Basic Fortran I/O Concepts

Transcription

1 Basic Fortran I/O Concepts LECTURE OUTLINE Free versus Directed I/O Edit Descriptors Carriage Control Numeric Control Character Control Spacing Control Repeat Specifier Read and Write to Files Examples!! Assignment Free vs. List Directed I/O Free Formatting A simplified method for input and output without detailed formatting instructions Output variables and strings are sent to the output device without instructions. An asterisk is used in place of the format specifier The computer formats the output according to internal rules You specify the output, the computer specifies the format Simple, but it provides NO flexibility Free-format example: WRITE(*,*) 'PROJECTILE VELOCITY =', VF free format specifier String label Variable value

2 List Directed Input and Output Provides the flexibility the free formatting doesn't have You specify the exact format of the output Method is a bit tedious, but you have complete control Utilizes a group of edit descriptors Example of List Directed output: WRITE(*,20) VO, VF 20 FORMAT (' ', T15, 'INITIAL PROJECTILE VELOCITY =', F10.4/& &T15, 'FINAL PROJECTILE VELOCITY =', F10.4) Examples: Formatted Write Statements WRITE (*,25) Q, LOAD 100 FORMAT (' THE MASS AND LOAD VALUES ARE ',F8.1,' AND ',F8.1) You are not limited to the format statement, the specifier can occur in numerous configurations: WRITE (*,30) Q, LOAD 30 FORMAT (T20,2F15.3) WRITE (*, '(T20,2F15.3)') Q, LOAD CHARACTER(LEN=35)::TEXT TEXT='(T20,2F15.3)' WRITE (*,TEXT) Q, LOAD!STANDARD FORMAT STATEMENT!FORMAT IN CHARACTER CONSTANT!FORMAT IN CHARACTER VARIABLE

3 Edit Descriptors What do they describe? The alignment of the output The number of decimal places to be written Spacing between the output, both horizontal and vertical Horizontal alignment Carriage Control Carriage control is a nearly obsolete feature of Fortran, but is kept for maximum portability of your code. What it does: Resets printer and announces a new line of text Controls vertical spacing, i.e. single space, double space, new page. Numeric Descriptors Integer Descriptor Real Descriptors for Double Precision I F, E, ES, G D Other Descriptors Function Descriptor Character A Tab to T New line / Grouping ( ) Repetition [coefficient]

4 Carriage Control Character Position: Start New Line Double Space New Page No New Line Initializes Printer for output First character in line to be printed determines line spacing action. (' ') ('0') ('1') ('+') Use on line printers to control paper feed and vertical spacing. Laser printers ignore this convention!! Keep for portability and to avoid problems if carriage control is encountered! Integer Output -- The I Descriptor Provide format for all integer output Format: riw or riw.m Example Source Code: INTEGER :: A=-25, B=5, C=-55, D=11 WRITE (*, 110) A,A+25,B,C,D WRITE (*, 120) A,A+25,B,C,D WRITE (*, 130) A,A+25,B,C,D 110 FORMAT (' ', 5I8)!40 spaces total 120 FORMAT (' ', 2I8.0, 2I8, I10.6 )!42 spaces total 130 FORMAT (' ', 2I8.3, 2I8, I5 )!37 spaces total Output: ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Field Spacing Illustrator

5 Real Output -- The F Descriptor Format: Provide format for all REAL output rfw.d Example Source Code: Note that space is required for decimal points and negative signs. And, if you want to separate data on the same row, best to allow a few extra spaces! Output: REAL :: P=-198.3, Q=9.43, R=787.4 WRITE (*, 110) P, Q, R WRITE (*, 120) P, Q, R 110 FORMAT (' ', 3F7.3 ) 120 FORMAT (' ', 3F10.2 ) ******* ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^Field Spacing Illustrator Real Output -- The E Descriptor Provide exponential notation for all REAL output Format: rew.d w d + 8 Example Source Code: Output: REAL :: N=6.023E23, BW=2.4E10, LIGHT=3.0E8,NEG=-8.0 WRITE (*, 100) N, BW, LIGHT, NEG WRITE (*, 101) N, BW, LIGHT, NEG WRITE (*, 102) N, BW, LIGHT, NEG 100 FORMAT (' ', 4E14.4 ) 101 FORMAT (' ', 4E10.4 ) 102 FORMAT (' ', 4E9.4 ) E E E E E E E E E E E+09********* ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

6 Real Output -- The ES Descriptor Provide SCIENTIFIC notation for all REAL output Format: resw.d w d + 8 Example Source Code: REAL :: N=6.023E23, BW=2.4E10, LIGHT=3.0E8,NEG=-8.0 WRITE (*, 100) N, BW, LIGHT, NEG 100 FORMAT (' ', 4ES14.4 ) Output: E E E E+00 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Horizontal Positioning -- The X and T Descriptor The X Descriptor Format: Provide horizontal spacing, one space at a time. Does not work on ELF90 nx The T Descriptor To advance to a specified field space, similar to a tab character in word processing Format: Tc c=column number

7 Horizontal Positioning -- The X and T Descriptor X Descriptor Example REAL:: MASS=1.529, VOLUME=0.65 DENSITY=MASS/VOLUME WRITE(*, 110) WRITE(*, 120) MASS, VOLUME, DENSITY 110 FORMAT (' TEST RESULTS FROM LABORATORY MEASUREMENT'/& &'MASS, KG VOLUME, CC DENSITY,G/CC) 120 FORMAT (' ',F7.2,5X,F10.2,7X,F12.2)!Or, a better, easier, and ELF90 compatible approach is below: 120 FORMAT (' ',F7.2,F15.2,F19.2) Output: TEST RESULTS FROM LABORATORY MEASUREMENT MASS, KG VOLUME, CC DENSITY,G/CC ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Horizontal Positioning -- The X and T Descriptor T Descriptor Example Output: WRITE(*,23) WRITE(*,19) TEMP1, X1, EXPAN1 WRITE(*,19) TEMP2, X2, EXPAN2 WRITE(*,19) TEMP3, X3, EXPAN3 WRITE(*,19) TEMP4, X4, EXPAN4 WRITE(*,19) TEMP5, X5, EXPAN5 23 FORMAT (' ', T4,'TEMP (oc)', T15, 'LENGTH OF & &BAR (CM)', T35, 'EXPANSION (%)') 19 FORMAT (' ', T5, F6.1, T15, F9.3, T33, F9.3) STOP END TEMP (oc) LENGTH OF BAR (CM) EXPANSION (%) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

8 Character Output -- The A Descriptor Format: Example Source Code: Output: Provide format for all Character output ran CHARACTER(LEN=50)::COURSE COURSE = 'INTRO TO COMPUTING FOR ENG' WRITE(*,100) COURSE 100 FORMAT (' ', A13)!PARTIAL VARIABLE WRITE(*,110) COURSE 110 FORMAT (' ', A50)!FULL VARIABLE INTRO TO COMP INTRO TO COMPUTING FOR ENG ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^Field Spacing Illustrator Reading and Writing to Files Places to store data --avoid reentry each time program is run Save and store output Fortran files are sequential access Key I/O File Statements are: Using Files: Some Essential commands Open a file, assign a unit number, and give status Close file, terminate association with unit number Read data from an established unit number/file. Write data to an established unit number/file. OPEN CLOSE READ WRITE

9 Unit Numbers Unit Number Conventions and Practices I/O number 5 is reserved for input from keyboard [Fortran convention, Used at Rutgers] I/O number 6 is reserved for output to screen [Fortran convention, Used at Rutgers] Pick any other number you like to name your files. 3, 28, 64, Your file will be named accordingly, i.e DATA.F03 The OPEN Statement Format: To associate a file with a I/O number OPEN (open_list) Open_list options UNIT= int_expr Any integer except 5 or 6 FILE= char_expr Any name you want STATUS= char_expr Old, New, Replace ACTION= char_expr Read or Write IOSTAT= int_var Not Input, the computer returns this one: Open successful, then IOSTAT=0 EOF on READ, then IOSTAT < 0 Other Error, IOSTAT > 0 = Error Code

10 Using Files with Fortran90 Fortran 90 has excellent file access capability. One of the most important features is the IOSTAT=clause feature, which enables you to trap file access errors and prevent your program from crashing during execution. An example of the form for IOSTAT is: READ (20,*,IOSTAT=STATUS) where the READ statement accesses data from a disk file associated with the unit number 20, and free-form I/O is utilized as represented by the asterisk, *. The system returns a value for IOSTAT that indicates success or failure in accessing the file. Using Files with Fortran90 The use of this construction is shown well in an example given by Chapman in the textbook. Refer to page 191, example 4-3. The problem has two objectives: To read real values from a data file To detect any errors and the end of file when it occurs.

11 Using Files with Fortran90 PSEUDOCODE!! Get file name from user Assign unit number and open the file Check for errors on open If no open error then read data Continue to check IOSTAT for error flag as data is read If error, exit with error message If end of file, exit with completion message Using Files with Fortran90 Start THE ALGORITHM -- In Flowchart Format Num = 0 READ value READ filename OPEN filename Status = = 0? true Num = Num + 1 false Status > 0? true WRITE 'Error Reading Line' false WRITE 'End of file', num Status = = 0? true WRITE num, value false WRITE 'Error Opening File, Status =' Stop Source: Fortran 90/95 for Scientists and Engineers by Stephen Chapman. Published by McGraw Hill, 1998, page 193.

12 Example Problem [4-3, page 191] THE PROGRAM -- Setup program PROGRAM READ IMPLICIT NONE! DECLARE VARIABLES CHARACTER(LEN=20) :: FILENAME! NAME OF FILE TO OPEN INTEGER :: NVALS = 0! NUMBER OF VALUES READ INTEGER :: STATUS! I/O STATUS REAL :: VALUE! THE REAL VALUE READ IN! GET THE FILE NAME AND ECHO IT BACK TO THE USER. WRITE (*,*) 'PLEASE ENTER INPUT FILE NAME: ' READ (*,*) FILENAME WRITE (*,1000) FILENAME 1000 FORMAT (' ','THE INPUT FILE NAME IS: ', A) Example Problem [4-3, page 191] THE PROGRAM -- Open file and read values! OPEN THE FILE AND CHECK FOR ERRORS ON OPEN. OPEN (UNIT=3, FILE=FILENAME, STATUS=' OLD ', ACTION='READ', & IOSTAT=STATUS ) OPENIF: IF ( STATUS == 0 ) THEN! OPEN WAS OK. READ VALUES. READLOOP: DO READ (3,*,IOSTAT=STATUS) VALUE! GET NEXT VALUE IF ( STATUS /= 0 ) EXIT! EXIT IF NOT VALID NVALS = NVALS + 1! VALID: INCREASE WRITE (*,1010) NVALS, VALUE! ECHO TO SCREEN 1010 FORMAT (' ','LINE ', I6, ': VALUE = ',F10.4) END DO READLOOP! THE WHILE LOOP HAS TERMINATED. WAS IT BECAUSE OF A READ! ERROR OR BECAUSE OF THE END OF THE INPUT FILE?

13 Example Problem [4-3, page 191] THE PROGRAM -- Read has terminated, report why --Finish up READIF: IF ( STATUS > 0 ) THEN! A READ ERROR OCCURRED. TELL USER WRITE (*,1020) NVALS FORMAT ('0','AN ERROR OCCURRED READING LINE ', I6) ELSE! THE END OF THE DATA WAS REACHED. TELL USER. WRITE (*,1030) NVALS 1030 FORMAT ('0','END OF FILE REACHED. THERE WERE ', I6, & ' VALUES IN THE FILE.') END IF READIF ELSE OPENIF WRITE (*,1040) STATUS 1040 FORMAT (' ',' ERROR OPENING FILE: IOSTAT = ', I6 ) END IF OPENIF!CLOSE FILE CLOSE ( UNIT=3 ) STOP END PROGRAM READ Key Fortran90 Programming Issues I/O Format Descriptors Use carriage control by placing the correct descriptor in column one of the output field. Match data type with format specifier type Use asterisk rather than 5 or 6 when indicating default devices Use STATUS='OLD' and ACTION='READ' for input files to protect data. Use STATUS='NEW' for all output files to protect existing files. Use IOSTAT= with file read commands to detect errors and EOF Source: Fortran 90/95 for Scientists and Engineers by Stephen Chapman. Published by McGraw Hill, 1998, page 206.

14 Homework Assignment -- Week 4 Read Chapter 4 -- pp Do problems: 3 (p. 209) -- Topic: Getting Started 9 (p. 210) -- Topic: Real variable format 13 (p. 211) -- Topic: 24 h clock program Assignments are due next week in lecture