Program read_gridded

       integer temp(36,72)
       integer i
       integer imonth,jmonth,iyear,jyear
       integer datachk
       integer openfile
       integer loopcnt
       real lat,lon
       character*110 infile,outfile

C      INITIALIZE VARIABLES
       datachk = 0
       openfile = 0
       loopcnt = 0

C      ASK USER FOR INPUT GRID SET 
       write(*,*) 'Enter gridded dataset filename. '
       read(*,10) infile
C      OPEN INPUT FILE
       open(10,file=infile,status='old',err=500)
       openfile = 1

C      GRIDDED FILENAME NOT VALID
 500   do while(openfile .eq. 0)
         write(*,*) 'Cannot open ',infile(1:lnblnk(infile))
         write(*,*) 'Reenter gridded dataset filename.'
         read(*,10) infile
         open(10,file=infile,status='old',err=500)
         openfile = 1
       enddo

C      ASK USER FOR OUTPUT FILENAME
       write(*,*) 'Enter output filename for gridded subset. '
       read(*,10) outfile
C      OPEN OUTPUT FILE
       open(11,file=outfile)

C      ASK USER FOR MONTH AND YEAR TO EXTRACT
       write(*,*) 'Enter month to extract',
     *'- integer format (1=>Jan, 2=>Feb, etc.)'
       read(*,40) imonth
       write(*,*) 'Enter year to extract'
       read(*,40) iyear

 10    format(a110)
 40    format(i6)

C      LOOP OVER ALL MONTHS AND YEARS UNTIL FIND REQUESTED DATA
       do while(datachk .ne. 1)
         read(10,100,end=900) jmonth, jyear
         loopcnt = loopcnt + 1
C      IF FOUND, THEN READ IN GRIDDED DATA
         if(imonth .eq. jmonth .and. iyear .eq. jyear) then
           write(*,50) imonth, iyear
 50        format(' Reading gridded data for month ', i2.2,
     *' and year ',i4)
          
          do i=1,36
            do j=1,6
              read (10,20) (temp(i,k),k=j*12-11,j*12)
            enddo
          enddo
C      SET BOOLEAN TO TRUE
           datachk = 1
C      NOT CORRECT MONTH AND YEAR, SO SKIP GRIDDED DATA FIELDS
         else
           if(loopcnt.eq.1) then
             write(*,110) imonth,iyear
 110   format(' Searching for month ', i2.2, ' and year ',i4)
           endif
          do i=1,36
            do j=1,6
              read(10,*)
             enddo
           enddo
         endif
        enddo

 100   format(2i6)

C      WRITE DATA TO OUTPUT FILE
       write(*,*)'Writing output data to file'
       do i = 1,36
        if(i.le.18) lat = 92.5 - i * 5.0
        if(i.gt.18) lat = ((i-18) * -5.0) + 2.5
        do j = 1,72
          if(temp(i,j) .ne. -9999) then
            if(j.le.36) lon = (182.5 - (j * 5.0)) * (-1)
            if(j.gt.36) lon = ((j-36) * 5.0) - 2.5
            write(11,30) lat,lon,temp(i,j)/100.
          endif
        enddo
       enddo

C      CLOSE INPUT AND OUTPUT FILES
       close(10,err=540)
       close(11,err=560)

       GO TO 999

 20    format(12i7)
 30    format(f5.1,2x,f6.1,2x,f6.2)

 540   write(*,*) 'Cannot close ',infile(1:lnblnk(infile))
 560   write(*,*) 'Cannot close output file' 
 900   write(*,*) 'Requested Data Not Available!!!!'
       write(*,*) 'Sorry - Try Again. Exiting Program!!!!'

 999   STOP
       END