How to jump to a specific line in a text file and return the line in a string (Views: 28)
Problem/Question/Abstract: I'm trying to write a function that, given a FileName and a line number, returns the entire line in a string. Answer: This technique is useful for high-speed processing. Save the sample program file with a .pas or .dpr file name and compile it with dcc32: {$APPTYPE CONSOLE} uses SysUtils, Classes; function GrabLine(const AFileName: string; ALine: Integer): string; var fs: TFileStream; buf: packed array[0..4095] of Char; bufRead: Integer; bufPos: PChar; lineStart: PChar; tmp: string; begin fs := TFileStream.Create(AFileName, fmOpenRead); try Dec(ALine); bufRead := 0; bufPos := nil; { read the first line specially } if ALine = 0 then begin bufRead := fs.Read(buf, SizeOf(buf)); if bufRead = 0 then raise Exception.Create('Line not found'); bufPos := buf; end else while ALine > 0 do begin { read in a buffer } bufRead := fs.Read(buf, SizeOf(buf)); if bufRead = 0 then raise Exception.Create('Line not found'); bufPos := buf; while (bufRead > 0) and (ALine > 0) do begin if bufPos^ = #10 then Dec(ALine); Inc(bufPos); Dec(bufRead); end; end; { Found the beginning of the line at bufPos... scan for end. Two cases: 1) we'll find it before the end of this buffer 2) it'll go beyond this buffer and into n more buffers } lineStart := bufPos; while (bufRead > 0) and (bufPos^ <> #10) do begin Inc(bufPos); Dec(bufRead); end; { if bufRead is positive, we'll have found the end and we can leave. } SetString(Result, lineStart, bufPos - lineStart); { determine if there are more buffers to process } while bufRead = 0 do begin bufRead := fs.Read(buf, SizeOf(buf)); lineStart := buf; bufPos := buf; while (bufRead > 0) and (bufPos^ <> #10) do begin Inc(bufPos); Dec(bufRead); end; SetString(tmp, lineStart, bufPos - lineStart); Result := Result + tmp; end; finally fs.Free; end; end; function GrabLine2(const s: string; ALine: Integer): string; var sl: TStringList; begin sl := TStringList.Create; try sl.LoadFromFile(s); Result := sl[ALine - 1]; { index off by one } finally sl.Free; end; end; begin Writeln(GrabLine(ParamStr(1), StrToInt(ParamStr(2)))); Writeln(GrabLine2(ParamStr(1), StrToInt(ParamStr(2)))); end. Call it like 'getline testfile.txt 20000', depending on what you call the .pas (or .dpr) file. For large (i.e. tens of megabytes) files, the (rather complex) scanning function easily beats the memory expensive StringList version. |