WRITING TO AND READING FROM FILES MET 50

Files To read from a file, we still use the READ statement. To write output to a file, we use the WRITE statement. 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 2

Files WRITE (mm, nn) (things to write) 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 3 Tells us the FORMAT of what we write Tells us where we write the output to

Files READ (mm, nn) (things to write) 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 4 Tells us the FORMAT of what we read Tells us where we read the input from

Files Example… WRITE (25, 16) X1, X2, X3 16 FORMAT (1X, 3F10.2) Also… WRITE (mm, nn) X1, X2, X3 “mm” and “nn” must be integer WRITE (*, nn) directs output to screen 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 5

Files Guidance for values of “mm” … In the old days, WRITE (6, nn) - directed output to the printer READ (5, nn) - read input from the screen Caution: the values ‘5’ and ‘6’ might be hard-wired into old code. 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 6

Files Files… A file is like a book. It needs to be opened, closed, flipped thru etc. We have commands for this! 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 7

Files Opening a file… OPEN (UNIT=mm, FILE=“name of file”, STATUS=“OLD or NEW or replace”, ACTION=“READ or WRITE or READWRITE”, POSITION=“REWIND or APPEND or ASIS”, IOSTAT=integer number) 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 8

Files Example… OPEN (UNIT=12, don’t need to write “UNIT” FILE=“temperature_data_1977.dat”, name STATUS=“OLD”, file already exists ACTION=“READWRITE”, usually omit POSITION=“ASIS”,often omit IOSTAT=integer number) see later… 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 9

Files Simplified example… OPEN (12, FILE=“temperature_data_1977.dat”, STATUS=“OLD”, IOSTAT=IOS) 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 10

Files Code example…running this… REAL :: DATE, TEMP add code here to generate some data to write! OPEN (12, FILE=“temperature_data_1977.dat”, STATUS=“NEW”, IOSTAT=IOS) WRITE (12,22) DATE, TEMP 22 FORMAT (1X, 2F12.4) After you compile and run this code, you would find that the file “temperature_data_1977.dat” has appeared in your directory! 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 11

Files And reading from a file? REAL :: DATE, TEMP OPEN (12, FILE=“temperature_data_1977.dat”, STATUS=“OLD”, IOSTAT=IOS) READ (12,22) DATE, TEMP 22 FORMAT (1X, 2F12.4) IF (IOS > 0) THEN PRINT*,’Bad data or missing data with IOS=‘, IOS ENDIF add some code to do something with this data!!! 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 12

Files Two three more things… (1) REWIND command Sometimes we may need to : OPEN a file READ the data READ it all again from the start 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 13

Files Code could include these lines… OPEN (12, file=‘name.dat’, status=‘OLD’) READ (12,100) X,Y,Z 12 FORMAT (bla bla bla) do some calculations REWIND (12) READ (12,100) X,Y,Z do some more calculations CLOSE (12) 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 14

Files (2) Forms in which data is stored Either:text data Or:binary data 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 15

Files text data Looks just like regular characters on the screen IS readable! Read: Example…online (/121) 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 16

Files binary data Looks unintelligible Is NOT! Example…online (/121) 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 17

Files (3) Reading & writing binary data This is NOT formatted! Space-saving solution for massive datasets. 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 18

Files Code would include lines like this… OPEN (12, file=‘name.dat’, status=‘OLD’, form=‘unformatted’) READ (12) X,Y,Z note no FORMAT statement! and WRITE (12) X,Y,Zditto! 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 19

Files Large datasets often arrive in binary form. A new paradigm in atmospheric science is “netCDF” 10/13/2011 MET 50, FALL 2011, CHAPTER 5 PART 2 20