CS1001 Lecture 25

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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

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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

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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

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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

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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

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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

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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)

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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

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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)

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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)

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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

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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

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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 (UNIT1, ACCESS 'sequential', FILE
Filename, FORM 'Formatted',
STATUS old') DO i 1, NumRecords
READ (1, 2, IOSTATStatus)
Names(i), Numbers(i), Active(i) 2
FORMAT (A20, A13,L1) IF (Status
lt 0 ) EXIT END DO ENFILE(1)
CLOSE (1) END SUBROUTINE ReadList
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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(912)
• READ (UNIT cYear, FMT (I4)) iYear
• or READ (cYear, (I4)) iYear
• or READ (cDate(912), (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

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OPEN Arguments

• Back up slides

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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

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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

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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

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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

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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

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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

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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

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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

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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

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READ Arguments

• Backup slides

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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

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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

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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

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WRITE arguments

• Backup slides

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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

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REWIND and BACKSPACE

• REWIND positions the file at the beginning of the
  files 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)

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Not Needed For This Class

• INQUIRE
• Merging Files
• External Sorting
• Unformatted Files