write in a text file at given line

[tsh73]

Creative misuse of file operations.
(inspired by
libertybasiccom.proboards.com/thread/322/on-line
)

Question:
Could we rewrite given line in text file?
Yes, right way:
read file line by line
 on specific line, write new content to new file
 on other lines, write these lines to new file

delete old file (may be BACK IT UP first)
rename new file to old name.

Now,
Could we do it without creating copy of a file?
- well, with restrictions

'tsh73 Oct 2018
'Task: replace line #3 in a file with new text.
'Attempt to do without rewriting whole file
'(OK of new text is shorter or could be truncated)

PRINT "creating data file"
fname$ = "sample.txt"
gosub [create]
gosub [read]
print

nLine=3
newLine$="Yep!"
print "try to rewrite line #";nLine
print "with a short line '";newLine$;"'"
call writeAtLine fname$,nLine,newLine$
'check
gosub [read]
print "=>Short line is padded with spaces"
print

nLine=3
newLine$="Old mcDonald had a ..."
print "with a long line '";newLine$;"'"
call writeAtLine fname$,nLine,newLine$
'check
gosub [read]
print "=>Long line is truncated"
print

nLine=10
newLine$="Not going to be written anyway"
print "try to rewrite non-existing line ";nLine
print "with a long line '";newLine$;"'"
call writeAtLine fname$,nLine,newLine$
'check
gosub [read]
print "=>File not changed"

end

[create]
'create file
open fname$ for output as #1
   for i =1 to 5
       print #1, "Line ";space$(i);i
   next
close #1
print "File ";fname$;" (re) created"
return

[read]
'retrieve file, line by line
print "*reading*"
open fname$ for input as #2
   i = 0
   while not (eof(#2))
       i = i+1
       line input #2, aLine$
       print using("##",i);">";aLine$;"<"
   wend
close #2
print "File ";fname$;" read - ";i;" lines"
return

'---------------------------------------------
'finds line lineNum in sequential file fileName$
'replaces it with newLine$ *inplace*
'    pads it with spaces
'    or truncates it so not to spoil next lines
'if no such line: do nothing
sub writeAtLine fileName$,lineNum,newLine$
   open fileName$ for binary as #writeAtLine    'both input/output
       'skip (lineNum-1) lines
       for i = 1 to lineNum-1
           if eof(#writeAtLine) then [writeAtLine.Exit]
           line input #writeAtLine, aLine$
       next
       startPos=LOC(#writeAtLine)
       if eof(#writeAtLine) then [writeAtLine.Exit]
       line input #writeAtLine, aLine$
       aLen=len(aLine$)
       'rewind to start of the line and rewrite it
       seek #writeAtLine, startPos
       print #writeAtLine, padr$(newLine$,aLen)
[writeAtLine.Exit]
   close #writeAtLine
end sub

'---------------------------------------------
'adds spaces from the left until 'n' symbols
'if n<len(a$) returns left$(a$,n)
function padl$(a$,n)
   padl$ = left$(space$(n-len(a$))+a$,n)
end function

'adds spaces from the right until 'n' symbols
'if n<len(a$) returns left$(a$,n)
function padr$(a$,n)
   padr$ = left$(a$+space$(n-len(a$)),n)
end function


[B+]

Save this file as "this file.txt", run and then check line 3.

'this file.txt for JB v2.0 b+ 2018-10-14
' replace line 3 with something else
'This is line number 3.
fn$ = "this file.txt"
open fn$ for input as #1
while eof(#1) = 0
   line input #1, fline$
   b$ = b$ + fline$ + chr$(13)+chr$(10)
wend
close #1


open fn$ for output as #1
i = 1
while word$(b$, i, chr$(13) + chr$(10)) <> ""
   if i = 3 then
       print #1, "'This is the replacement for line number 3."
   else
       if word$(b$, i, chr$(13) + chr$(10)) <> chr$(13) + chr$(10) then
           print #1, word$(b$, i, chr$(13) + chr$(10))
       else
           print #1, ""
       end if
   end if
   i = i + 1
wend
close #1


Make sure there are 2 blank lines in the edited file, I had to fix for blank lines.

Well it copies the file but it seems more straight forward.

[tsh73]

Wow! Nice idea.
Then if we read file line by line anyway, why not change that line *when*?
So we don't need parse lines from memory.
Here it is in same format as mine
sub writeAtLine fileName$,lineNum,newLine$
   CRLF$= chr$(13) + chr$(10)
   open fileName$ for input as #writeAtLine    'input, bild a file in memory
       txt$="": i=0
       while not(eof(#writeAtLine))
           i=i+1
           line input #writeAtLine, aLine$
           if i = lineNum then aLine$=newLine$  'change line needed
           txt$=txt$+aLine$+CRLF$
       wend
   close #writeAtLine
   open fileName$ for output as #writeAtLine
       print #writeAtLine, txt$;   '(;) prevents extra CRLF
   close #writeAtLine
end sub


[B+]

Yes, even better to write file in one go!

[B+]

OK how about this: take in the file in one gulp, process the file only until reach the string in question using INSTR to find the end of the last line and beginning of next then glue the file together from 3 pieces: Head$ + Middle$ + Tail$ method, then save the file in one gulp.


REM test first line again.
'Test file.bas for JB v 2.0 B+ mod of tsh73 sub 2018-10-16
'see if can load and save file in one gulp, and not have to process whole file
' Test this line 4 again.

call writeAtLine "Test file.bas", 1, "REM test first line again."
print "Done"
end

sub writeAtLine fileName$, lineNum, newLine$
   CRLF$ = chr$(13) + chr$(10)
   open fileName$ for input as #writeAtLine
   f$ = input$(#writeAtLine, lof(#writeAtLine))
   close #writeAtLine

   while i < lineNum
       lastPos = pos
       pos = instr(f$, CRLF$, lastPos + 1)
       if pos then i = i + 1
   wend
   if lineNum = 1 then txt$ = "" else txt$ = mid$(f$, 1, lastPos + 1)
   txt$ = txt$ + newLine$ + mid$(f$, pos)

   open fileName$ for output as #writeAtLine
       print #writeAtLine, txt$;   '(;) prevents extra CRLF
   close #writeAtLine
end sub


EDIT: did find a little problem with first line edits, think I have it fixed.