如何使用Delphi读取文本文件中的最后一行
我需要读取一些非常大的文本文件中的最后一行（从数据中获取时间戳）。TStringlist是一种简单的方法，但它返回内存不足错误。我试图使用seek和blockread，但是缓冲区中的字符都是无意义的。这与unicode有关吗如何使用Delphi读取文本文件中的最后一行,delphi,ascii,delphi-xe2,pascal,Delphi,Ascii,Delphi Xe2,Pascal,我需要读取一些非常大的文本文件中的最后一行（从数据中获取时间戳）。TStringlist是一种简单的方法，但它返回内存不足错误。我试图使用seek和blockread，但是缓冲区中的字符都是无意义的。这与unicode有关吗 Function TForm1.ReadLastLine2(FileName: String): String; var FileHandle: File; s,line: string; ok: 0..1;
Function TForm1.ReadLastLine2(FileName: String): String;
var
FileHandle: File;
s,line: string;
ok: 0..1;
Buf: array[1..8] of Char;
k: longword;
i,ReadCount: integer;
begin
AssignFile (FileHandle,FileName);
Reset (FileHandle); // or for binary files: Reset (FileHandle,1);
ok := 0;
k := FileSize (FileHandle);
Seek (FileHandle, k-1);
s := '';
while ok<>1 do begin
BlockRead (FileHandle, buf, SizeOf(Buf)-1, ReadCount); //BlockRead ( var FileHandle : File; var Buffer; RecordCount : Integer {; var RecordsRead : Integer} ) ;
if ord (buf[1]) <>13 then //Arg to integer
s := s + buf[1]
else
ok := ok + 1;
k := k-1;
seek (FileHandle,k);
end;
CloseFile (FileHandle);
// Reverse the order in the line read
setlength (line,length(s));
for i:=1 to length(s) do
line[length(s) - i+1 ] := s[i];
Result := Line;
end;
只是想到了一个新的解决方案 同样，可能会有更好的，但这是我想到的最好的
function GetLastLine(textFilePath: string): string;
var
list: tstringlist;
begin
list := tstringlist.Create;
try
list.LoadFromFile(textFilePath);
result := list[list.Count-1];
finally
list.free;
end;
end;
您的字符类型是两个字节，所以缓冲区是16个字节。然后使用blockread将sizeof（buffer）-1字节读入其中，并检查前2字节字符是否等于#13 sizeof（buffer）-1是不可靠的（这个-1来自哪里？），其余的都是有效的，但只有当您的输入文件是utf16时才有效 同时，每次读取8（或16）个字符，但只比较一个字符，然后再次进行搜索。这也不是很合乎逻辑
如果您的编码不是utf16，我建议您将缓冲区元素的类型更改为ansichar，并删除-1。作为对kopiks建议的响应，我找到了使用TFilestream的方法，它可以与简单的测试文件一起工作，尽管在各种csv文件上使用它时可能会有一些进一步的问题。而且，我也不认为这是最有效的方法
procedure TForm1.Button6Click(Sender: TObject);
Var
StreamSize, ApproxNumRows : Integer;
TempStr : String;
begin
if OpenDialog1.Execute then begin
TempStr := ReadLastLineOfTextFile(OpenDialog1.FileName,StreamSize, ApproxNumRows);
// TempStr := ReadFileStream('c:\temp\CSVTestFile.csv');
ShowMessage ('approximately '+ IntToStr(ApproxNumRows)+' Rows');
ListBox1.Items.Add(TempStr);
end;
end;
Function TForm1.ReadLastLineOfTextFile(const FileName: String; var StreamSize, ApproxNumRows : Integer): String;
const
MAXLINELENGTH = 256;
var
Stream: TFileStream;
BlockSize,CharCount : integer;
Hash13Found : Boolean;
Buffer : array [0..MAXLINELENGTH] of AnsiChar;
begin
Hash13Found := False;
Result :='';
Stream := TFileStream.Create(FileName, fmOpenRead or fmShareDenyWrite);
StreamSize := Stream.size;
if StreamSize < MAXLINELENGTH then
BlockSize := StreamSize
Else
BlockSize := MAXLINELENGTH;
// for CharCount := 0 to Length(Buffer)-1 do begin
// Buffer[CharCount] := #0; // zeroing the buffer can aid diagnostics
// end;
CharCount := 0;
Repeat
Stream.Seek(-(CharCount+3), 2); //+3 misses out the #0,#10,#13 at the end of the file
Stream.Read( Buffer[CharCount], 1);
Result := String(Buffer[CharCount]) + result;
if Buffer[CharCount] =#13 then
Hash13Found := True;
Inc(CharCount);
Until Hash13Found OR (CharCount = BlockSize);
ShowMessage(Result);
ApproxNumRows := Round(StreamSize / CharCount);
end;
程序TForm1.按钮6单击（发送方：TObject）；
变量
StreamSize，ApproxNumRows：整数；
TempStr：字符串；
开始
如果是OpenDialog1.Execute，则开始
TempStr:=ReadLastLineOfTextFile（OpenDialog1.FileName、StreamSize、ApproxNumRows）；
//TempStr:=ReadFileStream（'c:\temp\CSVTestFile.csv'）；
ShowMessage（'大约'+IntToStr（近似numrows）+'行'）；
ListBox1.Items.Add（TempStr）；
结束；
结束；
函数TForm1.ReadLastLineOfTextFile（常量文件名：String；var StreamSize，ApproxNumRows:Integer）：String；
常数
MAXLINELENGTH=256；
变量
流：TFileStream；
块大小，字符数：整数；
Hash13Found：布尔型；
缓冲区：AnsiChar的数组[0..MAXLINELENGTH]；
开始
Hash13Found:=False；
结果：=''；
Stream:=TFileStream.Create（文件名、fmOpenRead或fmShareDenyWrite）；
StreamSize:=Stream.size；
如果StreamSizeReadLn- CR/LF aka#13#10=^M^J-DOS/Windows风格
- CR不带LF-仅#13=^M-经典MacOS文件
- LF不带CR-just#10=^J-UNIX风格，包括MacOS版本10
uses System.IOUtils, System.Math, System.Classes;
type FileChar = AnsiChar; FileString = AnsiString; // for non-Unicode files
// type FileChar = WideChar; FileString = UnicodeString;// for UTF16 and UCS-2 files
const FileCharSize = SizeOf(FileChar);
// somewhere later in the code add: Assert(FileCharSize = SizeOf(FileString[1]);
function ReadLastLine(const FileName: String): FileString; overload; forward;
const PageSize = 4*1024;
// the minimal read atom of most modern HDD and the memory allocation atom of Win32
// since the chances your file would have lines longer than 4Kb are very small - I would not increase it to several atoms.
function ReadLastLine(const Lines: TStringDynArray): FileString; overload;
var i: integer;
begin
Result := '';
i := High(Lines);
if i < Low(Lines) then exit; // empty array - empty file
Result := Lines[i];
if Result > '' then exit; // we got the line
Dec(i); // skip the empty ghost line, in case last line was CRLF-terminated
if i < Low(Lines) then exit; // that ghost was the only line in the empty file
Result := Lines[i];
end;
// scan for EOLs in not-yet-scanned part
function FindLastLine(buffer: TArray<FileChar>; const OldRead : Integer;
const LastChunk: Boolean; out Line: FileString): boolean;
var i, tailCRLF: integer; c: FileChar;
begin
Result := False;
if Length(Buffer) = 0 then exit;
i := High(Buffer);
tailCRLF := 0; // test for trailing CR/LF
if Buffer[i] = ^J then begin // LF - single, or after CR
Dec(i);
Inc(tailCRLF);
end;
if (i >= Low(Buffer)) and (Buffer[i] = ^M) then begin // CR, alone or before LF
Inc(tailCRLF);
end;
i := High(Buffer) - Max(OldRead, tailCRLF);
if i - Low(Buffer) < 0 then exit; // no new data to read - results would be like before
if OldRead > 0 then Inc(i); // the CR/LF pair could be sliced between new and previous buffer - so need to start a bit earlier
for i := i downto Low(Buffer) do begin
c := Buffer[i];
if (c=^J) or (c=^M) then begin // found EOL
SetString( Line, @Buffer[i+1], High(Buffer) - tailCRLF - i);
exit(True);
end;
end;
// we did not find non-terminating EOL in the buffer (except maybe trailing),
// now we should ask for more file content, if there is still left any
// or take the entire file (without trailing EOL if any)
if LastChunk then begin
SetString( Line, @Buffer[ Low(Buffer) ], Length(Buffer) - tailCRLF);
Result := true;
end;
end;
function ReadLastLine(const FileName: String): FileString; overload;
var Buffer, tmp: TArray<FileChar>;
// dynamic arrays - eases memory management and protect from stack corruption
FS: TFileStream; FSize, NewPos: Int64;
OldRead, NewLen : Integer; EndOfFile: boolean;
begin
Result := '';
FS := TFile.OpenRead(FileName);
try
FSize := FS.Size;
if FSize <= PageSize then begin // small file, we can be lazy!
FreeAndNil(FS); // free the handle and avoid double-free in finally
Result := ReadLastLine( TFile.ReadAllLines( FileName, TEncoding.ANSI ));
// or TEncoding.UTF16
// warning - TFIle is not share-aware, if the file is being written to by another app
exit;
end;
SetLength( Buffer, PageSize div FileCharSize);
OldRead := 0;
repeat
NewPos := FSize - Length(Buffer)*FileCharSize;
EndOfFile := NewPos <= 0;
if NewPos < 0 then NewPos := 0;
FS.Position := NewPos;
FS.ReadBuffer( Buffer[Low(Buffer)], (Length(Buffer) - OldRead)*FileCharSize);
if FindLastLine(Buffer, OldRead, EndOfFile, Result) then
exit; // done !
tmp := Buffer; Buffer := nil; // flip-flop: preparing to broaden our mouth
OldRead := Length(tmp); // need not to re-scan the tail again and again when expanding our scanning range
NewLen := Min( 2*Length(tmp), FSize div FileCharSize );
SetLength(Buffer, NewLen); // this may trigger EOutOfMemory...
Move( tmp[Low(tmp)], Buffer[High(Buffer)-OldRead+1], OldRead*FileCharSize);
tmp := nil; // free old buffer
until EndOfFile;
finally
FS.Free;
end;
end;
使用System.IOUtils、System.Math、System.class；
类型FileChar=AnsiChar；FileString=AnsiString；//对于非Unicode文件
//类型FileChar=WideChar；FileString=UnicodeString；//对于UTF16和UCS-2文件
const FileCharSize=SizeOf（FileChar）；
//在代码后面的某个地方添加：Assert（FileCharSize=SizeOf（FileString[1]）；
函数ReadLastLine（constfilename:String）：FileString；重载；转发；
常量页面大小=4*1024；
//最现代HDD的最小读取原子和Win32的内存分配原子
//因为你的文件有超过4Kb的行的可能性非常小，我不会把它增加到几个原子。
函数ReadLastLine（常量行：TStringDynArray）：FileString；重载；
varⅠ：整数；
开始
结果：=''；
i:=高（行）；
如果i''，则退出；//我们得到了行
Dec（i）；//跳过空的重影行，以防最后一行被CRLF终止
如果i=低（缓冲区））和（缓冲区[i]=^M），则单独或在LF之前开始//CR
公司（tailCRLF）；
结束；
i:=高（缓冲）-最大（OldRead，tailCRLF）；
如果i-Low（Buffer）<0，则退出；//没有要读取的新数据-结果将与以前类似
如果OldRead>0，则Inc（i）；//CR/LF对可以在新缓冲区和以前的缓冲区之间切片-因此需要稍微开始一点
uses System.IOUtils, System.Math, System.Classes;
type FileChar = AnsiChar; FileString = AnsiString; // for non-Unicode files
// type FileChar = WideChar; FileString = UnicodeString;// for UTF16 and UCS-2 files
const FileCharSize = SizeOf(FileChar);
// somewhere later in the code add: Assert(FileCharSize = SizeOf(FileString[1]);
function ReadLastLine(const FileName: String): FileString; overload; forward;
const PageSize = 4*1024;
// the minimal read atom of most modern HDD and the memory allocation atom of Win32
// since the chances your file would have lines longer than 4Kb are very small - I would not increase it to several atoms.
function ReadLastLine(const Lines: TStringDynArray): FileString; overload;
var i: integer;
begin
Result := '';
i := High(Lines);
if i < Low(Lines) then exit; // empty array - empty file
Result := Lines[i];
if Result > '' then exit; // we got the line
Dec(i); // skip the empty ghost line, in case last line was CRLF-terminated
if i < Low(Lines) then exit; // that ghost was the only line in the empty file
Result := Lines[i];
end;
// scan for EOLs in not-yet-scanned part
function FindLastLine(buffer: TArray<FileChar>; const OldRead : Integer;
const LastChunk: Boolean; out Line: FileString): boolean;
var i, tailCRLF: integer; c: FileChar;
begin
Result := False;
if Length(Buffer) = 0 then exit;
i := High(Buffer);
tailCRLF := 0; // test for trailing CR/LF
if Buffer[i] = ^J then begin // LF - single, or after CR
Dec(i);
Inc(tailCRLF);
end;
if (i >= Low(Buffer)) and (Buffer[i] = ^M) then begin // CR, alone or before LF
Inc(tailCRLF);
end;
i := High(Buffer) - Max(OldRead, tailCRLF);
if i - Low(Buffer) < 0 then exit; // no new data to read - results would be like before
if OldRead > 0 then Inc(i); // the CR/LF pair could be sliced between new and previous buffer - so need to start a bit earlier
for i := i downto Low(Buffer) do begin
c := Buffer[i];
if (c=^J) or (c=^M) then begin // found EOL
SetString( Line, @Buffer[i+1], High(Buffer) - tailCRLF - i);
exit(True);
end;
end;
// we did not find non-terminating EOL in the buffer (except maybe trailing),
// now we should ask for more file content, if there is still left any
// or take the entire file (without trailing EOL if any)
if LastChunk then begin
SetString( Line, @Buffer[ Low(Buffer) ], Length(Buffer) - tailCRLF);
Result := true;
end;
end;
function ReadLastLine(const FileName: String): FileString; overload;
var Buffer, tmp: TArray<FileChar>;
// dynamic arrays - eases memory management and protect from stack corruption
FS: TFileStream; FSize, NewPos: Int64;
OldRead, NewLen : Integer; EndOfFile: boolean;
begin
Result := '';
FS := TFile.OpenRead(FileName);
try
FSize := FS.Size;
if FSize <= PageSize then begin // small file, we can be lazy!
FreeAndNil(FS); // free the handle and avoid double-free in finally
Result := ReadLastLine( TFile.ReadAllLines( FileName, TEncoding.ANSI ));
// or TEncoding.UTF16
// warning - TFIle is not share-aware, if the file is being written to by another app
exit;
end;
SetLength( Buffer, PageSize div FileCharSize);
OldRead := 0;
repeat
NewPos := FSize - Length(Buffer)*FileCharSize;
EndOfFile := NewPos <= 0;
if NewPos < 0 then NewPos := 0;
FS.Position := NewPos;
FS.ReadBuffer( Buffer[Low(Buffer)], (Length(Buffer) - OldRead)*FileCharSize);
if FindLastLine(Buffer, OldRead, EndOfFile, Result) then
exit; // done !
tmp := Buffer; Buffer := nil; // flip-flop: preparing to broaden our mouth
OldRead := Length(tmp); // need not to re-scan the tail again and again when expanding our scanning range
NewLen := Min( 2*Length(tmp), FSize div FileCharSize );
SetLength(Buffer, NewLen); // this may trigger EOutOfMemory...
Move( tmp[Low(tmp)], Buffer[High(Buffer)-OldRead+1], OldRead*FileCharSize);
tmp := nil; // free old buffer
until EndOfFile;
finally
FS.Free;
end;
end;