Fortran Read Last Line From File
In general writing in an existing file will destroy that file. The simplest and most robust solution is to write a copy of the file, inserting/substituting the new values as you go along. If the file can be treated as a direct-access (formatted) file you can in fact overwrite parts of it without damaging the rest.
The format string * will cause Fortran will use 'default' printing. Repeating the last format descriptor. And the file can be read again from the start. Each Fortran read statement, by default, reads a list of values and then advances to the beginning of the next line. Think of read as moving a cursor through the input file as it works. So your statement. Read(100,*) test does what you expect when the numbers in the input file are on separate lines. Read a specific line from a file. Fortran's file reading counts a null line as. Note that this code assumes that the last character in the file is the line.
But that is a tricky route: it requires that all records as viewed by the program have the same length (if you are stubborn enough, I guess you could open the file with a record length of 1, thus allowing you to rewrite the file at the smallest granularity). Regards, Arjen. Quoting I can write a copy of the input file no problem.
But how do I replace specific, small parts of it, on various lines, with text from somewhere else. Each line contains multiple values corresponding with multiple parameters, but I only need to replace one value on a given line: how can I specify the position on the line to begin the write? The level of file control you seek is straightforward, but mostly beyond the limited capabilities of Fortran file i/o (which is way behind the times, language-wise). If you are able to sacrifice portability for functionality, you can easily do all your file i/o using Win32 API functions which provide exactly the needed level of granular control. Here are some sample wrapper functions, which illustrate read/write operations wherein an arbitrary amount of data, denominated in bytes, can be positioned at any offset in the file: SUBROUTINE rw_file (rwmode, ihandl, nbytes, loc_pointer, offset) IMPLICIT NONE CHARACTER(LEN=1), INTENT(IN):: rwmode INTEGER(HANDLE), INTENT(IN):: ihandl INTEGER, INTENT(IN):: nbytes, loc_pointer INTEGER, INTENT(IN), OPTIONAL:: offset INTEGER:: nact!
Fortran Read Text From File
Position pointer if offset is provided IF (PRESENT(offset)) nact = SetFilePointer (ihandl, offset, NULL,FILE_BEGIN) IF (rwmode == 'R') THEN IF (.NOT.ReadFile (ihandl, &! File handle loc_pointer, &! Address of data nbytes, &!
Byte count to read LOC(nact), &! Actual bytes read NULL_OVERLAPPED)) THEN! Read error END IF ELSE IF (.NOT.WriteFile(ihandl, &! File handle loc_pointer, &! Address of data nbytes, &! Byte count to write LOC(nact), &! Actual bytes written NULL_OVERLAPPED)) THEN!write error END IF END IF END SUBROUTINE rw_file SUBROUTINE Set_File_Pointer (ihandl, offset, truncate) IMPLICIT NONE INTEGER(HANDLE), INTENT(IN):: ihandl INTEGER, INTENT(IN):: offset LOGICAL, INTENT(IN), OPTIONAL:: truncate INTEGER:: rslt rslt = SetFilePointer (ihandl, MAX0(offset,0), NULL, FILE_BEGIN) IF (PRESENT(truncate)) rslt = SetEndOfFile (ihandl) END SUBROUTINE Set_File_Pointer.
Arjen, Paul - thanks for the assistance. I originally began attempting this task in python, which I thought would be able to handle it without too much trouble.
I have yet to get anywhere on this problem using python though. That being said, I am obviously open to other options besides Fortran, though I frankly do not have the time to familiarize myself with another language right now - I have just spent three weeks trying to come to terms with DOS and BASH scripting as well as Python.
This is a link to the python forum I originally posted this question on - it contains the input file I am talking about as an attachment, and some conversation that clarifies just what I am trying to achieve. I am very surprised by how seemingly difficult this is turning out to be - all I want is to be able to pinpoint a position in a text file, based on co-ordinates of line number and position on that line, and write the value of a variable over whatever is at that point. '.all I want is to be able to pinpoint a position in a text file, based on co-ordinates of line number and position on that line, and write the value of a variable over whatever is at that point' If each line (ie, record) in the file is the same length, presumably with a terminal, then any specific line+column location is a simple calculation of (n-1)*(recordlength + 2) + column_position, and you can simply open the file with that offset and (over)write as needed. In the more general case where the file has lines of varying length, again each with a terminal, the easy calculation will not work and you will have to read the entire file into a buffer array of records (ie, lines), find/change the substring of interest, and finally rewrite the entire file line by line, sending only the actual filled length of each record +. Quoting I am very surprised by how seemingly difficult this is turning out to be - all I want is to be able to pinpoint a position in a text file, based on co-ordinates of line number and position on that line, and write the value of a variable over whatever is at that point. Declare variables to hold a datarecord and a record count initialise the record count to zero open the input file open the output file read a record from the input file into the datarecord did we get end-of-file? If 'yes' then go to 'done' increment record counter is this the record, or one of the records, we wish to modify?
If 'yes' then find in the data record the specificsubstring we wish to modify (hint use INDEX? Unlessyou already knowthe start and length/end position of the substring) replace the substring of datarecord containing that data with the new data endif write the datarecord to the output file go back and read the next datarecord 'done' close both files Les.
Fortran Read Write Format
Contents • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • [ ] This program uses OS QSAM I/O macros (OPEN,CLOSE,GET,PUT,DCB). * Read a file line by line READFILE CSECT SAVE (14,12) save registers on entry PRINT NOGEN BALR R12,0 establish addressability USING *,R12 set base register ST R13,SAVEA+4 link mySA->prevSA LA R11,SAVEA mySA ST R11,8(R13) link prevSA->mySA LR R13,R11 set mySA pointer OPEN (INDCB,INPUT) open the input file OPEN (OUTDCB,OUTPUT) open the output file LOOP GET INDCB,PG read record CLI EOFFLAG,C'Y' eof reached? BE EOF PUT OUTDCB,PG write record B LOOP EOF CLOSE (INDCB) close input CLOSE (OUTDCB) close output L R13,SAVEA+4 previous save area addrs RETURN (14,12),RC=0 return to caller with rc=0 INEOF CNOP 0,4 end-of-data routine MVI EOFFLAG,C'Y' set the end-of-file flag BR R14 return to caller SAVEA DS 18F save area for chaining INDCB DCB DSORG=PS,MACRF=PM,DDNAME=INDD,LRECL=80, * RECFM=FB,EODAD=INEOF OUTDCB DCB DSORG=PS,MACRF=PM,DDNAME=OUTDD,LRECL=80, * RECFM=FB EOFFLAG DC C'N' end-of-file flag PG DS CL80 buffer YREGS END READFILE [ ] 'path/to/file' f:open (.
Cr ) f:eachline f:close [ ]. 
Works with: version Any - tested with release. File:./Read_a_file_line_by_line.a68 #!/usr/local/bin/a68g --script # FILE foobar; INT errno = open (foobar, 'Read_a_file_line_by_line.a68', stand in channel ); STRING line; FORMAT line fmt = $gl$; PROC mount next tape = ( REF FILE file ) BOOL: ( print ( 'Please mount next tape or q to quit' ); IF read char = 'q' THEN done ELSE TRUE FI ); on physical file end (foobar, mount next tape ); on logical file end (foobar, ( REF FILE skip ) BOOL: done ); FOR count DO getf (foobar, (line fmt, line ) ); printf ( ($g (0 ) ': '$, count, line fmt, line ) ) OD; done: SKIP.
Output: prompt$./readlines 10 lines in readlines.bac [ ] 10 OPENIN 'foo.txt' 20 WHILE NOT EOF 30 LINE INPUT# 9,i$ 40 PRINT i$ 50 WEND [ ] The tape recorder interface does not support fragmented reads, because tape recorder start and stop is not atomic, (and a leadin is required for tape input). However, the microdrive does support fragmented reads. In the following example, we read a file line by line from a file on microdrive 1. 10 REM open my file for input 20 OPEN #4;'m';1;'MYFILE': REM stream 4 is the first available for general purpose 30 INPUT #4; LINE a$: REM a$ will hold our line from the file 40 REM because we do not know how many lines are in the file, we need an error trap 50 REM to gracefully exit when the file is read. (omitted from this example) 60 REM to prevent an error at end of file, place a handler here 100 GOTO 30 [ ] This takes account on the blank lines, because FOR ignores blank lines when reading a file. @ off rem delayed expansion must be disabled before the FOR command.