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MET 50

MET 50. WRITING TO AND READING FROM FILES. Files. To read from a file, we still use the READ statement. To write output to a file, we use the WRITE statement. Files. WRITE ( mm , nn ) (things to write). Tells us the FORMAT of what we write. Tells us where we

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MET 50

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  1. MET 50 WRITING TO AND READING FROM FILES

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

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

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

  5. 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 MET 50, FALL 2011, CHAPTER 5 PART 2

  6. 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. MET 50, FALL 2011, CHAPTER 5 PART 2

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

  8. 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) MET 50, FALL 2011, CHAPTER 5 PART 2

  9. 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… MET 50, FALL 2011, CHAPTER 5 PART 2

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

  11. 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! MET 50, FALL 2011, CHAPTER 5 PART 2

  12. 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!!! MET 50, FALL 2011, CHAPTER 5 PART 2

  13. 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 MET 50, FALL 2011, CHAPTER 5 PART 2

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

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

  16. Files text data Looks just like regular characters on the screen IS readable! Read: http://en.wikipedia.org/wiki/Data_file Example…online (/121) MET 50, FALL 2011, CHAPTER 5 PART 2

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

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

  19. 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,Z ditto! MET 50, FALL 2011, CHAPTER 5 PART 2

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

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