extractmap <- function(x, y, data, imonth, iyear, tmonth, tyear ,xmin=min(x), 
                    xmax=max(x), ymin=min(y), ymax=max(y)) {


# Extract a matrix (map) on a specific date from a three-dimensional array 
# containing monthly data with first dimension p (longitude), second dimension
# q (latitude) and third dimension n (time) 
#
# Description:
#
#      Returns the data of the specific date
#
# Usage:
#
#      extractmap(x, y, data, imonth, iyear, tmonth, tyear ,
#                 xmin=min(x),xmax=max(x), ymin=min(y), ymax=max(y))
#
# Arguments:
#
#      x: Vector of longitudes 
#
#      y: Vector of latitudes
#
#      data: Matrix or three-dimensional array of data. Space dimensions of
#            `data' must be `length(x)' by `length(y)' respectively. 
#            Three-dimensional array must have the following dimension order:
#            first dimension for longitude, second dimension for latitude,
#            and third dimention for time
#
#      imonth: Number of the first month of the time series of 'data' 
#
#      iyear:  Number of the first year of the time series of 'data'
#
#      tmonth: Number of the target month to be extracted from the 
#              time series of 'data' 
#
#      tyear:  Number of the target year to be extracted from the 
#              time series of 'data' 
#
#      xmin: Lower bound longitude from where to start extracting data
#
#      xmax: Upper bound longitude up to where to extract data
#
#      ymin: Lower bound latitude from where to start extracting data
#
#      ymax: Upper bound latitude up to where to extract data
#
# Output:
#
#      A list with three components is returned invisibly:
#
#         lon: Vector of longitudes within the selected range
#
#         lat: Vector of latitudes within the selected range
#
#         data: Matrix or three-dimentional array containing the selected data
#
# Authors:
#       
#      Caio Coelho <c.a.d.s.coelho@reading.ac.uk> 3 November 2005
#      Chris Ferro <c.a.t.ferro@reading.ac.uk> 
#
# Examples:
#
#      # Extract data from a three-dimentional array (field)
#      x <- seq(-20, 20, 5)
#      y <- seq(30, 60, 5)
#      dim <- c(length(x), length(y), 100)
#      data <- array(rnorm(prod(dim)), dim)
#      output <- extractmap(x, y,data,1,1948,9,1951)
#      output <- extractmap(x, y,data,1,1948,9,1951,xmin=-12,xmax=12,ymin=32,ymax=58)
#


if (imonth == 1 & tmonth == 1) {t<-((tyear-iyear)*12)+1}
if (imonth == 1 & tmonth != 1) {t<-((tyear-iyear)*12)+tmonth}
if (imonth != 1) {t<-12-imonth+1+((tyear-iyear)*12)+tmonth-12}

xnew <- x
ynew <- y
tnew <- t
xnew <- x[x >= xmin & x <= xmax]
ynew <- y[y >= ymin & y <= ymax]
datanew <- data[match(xnew, x), match(ynew, y), t, drop = TRUE]
cat(paste("Extracted data for month",as.character(tmonth),"of",as.character(tyear),"\n",sep=" "))
invisible(list(lon = xnew, lat = ynew, data = datanew))
}