <- cut(X$V1, br=5)
X$V2 <- cut(X$V2, br=5)
X$V3 <- cut(X$V3, br=5)
X$V4 <- cut(X$V4, br=5)
X$V5 <- cut(X$V5, br=5)
tab <- table(X)
which(tab>0) -> cells
length(cells)

[1] 94

thus, of course, 94 cells over 5^5 = 3125 are non empty.
I would like a smart way (without reimplementing table/tabulate) to
get the list of length 94 which contains the indexes of the obs in
each cell
Or, viceversa, a vector of length n which tells, observation by
observation,  which cell (out of the 3125) the observation is in.
stefano

Suppose X is a data.frame with n obs and k vars, all variables are
factors.

tab <- table(X)

containes a k-dim array

I would like to get a list from tab. This list is such that, each
element contain the indexes corresponding to the observations which
are in the same cell of this k-dim array. Of course, only for non
empty cell.

E.g.

set.seed(123)
X <- as.data.frame(matrix(rnorm(5000),100,5))
X$V1 <- cut(X$V1, br=5)
X$V2 <- cut(X$V2, br=5)
X$V3 <- cut(X$V3, br=5)
X$V4 <- cut(X$V4, br=5)
X$V5 <- cut(X$V5, br=5)
tab <- table(X)
which(tab>0) -> cells
length(cells)

[1] 94

thus, of course, 94 cells over 5^5 = 3125 are non empty.
I would like a smart way (without reimplementing table/tabulate) to
get the list of length 94 which contains the indexes of the obs in
each cell
Or, viceversa, a vector of length n which tells, observation by
observation,  which cell (out of the 3125) the observation is in.
stefano

Stefano,

Try this

XX <- as.numeric(X[[1]])
for (i in 2:length(X)) XX <- 10*XX + as.numeric(X[[i]])
split(seq(along=XX), XX)

You can read off the cell from the decimal expansion of the label.
And XX goes from observations to cells.

The hard work is done by unique() under the skin (split makes XX  
into a factor).

Brian

On Wed, 1 Feb 2006, stefano iacus wrote:

Suppose X is a data.frame with n obs and k vars, all variables are
factors.

tab <- table(X)

containes a k-dim array

I would like to get a list from tab. This list is such that, each
element contain the indexes corresponding to the observations which
are in the same cell of this k-dim array. Of course, only for non
empty cell.

E.g.

set.seed(123)
X <- as.data.frame(matrix(rnorm(5000),100,5))
X$V1 <- cut(X$V1, br=5)
X$V2 <- cut(X$V2, br=5)
X$V3 <- cut(X$V3, br=5)
X$V4 <- cut(X$V4, br=5)
X$V5 <- cut(X$V5, br=5)
tab <- table(X)
which(tab>0) -> cells
length(cells)

[1] 94

thus, of course, 94 cells over 5^5 = 3125 are non empty.
I would like a smart way (without reimplementing table/tabulate) to
get the list of length 94 which contains the indexes of the obs in
each cell
Or, viceversa, a vector of length n which tells, observation by
observation,  which cell (out of the 3125) the observation is in.
stefano

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595