xindexfield <- function(y,u,threshold = FALSE) {

  # Description:
  #
  #   Computes the intervals estimator for the extremal index, an index for
  #   time clusters, at each grid point of a three-dimensional array of data
  #   with first two space dimensions (e.g. longitude and latitude) and 
  #   third time dimension
  #
  # Usage:
  #
  #   xindexfield(y,u,threshold=FALSE)
  #
  # Arguments:
  #
  #   y: a three-dimensional array, with first two space dimensions (e.g. 
  #      longitude and latitude) and third time dimension
  #
  #   u: Threshold. Can be either a quantile (e.g. 0.8) or a threshold value
  #      (see definition of theshold below).
  #
  #   threshold: Logical. If FALSE (the default) u must be a quantile value.
  #              If TRUE, then u must be a threshold value.
  #
  # Value:
  # 
  #   Estimate of the extremal index at each grid point.
  #
  # Details:
  #
  # Warning:
  #
  #   The estimator is not constrained to lie in [0,1] and a
  #   default value of 1 is returned if there are fewer than two
  #   extreme values.
  #
  # Authors:
  #
  #   Christopher Ferro <c.a.t.ferro@reading.ac.uk> 3 Jan 2006
  #   Caio Coelho <c.a.d.s.coelho@reading.ac.uk> 
  # 
  # References:
  #
  #   Ferro CAT & Segers J (2003) Inference for clusters of extreme
  #   values. Journal of the Royal Statistical Society B 65, 545-556.
  #
  # Examples:
  #
  #      x <- seq(-20, 20, 5)
  #      y <- seq(30, 60, 5)
  #      dim <- c(length(x), length(y), 100)
  #      data <- array(rnorm(prod(dim),25,2), dim)
  #      xindexfield(data,0.9)
  #      xindexfield(data,28,threshold=TRUE)


apply(y,c(1,2),xindex,u,threshold)

}