extractwindow <- function(x, y, data, imonth, iyear, month1, year1,
                         month2, year2 ,xmin=min(x), 
                         xmax=max(x), ymin=min(y), ymax=max(y)) {

# Extract field of monthly data for a given time period from a three-dimensional
# array  with first dimension p (longitude), second dimension q (latitude) and third 
# dimension n (time) 
#
# Description:
#
#      Returns field of data for a given time period
#
# Usage:
#
#      extractwindow(x, y, data, imonth, iyear, month1, year1,
#                 month2, year2 ,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'
#
#      month1: Number of the month from where to start extracting data
#
#      year1:  Number of the year from where to start extracting data
#
#      month2: Number of the month up to where to extract data
#
#      year2:  Number of the year up to where to extract data
#
#      xmin: Lower bound longitude from where to start extracting data
#            Default is the minimum value of x
#
#      xmax: Upper bound longitude up to where to extract data
#            Default is the maximum value of x
#
#      ymin: Lower bound latitude from where to start extracting data
#            Default is the minimum value of y
#
#      ymax: Upper bound latitude up to where to extract data
#            Default is the maximum value of y
#
#
# Output:
#
#      A list with three components is returned invisibly:
#
#         x: Vector of longitudes for the extracted data
#
#         y: Vector of latitudes for the extracted data
#
#         data: Extracted three-dimensional array (field) 
#
#
# 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-dimensional 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 <- extractwindow(x, y,data,1,1948,3,1948,5,1949)
#      output <- extractwindow(x, y,data,1,1948,3,1948,5,1949,xmin=-12,xmax=12,ymin=32,ymax=58)
#      output <- extractwindow(x, y,data,9,1948,11,1948,5,1949,xmin=-12,xmax=12,ymin=32,ymax=58)
#


if (imonth == 1 & month1 == 1) {
t1<-((year1-iyear)*12)+1

if (month2 ==1){
t2<-((year2-iyear)*12)+1
}
else{
t2<-((year2-iyear)*12)+month2
}

}

if (imonth == 1 & month1 != 1) {
t1<-((year1-iyear)*12)+month1

if (month2 ==1){
t2<-((year2-iyear)*12)+1
}
else{
t2<-((year2-iyear)*12)+month2
}

}

if (imonth != 1) {
t1<-12-imonth+1+((year1-iyear)*12)+month1-12
t2<-12-imonth+1+((year2-iyear)*12)+month2-12
}



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), t1:t2, drop = FALSE]
cat(paste("Extracted data from month",as.character(month1),"of",as.character(year1),"to month",as.character(month2),"of",as.character(year2),"\n",sep=" "))
invisible(list(x = xnew, y = ynew, data = datanew))

}