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Published on March 9, 2014

Author: fyjordan9

Source: slideshare.net

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Basic Scientific Programming File Processing

File Processing    Applications involving large data sets can be processed more conveniently if the data is stored in a file. The data in the file to be used as an input file must be arranged in a form suitable for reading by a read statement. These values are read during program execution, just like data entered by the user from the keyboard.

Opening Files  form OPEN ( open_list ) opens the file associated with specified unit number so that Input/Output can take place.

Open List  Open_list includes: 1) a unit specifier indicating a unit number connected to the file being opened. The unit specifier has the form described for the write statement. 2) FILE = Character_expression giving the name of the file being opened.

Open List  3) Status = character_expression where character expression is one of the following “old”,”new”, or “replace”. *Old means the file already exists. *New means the file does not yet exist and is being created. *Replace creates a new file, replacing the old one if one already exists, and changes its status to old

Open List  Action = i-o-action where i-o-action is a character expression whose value is one of the following: “Read”: file opened for reading only “Write”: file opened for writing only “Readwrite”: file opened for both reading and writing.

Open List  Position = character_expression where the character expression is on of “Rewind”: position the file at its initial point. “Append”: position the file at the end of file. “ASIS”: leaves the position unchanged..

Open List  IOSTAT = Status_variable where status variable is an integer variable to which the value zero is assigned if the file is opened successfully and a positive value is assigned otherwise.

Ex:  To open a file named info.dat from which data values are to be read: open(unit=12,FILE=“info.dat”,status=“old”,& action=“read”, Position=“rewind”,& IOSTAT = OpenStauts) where openstatus is an integer variable equal to zero if the file is successfully opened or some positive value if the file cannot be open.

 If the program is to used with various data files, it’s better not to assign the file name directly but to use a character variable. Character(12):: filename Write(*,’(1X,A)’,ADVANCE=“NO”) & “Enter name of data file” read *, filename open(unit=12,FILE=filename,status=“old”,& action=“read”, Position=“rewind”,& IOSTAT = OpenStauts)

  To open a new file named Report to which values are to be written; open(unit=13,File=“Report”,status=“new”,& action=“write”, IOSTAT= openstatus) To write values to the file, we use the write statement Write(13,’(1X,I3)’) Code

Closing Files  The CLOSE statement has a function opposite that of the OPEN statement and is used to disconnect a file from its unit number. Form close (unit_number)

Notes   After a close statement is executed, the closed file may be reopened by using an open statement; the same unit number may be connected to it or a different on may be used. All files that are not explicitly closed by means of a close statement are automatically closed when and End or Stop statement is executed.

File Input/Output   Once a file has been connected to a unit number, data can be written to or form that file using the general WRITE and READ statements. A general read statement may contain an IOSTAT= clause to detect an end-offile condition or an input error.

  IOSTAT = status_variable, Status_variable is assigned    A positive value if an input error occurs. A negative value if the end of data is encountered but no input errors occurs. Zero if neither an input error not the end of data occurs.

Ex:  Do Read(12,*,IOSTAT= Instat) pressure if (Instat >0) stop “ input error” if (Instat <0) exit !end of file ! Otherwise continue processing data sum = sum + pressure End Do

 An alternative method of detecting an end-of-file condition is to use an END=clause in the control list of a general read statement. Form END= statement_number.

Ex:  Do Read(12,*,End=20) pressure sum = sum + pressure End Do 20 Continue . . .

File Positioning Statements   Rewind Statement: Rewind unit_number positions the file at the beginning. Backspace Statement: Backspace unit_number positions the file at the beginning of the preceding line.

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