### cvsegments.R: A utility function to generate segments for k-fold### cross-validation.### $Id: cvsegments.R 75 2006-07-17 21:05:34Z bhm $cvsegments <- function(N, k, length.seg = ceiling(N / k),                       type = c("random", "consecutive", "interleaved")) {    ## length.seg overrides k:    if (!missing(length.seg)) k <- ceiling(N / length.seg)    incomplete <- k * length.seg - N    # Number of incomplete segments    complete <- k - incomplete          # Number of complete segments    ## Check arguments:    if (N < k) stop("More segments than observations requested")    ## The idea is to generate a k times length.seg matrix with indices, and    ## use each row as a segment.  If k*length.seg > N, the last element of    ## the N - k*length.seg last rows will be NA.  Any NAs are stripped when    ## the matrix is converted to a list of vectors.    type <- match.arg(type)    switch(type,            random = {               inds <- matrix(c(sample(1:N), rep(NA, incomplete)),                              nrow = length.seg, byrow = TRUE)           },           consecutive = {               if (complete < k) {                   inds <- cbind(matrix(1:(length.seg*complete),                                        nrow = length.seg),                                  rbind(matrix((length.seg*complete+1):N,                                              nrow = length.seg-1), NA))               } else {                   inds <- matrix(1:N, nrow = length.seg)               }           },           interleaved = {               inds <- matrix(c(1:N, rep(NA, incomplete)),                              nrow = length.seg, byrow = TRUE)           }           )    res <- lapply(as.data.frame(inds), function(x) c(na.omit(x)))    attr(res, "incomplete") <- incomplete    attr(res, "type") <- if (length.seg == 1) "leave-one-out" else type    res}