File Input and Output

Transcription

1 5. Input and Output

2 File Input and Output read(i,j) write(i,j) j is the statement number of format statement. i is the I/O unit or logical unit associated with device or file. non-negative integer from 0 to (the maximum 4-byte integer value). Usually, I/O unit 5 is reserved for default input device, 6 is reserved for default output device, 0 is reserved for error display. If I/O unit i is not associated with any device or file, fort.i will be the external filename.

3 Example: program test_io_unit implicit none real :: pi = write(*,*) 'I/O unit=*', pi write(0,*) 'I/O unit=0', pi write(6,*) 'I/O unit=6', pi write(91,*) 'I/O unit=91', pi write( ,*) 'I/O unit= ', pi end program Output on screen: I/O unit=* I/O unit= I/O unit= Output to two files: fort.91 I/O unit= fort I/O unit=

4 Formats and Formatted WRITE statements Free format WRITE write(*,*) 'The result for iteration ', iter, ' is ', result Output on screen: The result for iteration 21 is Formatted WRITE write(*,100) iter, result 100 format (' The result for iteration ', I3, ' is ', F7.3) Output on screen: The result for iteration 21 is I3 F7.3 format descriptors for the integer variable iter and real variable result

5 Format in FORMAT statement : write(*,120) ' The result for iteration ', iter, ' is ', result 120 format (A26,I3,A4,F7.3) write(*,110) iter, result 110 format (' The result for iteration ', I3, ' is ', F7.3) Format in character constant : write(*,'(a26,i3,a4,f7.3)') & ' The result for iteration ', iter, ' is ', result Format in character variable : character(len=20) :: string string = '(A26,I3,A4,F7.3)' write(*,string) ' The result for iteration ', iter, ' is ', result

6 Format Descriptors I descriptor riw[.m] Example: r: repeat count w: field width m: minimum number of digits to be displayed integer :: m = -12, n = 0, k = write(*,200) m, n, k 200 format(2i5, I10) write(*,201) m, n, k 201 format(2i5.0, I10.8) write(*,202) m, n, k 202 format(2i5.3, I5) * * * * *

7 F descriptor rfw.d r: repeat count w: field width d: number of digits to right of decimal place Example: real :: pi = write(*,101) pi, pi 101 format(f7.3, F10.8)

8 Example: real :: a = -12.3, b =.123, c = write(*,200) a, b, c 200 format(3f10.3) write(*,201) a, b, c 201 format(3f10.2) write(*,202) a, b, c 202 format(2f6.3, F8.3) * * * * * *

9 E descriptor rew.d r: repeat count w: field width d: number of digits to right of decimal place ±0.ddddE±ee (w d+7) Example: a = E6 b = c = -12.3E10 write(*,'(e14.4)') a write(*,'(e14.4)') b write(*,'(e13.6)') c write(*,'(e11.6)') c E E E * * * * * * * * * * *

10 ES descriptor (scientific notation) resw.d r: repeat count w: field width d: number of digits to right of decimal place ±x.dddde±ee (w d+7) Example: a = E6 b = c = -12.3E10 write(*,'(es14.4)') a write(*,'(es14.4)') b write(*,'(es12.6)') c E E * * * * * * * * * * * *

11 A descriptor (for character I/O) ra[w] r: repeat count w: the width of field (is the same as the number of characters) Example: character(len=17) :: string = 'This is a string.' write(*,'(a)') string write(*,'(a20)') string writ(*,'(a6)') string T h i s i s a s t r i n g. T h i s i s a s t r i n g. T h i s i

12 X and T descriptor (for horizontal position) nx n: number of blanks to insert Tc c: column number to go to Example: character(len=10) :: first_name = 'James ' character :: initial = 'R' character(len=16) :: last_name = 'Johnson ' character(len=9) :: class = 'ESOE 2015' integer :: grade = 92 write(*,100) first_name, initial, last_name, grade, class 100 format(a10, 1X, A1, 1X, A10, 4X, I3, T50, A9) James R Johnson 92 ESOE A10 1X 1X A10 4X I3 A9 A1 T50

13 Notes: Repeating groups of format descriptors 320 format(1x, I6, I6, F10.2, F10.2, I6, F10.2, F10.2) 320 format(1x, I6, 2(I6, 2F10.2)) Changing output lines: "/" descriptor i = 4 j = 5 write(*,'(i3,i3)') i, j write(*,'(i3/i3)') i, j

14 5a. More on Input and Output

15 Data Files and File processing READ(i,j) WRITE(i,j) j is the statement number of format statement i is the I/O unit or logical unit associated with device or file. Usually, I/O unit 5 is reserved for default input device, 6 is reserved for default output device, 0 is reserved for error display. If the I/O unit i is not associated with any device or file, fort.i will be the external filename. To use external file with name different from fort.i, we need to select the data file and to read from or write to the file. These statements include: OPEN: associate a file with a I/O unit number CLOSE: end the association of a file with the I/O unit number REWIND: move to the beginning of a file BACKSPACE: move back one record in a file

16 OPEN & CLOSE statements open a file for input: Ex: integer :: ierror real :: a(10) open(unit=8, file='input.dat', status='old', & & action='read', iostat=ierror) read(8,*) (a(i), i = 1,10)! read a(1) to a(10) from input.dat close(unit=8) open a file for output: Ex: integer :: ierror, n_unit=9 real :: a(10) character(len=7) :: data_name='out.dat' open(unit=n_unit, file=data_name, status='new', & & action='write', iostat=ierror) write(n_unit,*) (a(i), i = 1,10)! write a(1) to a(10) to output.dat close(unit=n_unit)

17 OPEN (open_list) commonly used clauses of open_list : OPEN (unit=int_expr, file=char_expr1, status=char_expr2, & & action=char_expr3, iostat=int_var) unit=int_expr to indicate the I/O unit number file=char_expr1 to specify the filename to be input/output status=char_expr2 char_expr2 is one of 'OLD', 'NEW', 'REPLACE', 'SCRATCH', 'UNKNOW' action=char_expr3 char_expr3 is one of 'READ', 'WRITE', 'READWRITE' iostat=int_var if the open statement execute successfully, 0 is assigned to int_var, otherwise position number is assigned.

18 Ex: open(unit=8, file='input.dat', status='old', & & action='read', iostat=ierror) The existing file input.dat is opened and assigned to logical unit 8. status='old' indicates the file already exits; if it does not, an error code is assigned to ierror action='read' the file should be read-only; if an attempt is made to write to the file, an error code is assigned to ierror Ex: integer :: ierror, n_unit=9 character(len=7) :: data_name='out.dat' open(unit=n_unit, file=data_name, status='new', & & action='write', iostat=ierror) A new file OUT.DAT is opened and assigned to logical unit 9. status='new' indicates the file is new; if it already exits, an error code is assigned to ierror. If status='replace', a new file is open for output whether a file by the same name exits or not. action='write' the file should be write-only Ex: open(unit=12, status='scratch', iostat=ierror) status='scratch' means the file is a temporary file; it will be deleted automatically once the file is closed or the program terminates. If the file is for scratch, no file name should be specified in OPEN statement.

19 IOSTAT= clause in READ statement READ(i, j, iostat=int_var) iostat=int_var is not present in READ, the program will abort on an attempt to read beyond the end of a file. iostat=int_var is present in READ: if the READ statement is successfully, a 0 is assigned to the variable int_var; if read beyond the end of a file, a negative number is assigned and the program continues ; if file or format error occurs, a positive number is assigned and the program continues. Ex:! read unknown number of data from a file real :: a(1000), tmp integer :: nvar=0, iopen, iread open(unit=8, file='input.dat', status='old', iostat=iopen) if(iopen == 0) then! iopen=0 means open file successfully do read(8,*,iostat=iread) tmp! iread=0 means read successfully if(iread < 0) exit nvar=nvar+1 a(nvar)=tmp end do end if...

20 ! Open input data file. OPEN ( UNIT=9, FILE=filename, STATUS='OLD', ACTION='READ', & & IOSTAT=status ) Continued on next page... Example: read values from an input data set, sort the data into ascending order PROGRAM sort1!! Read in a input data set, sort it into ascending order using the! selection sort algorithm, and write the sorted data to the! standard output device.! IMPLICIT NONE INTEGER, PARAMETER :: MAX_SIZE = 1000! Maximum input data set size REAL, DIMENSION(MAX_SIZE) :: a! Data array to sort CHARACTER(len=20) :: filename! Input data file name INTEGER :: i! Loop index INTEGER :: iptr! Pointer to smallest value INTEGER :: j! Loop index INTEGER :: nvals = 0! Number of data values to sort INTEGER :: status! I/O status: 0 for success REAL :: temp! Temporary variable for swapping! Get the name of the file containing the input data. WRITE (*,1000) 1000 FORMAT ('Enter the file name with the data to be sorted: ') READ (*,'(A20)') filename

21 ! Was the OPEN successful? fileopen: IF ( status == 0 ) THEN! Read the data DO READ (9, *, IOSTAT=status) temp IF ( status /= 0 ) EXIT nvals = nvals + 1 a(nvals) = temp END DO! Open successful! Exit on end of data file! Number of data count! Save value in array! Sort the data outer: DO i = 1, nvals-1! Find the minimum value in a(i) through a(nvals) iptr = i inner: DO j = i+1, nvals minval: IF ( a(j) < a(iptr) ) THEN iptr = j END IF minval END DO inner! iptr points to the minimum, swap a(iptr) with a(i) if i /= iptr swap: IF ( i /= iptr ) THEN temp = a(i) a(i) = a(iptr) a(iptr) = temp END IF swap END DO outer Continued on next page...

22 ! Now write out the sorted data. WRITE (*,'(1X,A)') 'The sorted output data values are: ' WRITE (*,'(4X,F10.4)') ( a(i), i = 1, nvals ) ELSE fileopen! Else file open failed. Tell user. WRITE (*,1050) status 1050 FORMAT (1X,'File open failed--status = ', I6) END IF fileopen END PROGRAM sort1

23 Exercise Table of Logarithms : Write a Fortran program to generate a table of the base 10 logarithms between 1.0 and 10.9 in steps of 0.1. Output the table to file log_tab.dat. The table should be organized as follows: X. 0 X. 1 X. 2 X. 3 X. 4 X. 5 X. 6 X. 7 X. 8 X

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