Reading and Writing Files. Keeping Data

Transcription

1 Reading and Writing Files Keeping Data

2 Why do we use files? For permanently storing data. For dealing with information too large to fit in memory.

3 Sequential Access Files Think of files as being stored on tape. Sequential Access means you must pass over all the information in a file to get the stuff you want.

4 Illustration To get from here... Read Head To here... F1 F2 Tape Moves F3 F4 File 5 F6 File 7 We have to read all this.

5 Random Access Files Think of files stored on a disk. Random Access means you can have any information at any time. (Like in RAM!)

6 Illustration File One Move the head from one file to another ignoring everything in between. File Two

7 Random Access Files, Details WE WILL NOT COVER THESE IN THIS CLASS These files use indexes to locate information within them. It means dealing with the indexes, how the disk memory is used, special ways to delete files..

8 Opening Files Questions to Answer: Do you want to read from it, write to it, both? Does the file already exist? What shall we do with the file that already exists? Are we looking at binary data or text data? Where is the file?

9 File Opening A file must first be opened before it can be used. Opening a file does two things: Makes sure its OK to open the file Assigns an identifier to the file. Files stay open until we close them (or the program ends)

10 OK to Open File? You haven t opened it already You aren t violating the file s integrity Somebody else has it open You re already using for a different purpose.

11 Assigning an Identifier Allows us to identify which file to write to and read from. Useful if Multiple Files are Open

12 The Open Statement Open file For Mode As filenumber file is the Full pathname of the file to be opened. Mode is how the file will be used. filenumber is the number that identifies the file.

13 Modes Input Sequential access for reading, starts at beginning Output Sequential access for writing, erases old file and starts at beginning Append Sequential access - doesn t erase the file, starts writing at the end.

14 Two Modes we Won t Use Random Which we won t use in this class - This is the default! Binary Which we also won t use. Opens binary files.

15 The Open Statement, Con t Opening a non-existent file for Input causes an Error! We ll write a special function for handling this. Opening a non-existent file for Output causes it to be created. Opening an existing file for Output causes it to be erased. Opening an existing file for Append causes data to be entered at the end of the existing file.

16 Output to a File Write # filenum, variablelist filenum is the file identifying number variable list is a list of variables. This is the easiest file output function. Use it unless you are forced to use something else.

17 Format These are written out using commas between values and double quotes around strings. Here is a string,19,27.3, String More strings,20,15.2, More Text

18 Illustration of File Writing Dim fleeb As String, norb As Single Dim ski As Integer, i As Integer fleeb = Test1 norb = 98.6 ski = 100 Open file.txt For Output As 1 Write #1, fleeb, norb,, ski Close 1 file.txt Test1,98.6,100

19 Printing to a File Print # filenum, var ; var, var ; var filenumber is from the Open Statement var can be the function Spc(n), for n spaces, Tab(n) for nth column tab, a a variable the semicolon puts no space between items the comma prints items with spaces.

20 Input From a File Input # filenumber, variablelist filenumber is the file to read from Identified in the open statement. variables list is the list of variables to read the value into. numbers get numbers, strings get either double quoted strings or first word. Use this if at all possible.

21 Illustration of File Reading file.txt Test1,98.6,100 Test 2,93.1,100 Quiz 3,18,25 fleeb norb ski Dim fleeb(1 To 3) As String, Dim norb(1 To 3) As Single Dim ski(1 To 3) As Integer Dim i As Integer Open file.txt For Input As 1 For i = 1 to 3 Input #1, fleeb(i), norb(i),ski(i) Next i Close 1

22 Input From a File, Con t Input (n,filenumber filenumber) n n is the number of characters to read in. filenumber is the file to read from. This was identified in the Open statement. This is useful if you must read in a file created with the Print# function.

23 Close File Statement Close filenumber(s) File identification number or numbers. Closes file and removes number assignment to it. Will rewind sequential files to beginning.

24 Closing a File Good Thing Open f for Input As 1 For i = 1 to 10 Input#, a(i), b(i) Next i Close 1 Bad Thing For i = 1 to 10 Open f for Input As 1 Input#, a(i), b(i) Close 1 Next i

25 Formatting Data Write Format must be the same as Read Format to ensure that we have data integrity. Be very careful of this.

26 File Length We don t always know the file length We can use the EOF(filenumber filenumber) function. filenumber is the number you assigned to the file. EOF returns True at the File end. EOF returns False otherwise.

27 Reading an unknown length file. Open file.txt For Input As 1 While(Not EOF(1)) Input#1, fred, wilma Wend Close 1

28 FreeFile Function A useful function when you re opening a lot of files and you re not sure what number you want to assign to each. Identifies a number which has not yet been assigned to a file.

29 Input Files that Don t Exist Requires us to use the error handling VB provides. We ll put this in a function so that it doesn t cause problems. When an Error occurs in VB it looks for an error handler. If none is found, the program ends. (Bad Thing)

30 Using the Error Handler We can create an error handler by inserting the statement: On Error Goto label Where label is a word that occurs later in the code with a comma after it. Special rules apply to these error handlers.

31 Comments We ll tuck our error handler away in a function so that its use will be easy to control. This function will determine if a file exists. If it does it returns true Otherwise it returns false.

32 File_Exists Function Function File_Exists (FileName( as String) As Integer On Error Goto File_error Open FileName For Input As 1 Close 1 If it reaches this line File_Exists = True the file was found Exit Function File_error: If it wasn t found, control is File_Exists = False here via the On Error line. Exit Function End Function