String.Split works strange when last value is empty

问题

I'd like to split my string to array but it works bad when last "value" is empty. See my example please. Is it bug or feature? Is there any way how to use this function without workarounds?

var
  arr: TArray<string>;

  arr:='a;b;c'.Split([';']); //length of array = 3, it's OK
  arr:='a;b;c;'.Split([';']); //length of array = 3, but I expect 4
  arr:='a;b;;c'.Split([';']); //length of array = 4 since empty value is inside
  arr:=('a;b;c;'+' ').Split([';']); //length of array = 4 (primitive workaround with space)

回答1:

This behaviour can't be changed. There's no way for you to customise how this split function works. I suspect that you'll need to provide your own split implementation. Michael Erikkson helpfully points out in a comment that System.StrUtils.SplitString behaves in the manner that you desire.

The design seems to me to be poor. For instance

Length('a;'.Split([';'])) = 1

and yet

Length(';a'.Split([';'])) = 2

This asymmetry is a clear indication of poor design. It's astonishing that testing did not identify this.

The fact that the design is so clearly suspect means that it may be worth submitting a bug report. I'd expect it to be denied since any change would impact existing code. But you never know.

My recommendations:

Use your own split implementation that performs as you require.
Submit a bug report.

Whilst System.StrUtils.SplitString does what you want, its performance is not great. That very likely does not matter. In which case you should use it. However, if performance matters, then I offer this:

{$APPTYPE CONSOLE}

uses
  System.SysUtils, System.Diagnostics, System.StrUtils;

function MySplit(const s: string; Separator: char): TArray<string>;
var
  i, ItemIndex: Integer;
  len: Integer;
  SeparatorCount: Integer;
  Start: Integer;
begin
  len := Length(s);
  if len=0 then begin
    Result := nil;
    exit;
  end;

  SeparatorCount := 0;
  for i := 1 to len do begin
    if s[i]=Separator then begin
      inc(SeparatorCount);
    end;
  end;

  SetLength(Result, SeparatorCount+1);
  ItemIndex := 0;
  Start := 1;
  for i := 1 to len do begin
    if s[i]=Separator then begin
      Result[ItemIndex] := Copy(s, Start, i-Start);
      inc(ItemIndex);
      Start := i+1;
    end;
  end;
  Result[ItemIndex] := Copy(s, Start, len-Start+1);
end;

const
  InputString = 'asdkjhasd,we1324,wqweqw,qweqlkjh,asdqwe,qweqwe,asdasdqw';

var
  i: Integer;
  Stopwatch: TStopwatch;

const
  Count = 3000000;

begin
  Stopwatch := TStopwatch.StartNew;
  for i := 1 to Count do begin
    InputString.Split([',']);
  end;
  Writeln('string.Split: ', Stopwatch.ElapsedMilliseconds);

  Stopwatch := TStopwatch.StartNew;
  for i := 1 to Count do begin
    System.StrUtils.SplitString(InputString, ',');
  end;
  Writeln('StrUtils.SplitString: ', Stopwatch.ElapsedMilliseconds);

  Stopwatch := TStopwatch.StartNew;
  for i := 1 to Count do begin
    MySplit(InputString, ',');
  end;
  Writeln('MySplit: ', Stopwatch.ElapsedMilliseconds);
end.

The output of a 32 bit release build with XE7 on my E5530 is:

string.Split: 2798
StrUtils.SplitString: 7167
MySplit: 1428

回答2:

The following is very similar to the accepted answer but i) it is a helper method and ii) it accepts an array of separators.

The method takes about 30% longer than David's for these reasons, but may be useful anyway.

program ImprovedSplit;

{$APPTYPE CONSOLE}

uses
  System.SysUtils;

type
  TStringHelperEx = record helper for string
  public
    function SplitEx(const Separator: array of Char): TArray<string>;
  end;

var
  TestString : string;
  StringArray : TArray<String>;


{ TStringHelperEx }

function TStringHelperEx.SplitEx( const Separator: array of Char ): TArray<string>;
var
  Str : string;
  Buf, Token : PChar;
  i, cnt : integer;
  sep : Char;
begin
  cnt := 0;
  Str := Self;
  Buf := @Str[1];
  SetLength(Result, 0);

  if Assigned(Buf) then begin

    for sep in Separator do begin
      for i := 0 to Length(Self) do begin
        if Buf[i] = sep then begin
          Buf[i] := #0;
          inc(cnt);
        end;
      end;
    end;

    SetLength(Result, cnt + 1);

    Token := Buf;
    for i := 0 to cnt do begin
      Result[i] := StrPas(Token);
      Token := Token + Length(Token) + 1;
    end;

  end;
end;

begin
  try
    TestString := '';
    StringArray := TestString.SplitEx([';']);
    Assert(Length(StringArray) = 0, 'Failed test for Empty String');

    TestString := 'a';
    StringArray := TestString.SplitEx([';']);
    Assert(Length(StringArray) = 1, 'Failed test for Single String');

    TestString := ';';
    StringArray := TestString.SplitEx([';']);
    Assert(Length(StringArray) = 2, 'Failed test for Single Separator');

    TestString := 'a;';
    StringArray := TestString.SplitEx([';']);
    Assert(Length(StringArray) = 2, 'Failed test for Single String + Single End-Separator');

    TestString := ';a';
    StringArray := TestString.SplitEx([';']);
    Assert(Length(StringArray) = 2, 'Failed test for Single String + Single Start-Separator');

    TestString := 'a;b;c';
    StringArray := TestString.SplitEx([';']);
    Assert(Length(StringArray) = 3, 'Failed test for Simple Case');

    TestString := ';a;b;c;';
    StringArray := TestString.SplitEx([';']);
    Assert(Length(StringArray) = 5, 'Failed test for Start and End Separator');

    TestString := '0;1;2;3;4;5;6;7;8;9;0;1;2;3;4;5;6;7;8;9;0;1;2;3;4;5;6;7;8;9;0;1;2;3;4;5;6;7;8;9';
    StringArray := TestString.SplitEx([';', ',']);
    Assert(Length(StringArray) = 40, 'Failed test for Larger Array');

    TestString := '0;1;2;3;4;5;6;7;8;9;0;1;2;3;4;5;6;7;8;9;0,1,2,3,4,5,6,7,8,9,0;1;2;3;4;5;6;7;8;9';
    StringArray := TestString.SplitEx([';', ',']);
    Assert(Length(StringArray) = 40, 'Failed test for Array of Separators');

    Writeln('No Errors');

  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;

  Writeln('Press ENTER to continue');
  Readln(TestString);

end.

来源：https://stackoverflow.com/questions/28410901/string-split-works-strange-when-last-value-is-empty

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