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CS1001 Lecture 25

CS1001 Lecture 25. Files Internal Files. File Access. OPEN the file READ from an existing file WRITE to a new file CLOSE the file. OPEN Statement. OPEN ( open-list ) where open-list is: Must include Unit Specifier Also includes: FILE = Clause STATUS = Clause ACTION = Clause

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CS1001 Lecture 25

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  1. CS1001 Lecture 25 • Files • Internal Files

  2. File Access • OPEN the file • READ from an existing file • WRITE to a new file • CLOSE the file

  3. OPEN Statement • OPEN (open-list) where open-list is: Must include Unit Specifier Also includes: • FILE = Clause • STATUS = Clause • ACTION = Clause May include • POSITION = Clause • IOSTAT = Clause May include : • ERR = Clause • ACCESS = Clause • FORM = Clause • RECL = Clause • BLANK = Clause • PAD = Clause • DELIM = Clause

  4. Unit Specifier • Required • May not be 0, 5, or 6 • 0 is standard error output, mapped to unit 6 • 5 is standard input, your keyboard • 6 is standard output, your monitor • Has the form UNIT = integer-expression or simply integer-expression FILE = Clause • FILE = character-expression, where character-expression is the filename e.g., FILE = “file.out”

  5. STATUS = Clause • STATUS = character-expression, where character-expression can have one of the following values: • OLD meaning the file already exists • NEW meaning the file is being created • REPLACE creates a new file to replace an old file • SCRATCH no name for FILE= given, creates a work file that is deleted upon CLOSE • UNKNOWN default if STATUS= omitted e.g., STATUS = “NEW”

  6. IOSTAT = Clause • IOSTAT = status-variable, where status-variable is an integer variable to which is assigned: • 0 if the file is opened successfully • Positive value otherwise, representing the number of the error message in a list found in the system manuals e.g., IOSTAT = iOpenError then later on, check iOpenError

  7. ERR = Clause • ERR = n, where n is the label of an executable statement that is the next statement executed if an error occurs in trying to open the file e.g., ERR = 30 30 PRINT *, “File Open Error = “, OpenError

  8. READ Statement • READ (control-list) input-list • Must have a Unit Specifier • Must have a Format Specifier • Optional ADVANCE =Clause • Optional IOSTAT = Clause • Optional END = Clause • Optional ERR = Clause, same as OPEN • Optional REC = Clause for direct-access files e.g., READ (12, 10) iVariable, rVariable, cVariable 10 FORMAT (I4, F7.2, A12)

  9. END = Clause • END = n, where n is the label of a statement to be executed when the end of a sequential file is encountered e.g., END = 40 40 PRINT *, “End of file reached”

  10. Comment on File Reading • In a file like “student.dat” on page A17, to read in a last name, like “Peters”, you must read in: • All of the lines (records) up to that line • All of the line itself READ (12, 10) iStudNum, cLName, cName, cInit, & cAddress, iPhone, cGender, iClass, cMajor, iCredits, & iGPA 10 FORMAT (I5, A15, A15, A1, A22, I7, A1, I1, A4, I3, I3)

  11. WRITE Statement • WRITE (control-list) output-list • Must have a Unit Specifier • Usually has a Format Specifier • Optional ERR = Clause • Optional IOSTAT = Clause • Optional REC = Clause e.g., WRITE (13, 10) iVar, rVar, cString 10 FORMAT (I4, F7.2, A12)

  12. ENDFILE Statement • ENDFILE writes into the file a special record called and end-of-file record • When encountered by a READ statement: • IOSTAT = clause condition can be detected • END = clause statement can be executed • Once encountered, no more data can be transferred to or from this file unless the file is repositioned using REWIND or BACKSPACE

  13. CLOSE Statement • CLOSE (close-list) where close-list is: • Must include Unit Specifier, same as OPEN • May include IOSTAT = Clause, same as OPEN • May include ERR = Clause, same as OPEN • May include STATUS = Clause • All files that are not closed by means of a CLOSE statement are automatically closed when an END or STOP statement is executed

  14. SUBROUTINE ReadList (Names, Numbers, Active, NumRecords) INTEGER, INTENT(IN) :: NumRecords CHARACTER *20, INTENT(OUT) :: Names(NumRecords) CHARACTER *13, INTENT(OUT) :: Numbers(NumRecords) LOGICAL, INTENT(OUT) :: Active(NumRecords) CHARACTER *20 :: Filename INTEGER :: i ,Status PRINT *, 'Enter filename of input file: ' READ *, Filename OPEN (UNIT=1, ACCESS = 'sequential', FILE = Filename, & FORM = 'Formatted', STATUS = ’old') DO i = 1, NumRecords READ (1, 2, IOSTAT=Status) Names(i), Numbers(i), Active(i) 2 FORMAT (A20, A13,L1) IF (Status <= 0 ) EXIT END DO ENFILE(1) CLOSE (1) END SUBROUTINE ReadList

  15. Internal Files • Sequence of memory locations containing information stored in character form and named by a character variable, array, or array element • Used to convert character information to numeric form • Used to convert character information into a character array • Internal File Example Given cDate = “JULY 4, 1776” cYear = cDate(9:12) READ (UNIT = cYear, FMT = ‘(I4)’) iYear or READ (cYear, ‘(I4)’) iYear or READ (cDate(9:12), ‘(I4)’) iYear or READ (cDate, ‘(8X, I4)’) iYear Each of these takes the last four characters in the string cDate and converts it to an integer format named iYear

  16. OPEN Arguments • Back up slides

  17. ACTION = Clause • ACTION = i-o-action, where i-o-action is a character expression whose value is one of: • “READ” which opens the file for reading only • “WRITE” which opens the file for writing only • “READWRITE” which opens the file for reading and writing e.g., ACTION = “READ”

  18. POSITION = Clause • POSITION = character-expression, where the value of character-expression is: • “REWIND” which positions the file at its initial point (beginning), default for new file • “APPEND” which positions the file at its end • “ASIS” leaves its position unchanged, default for an existing file already open e.g., POSITION = “REWIND”

  19. ACCESS = Clause • ACCESS = access-method, where access-method can be either: • “SEQUENTIAL” meaning a file is accessed sequentially, without defined records • “DIRECT” for record oriented files, where each record has a record number for access e.g., ACCESS = ‘“SEQUENTIAL”

  20. FORM = Clause • FORM = form-specifier, where form-specifier is a character expression that is either: • “FORMATTED” • ‘UNFORMATTED” • If omitted, file is assumed formatted if sequential, unformatted if direct-access e.g., FORM = “FORMATTED”

  21. PAD = Clause • Only applies to FORMATTED files • PAD = character-expression, which can have the values: • “YES”, means the input proceeds as if the actual record were padded with blanks between fields • “NO” e.g., PAD =“YES” is the default

  22. RECL = Clause • RECL = record-length, where record-length is an integer whose value must be positive • Only used for direct-access files and specifies the length of the records in the file • In a formatted file, the record length is the number of characters in a record e.g., RECL = 25

  23. BLANK = Clause • Only applies to FORMATTED files • BLANK = blank-specifier, which can be • “ZERO” • “NULL” • The first causes blanks in numeric fields to be interpreted as zeros, the other causes blanks to be ignored. • BLANK = “NULL” is default

  24. DELIM = Clause • Only applies to FORMATTED files • DELIM = character-expression, which can be: • “APOSTROPHE” is to be used as the delimiter for character strings written to a file • “QUOTE” is to be used as the delimiter for character strings written to a file • “NONE” • DELIM = “NONE” is the default

  25. STATUS = Clause • STATUS = character-expression • “KEEP” may not be used for SCRATCH files. Otherwise the state of existence of the file remains unchanged (not deleted). • “DELETE” is the default if this clause is omitted, however, it depends upon a PERMANENCE property which can be “TEMPORARY” or “PERMANENT” • STATUS is usually not used

  26. READ Arguments • Backup slides

  27. ADVANCE = Clause • ADVANCE = character-expression, which can have the values: • “YES” specifies that output should advance to a new line after the current output has been completed • “NO” does not advance the output • ADVANCE = “YES” is the default

  28. IOSTAT = Clause • IOSTAT = integer-variable, where • Positive value if an error occurs, usually the error number from a list in a manual • Negative value if the end of data occurs but no input error occurs • 0 if neither an input error nor an end of data occurs

  29. REC = Clause • REC = integer-expression, where integer-expression is positive and indicates the number of the record to be read from a direct-access file • Control list may not contain both an END = clause and a REC = clause e.g., REC = iPartNumber

  30. WRITE arguments • Backup slides

  31. File-Positioning Statements • REWIND unit or REWIND position-list • BACKSPACE unit or BACKSPACE position-list • ENDFILE unit or ENDFILE position-list • Where position-list • Must contain unit or UNIT = unit • May contain ERR = clause • May contain IOSTAT = clause

  32. REWIND and BACKSPACE • REWIND positions the file at the beginning of the file’s first record • BACKSPACE positions the file at the beginning of the preceding record or line • Neither has an affect if the file is already at the beginning e.g., REWIND 8 or REWIND (8, IOSTAT = iError)

  33. Not Needed For This Class • INQUIRE • Merging Files • External Sorting • Unformatted Files

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