function m = meanslices(fpat, ilist, ns, nps)

% Function m = meanslices(ilist, ns, nps)
% 
%
% INPUTS
% fpat     string containing file pattern
% ilist    list of iteration values
% ns       number of slices to process at at time
% nps      number of values per slice
%
% OUTPUTS
% m        output array of dimension ns*nps


if nargin ~= 4
  disp('Error: wrong number of arguments')
end

deltatT = 1200;                % model time step size (s)
tavefreq = 259200;             % averaging period (s)
startDate = datenum(1992,1,1); % model integration starting date

oldyear = -1;
newyear = oldyear;

is = 1;
for is = 1:ns
  
  disp(['Processing slice:  ' sprintf('%d',is)]);
  nm = 0;
  m = zeros(nps,1);
  
  i = 1;
  for i = 1:length(ilist)
    
    iter = ilist(i);
    str = datestr(startDate + (iter*deltatT-tavefreq/2)/60/60/24);
    dv = datevec(startDate + (iter*deltatT-tavefreq/2)/60/60/24);
    newyear = dv(1);
    
    fname = sprintf(fpat, ilist(i));    
    disp(['  ' fname '  ' str]);
    fid = fopen(fname, 'r', 'ieee-be');
    offset = (is - 1)*nps*4;
    fseek(fid, offset, 'bof');
    tmp = fread(fid, nps, 'real*4');
    fclose(fid);
    
    if (newyear ~= oldyear) && (nm > 0)
      fnout = sprintf('ave-%d', oldyear);
      disp(['  ==>  Writing file "' fnout '"']);
      fido = fopen(fnout, 'a', 'ieee-be');
      % fseek(fido, offset, 'bof');
      m = m / nm;
      fwrite(fido, m, 'real*4');
      fclose(fido);
      m = zeros(nps,1);
      nm = 0;
    end
    m = m + tmp;
    nm = nm + 1;
    
    oldyear = newyear;
    
  end
end