Author Topic: Random access for sequential files  (Read 735 times)

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Random access for sequential files
« on: October 16, 2018, 03:54:20 pm »
Add or Edit a line where you want. Sweet because strings are variable length. Here is JB code:
Code: [Select]
'Random access sequential files.txt for JB v2.0 B+ 2018-10-16

fName$ = "Test sequential file.dat"
[editAdd]
cls
call showFile fName$
print
input "(0 to quite) Enter a line number to edit > ";lNum
if lNum = 0 then print "Goodbye!" : end
line input "Enter the the line to edit or add > ";eaLine$
call editAddLine fName$, lNum, eaLine$
goto [editAdd]
end

sub editAddLine fileName$, lineNum, newLine$
    if lineNum = 0 then exit sub  'no good

    'avoid file exist erors
    open fileName$ for append as #1
    close #1

    CRLF$ = chr$(13) + chr$(10)  'constant line ender

    'inhale file
    open fileName$ for input as #writeAtLine
    f$ = input$(#writeAtLine, lof(#writeAtLine))
    close #writeAtLine

    'process: count and record positions of CRLF's in file contents
    if right$(f$, 2) <> CRLF$ then f$ = f$ + CRLF$  'balance line counts by CRLF$ count
    pos = instr(f$, CRLF$) 'find end of first line
    while pos 'find end positions of remaining lines until run out of CRLF's
        if len(NLpos$) = 0 then NLpos$ = str$(pos) else NLpos$ = NLpos$ + " " + str$(pos) 'record crlf positions
        count = count + 1 'count crlf's
        pos = instr(f$, CRLF$, pos + 1) 'get next crlf if there is one
    wend

    'edit a line or add one or many?
    if lineNum > count then 'add line(s) to lineNum spot
        if lineNum = 1 then
            txt$ = newLine$
        else
            txt$ = f$
            while count < lineNum - 1
                txt$ = txt$ + CRLF$
                count = count + 1
            wend
            txt$ = txt$ + newLine$
        end if
    else  'edit the line
        if lineNum = 1 then txt$ = "" else txt$ = mid$(f$, 1, val(word$(NLpos$, lineNum - 1)) + 1)
        txt$ = txt$ + newLine$ + mid$(f$, val(word$(NLpos$, lineNum)))
    end if
    'remove last CRLF
    if right$(txt$, 2) = CRLF$ then txt$ = left$(txt$, len(txt$) - 2)

    'write to file in one go
    open fileName$ for output as #writeAtLine
        print #writeAtLine, txt$;   '(;) prevents extra CRLF
    close #writeAtLine
end sub

sub showFile filename$
    'avoid file exist erors
    open filename$ for append as #1
    close #1
    print filename$;":"
    open filename$ for input as #1
    while eof(#1) = 0
        i = i + 1
        line input #1, fline$
        print right$("   ";str$(i);": ", 6);fline$
    wend
    close #1
end sub