QBasic/Files
From Wikibooks, the open-content textbooks collection
In this lesson, we will learn how to create and modify files. In doing so, we will create a portion of a text editor to handle reading a writing files to and from disk - the program won't be complete by the end of this chapter, but will be finished within Advanced Text Output.
Let's start by setting up out main procedure:
'$DYNAMIC ON ERROR GOTO handler ' Prepares the error handler DIM text(50) AS STRING ' Used to contain the text file. maxlines = 50 ' Contains the current size of the buffer. DO CLS 'clears the screen INPUT "Would you like to create a (N)ew file, (L)oad an existing one, or (E)xit the program"; choice$ SELECT CASE UCASE$(choice$) 'UCASE$ converts strings to UPPER CASE CASE "N" 'New file CASE "L" 'Load a file CASE "E" 'Exit CLS END END SELECT LOOP 'returns to the top of the program. handler: errorflag = ERR ' Keep track of the error that occurred. RESUME NEXT ' Proceeds with the next statement.
As you can see, we are using CASE rather than IF. IF statements can sometimes work better than case statements, but for now, we want to avoid spaghetti code (where there are too many GOTO's).
So far, we don't really have much, but it's a start. We've asked the user what they want to do, and finished 1/3 options. Not so shabby when you put it that way!
Contents |
[edit] The OPEN statement
The open statement allows either reading or writing information from the disk. In general, the open statement follows this pattern:
OPEN file$ FOR INPUT AS 1 OPEN file$ FOR OUTPUT AS 2
The file$ determines the filename to use. The FOR portion indicates how the file will be accessed or operated - it may be APPEND, BINARY, INPUT, OUTPUT, and RANDOM. The AS # is the identifier used for the file handle in question - this may be a variable if desired.
[edit] Input and output
When you need to access or write content to a file handle, the PRINT and INPUT statements expect a file handle to appear as the first parameter:
INPUT #1, a$ PRINT #2, a$
In some cases, you need to detect if you are going to reach the end of file - this is performed by the EOF function, which accepts a filehandle that takes input.
[edit] Reading the file from disk
We will now add a subroutine to read the file from disk:
SUB LoadFile SHARED filename$ SHARED lines, maxlines SHARED text() AS STRING SHARED errorflag INPUT "Enter filename: "; filename$ OPEN filename$ FOR INPUT AS 1 IF errorflag <> 0 THEN errorflag = 0 CLOSE PRINT "File not found - press return to continue." INPUT "", a$ EXIT SUB END IF ' Count the number of lines. lines = 0 DO WHILE NOT EOF(1) LINE INPUT #1, l$ lines = lines + 1 LOOP 'Allocate enough space for input. IF maxlines > lines THEN REDIM text(lines + 25) AS STRING maxlines = lines + 25 END IF SEEK #1, 1 ' Rewind to the beginning of the file. ' Read the lines into the buffer FOR cline = 1 TO lines LINE INPUT #1, text(cline) NEXT CLOSE 1 errorflag = 0 END SUB
[edit] Writing a file to the disk
The function for writing a file to disk is easier:
SUB SaveFile (outfile$) SHARED filename$ SHARED lines, maxlines SHARED text() AS STRING SHARED errorflag IF outfile$ = "" THEN LOCATE 1, 1 INPUT "Enter filename: "; outfile$ END IF OPEN outfile$ FOR OUTPUT AS 1 IF errorflag <> 0 THEN errorflag = 0 CLOSE PRINT "Couldn't save file - press return to continue." INPUT "", a$ EXIT SUB END IF ' Write each line to the file FOR cline = 1 TO lines PRINT #1, text(cline) NEXT CLOSE 1 errorflag = 0 filename$ = outfile$ END SUB
