flipmap <- function(lon,lat,data,dimension="lat") {

# Flip a two-dimensional map (matrix) or a three-dimensional array up side down,
# i.e. first line becomes last, last line becomes first and so on. In other 
# words, flip either latitude or longitude or both dimensions of a map or a 
# three-dimensional array (field), with first two space dimensions p for
# longitude and q for latitude, and third n time dimension.
#
# Description:
#
#      Returns a flipped map (matrix) or three-dimensional array
#
# Usage:
#
#      flipmap(lon,lat,data,dimension)
#
# Arguments:
#
#        lon: vector of longitudes 
#
#        lat: vector of latitudes 
#
#   	 data: A two-dimensional map (matrix) or three-dimensional array of data
#              values. If data is a matrix, it must
#              have the structure of longitude for columns and latitude for
#              rows. If data is a three-dimensional array, it must have the
#              longitude as the first dimension, latitude as the second 
#              dimension and time as the third dimension
#
#  	 dimension: A string indicating which dimension(s) will be flipped.
#                   It must be either "lat" (default), "lon" or "both"
#
#
# Output:
#
#	 $out: flipped map (matrix) or three-dimensional array
#
#	 $lon: longitude vector (flipped appropriately if necessary)
#
#	 $lat: latitude vector (flipped appropriately if necessary)
#
# Details:
#
#      A matrix or array is flipped into a format suitable for plotting. 
#
# Author:
#
#      Dag Johan Steinskog <dag.johan.steinskog@nersc.no> 25 October 2005
#      Caio Coelho <c.a.d.s.coelho@reading.ac.uk>
#      Chris Ferro <c.a.t.ferro@reading.ac.uk>
#
# Examples:
#
#      #Flip a map (matrix)
#      x <- array(1:8,dim=c(2,4))
#      lon <- seq(50,110,20)
#      lat <- seq(-10,10,20)
#      flipmap(lon,lat,x)$out # Defaults flips lat
#      flipmap(lon,lat,x)$lat 
#      flipmap(lon,lat,x)$lon 
#      flipmap(lon,lat,x,"lon")$out
#      flipmap(lon,lat,x,"both")$out
#
#      #Flip a three-dimensional array
#      y <- array(1:8,dim=c(2,2,2))
#      lonnew <- seq(50,70,20)
#      latnew <- seq(-10,10,20)
#      flipmap(lonnew,latnew,y)$out # Defaults flips lat
#      flipmap(lonnew,latnew,y)$lat
#      flipmap(lonnew,latnew,y)$lon
#      flipmap(lonnew,latnew,y,"lon")$out
#      flipmap(lonnew,latnew,y,"both")$out


index<-NULL
if(length(dim(data))==2){
   if(dimension == "lon") {flip <- c(F,T)}
   if(dimension == "lat") {flip <- c(T,F)}
   if(dimension == "both") {flip <- c(T,T)}
   for (i in 1:length(dim(data))){
       index[[i]] <- 1:dim(data)[i]
       if (flip[i]) {index[[i]] <- rev(index[[i]])}
   }
   out<-data[index[[1]],index[[2]]]
   lon<-lon[index[[2]]]
   lat<-lat[index[[1]]]
}
if(length(dim(data))==3){
   if(dimension == "lon") {flip <- c(F,T,F)}
   if(dimension == "lat") {flip <- c(T,F,F)}
   if(dimension == "both") {flip <- c(T,T,F)}
   for (i in 1:length(dim(data))){
       index[[i]] <- 1:dim(data)[i]
       if (flip[i]) {index[[i]] <- rev(index[[i]])}
   }
   out<-data[index[[1]],index[[2]],index[[3]]]
   lon<-lon[index[[2]]]
   lat<-lat[index[[1]]]
}
invisible(list(lon=lon,lat=lat,out=out))
}