Is any way to add 2 arrays into one?

问题

Is there any simple univesal way to add 2 arrays into one? In the case below it is not possible simply use C := A + B statement... I would like to avoid making algorhytm for it everytime .

TPerson = record
    Birthday: Tdate;
    Name, Surname:string;
end;

Tpeople = array of TPerson;

var A, B, C:Tpeople;

C:=A+B; // it is not possible

thanx

回答1:

Due to the two string fields in each TPerson record, you can't just use binary "move", since you'll mess the reference counting of string - especially in a multi-threaded environment.

You can do it manually - this is fast and nice:

TPerson = record
  Birthday: TDate;
  Name, Surname: string;
end;

TPeople = array of TPerson;

var A, B, C: TPeople;

// do C:=A+B
procedure Sum(const A,B: TPeople; var C: TPeople);
begin
var i, nA,nB: integer;
begin
  nA := length(A);
  nB := length(B);
  SetLength(C,nA+nB);
  for i := 0 to nA-1 do
    C[i] := A[i];
  for i := 0 to nB-1 do
    C[i+nA] := B[i];
end;

Or you can use our TDynArray wrapper, which has a method for handling such cases:

procedure AddToArray(var A: TPeople; const B: TPeople);
var DA: TDynArray;
begin
  DA.Init(TypeInfo(TPeople),A);
  DA.AddArray(B); // A := A+B
end;

The AddArray method can add a sub-port of the original array:

/// add elements from a given dynamic array
// - the supplied source DynArray MUST be of the same exact type as the
// current used for this TDynArray
// - you can specify the start index and the number of items to take from
// the source dynamic array (leave as -1 to add till the end)
procedure AddArray(const DynArray; aStartIndex: integer=0; aCount: integer=-1);

Note that with such records, it will use the System._CopyRecord RTL function, which is not so optimized for speed. I've written a faster version - see this blog article or this forum thread.

If you use dynamic arrays in functions/procedures, don't forget to use explicitly const or var parameters (as I coded above), otherwise it will make a temporary copy at each call, therefore it may be slow.

回答2:

There is nothing built in that allows dynamic arrays to be concatenated.

You may consider using one of the generic container classes found in Generics.Collections, TList.

In your case you would have 3 instances of TList, say A, B and C. Then you could write

A.Clear;
A.AddRange(B);
A.AddRange(C);

I think this is as close as you can get to what you want with what is delivered out of the box.

If you are prepared to do a bit of coding yourself then you could make use of operator overloading to use the exact syntax you requires. Declare a record containing an array of TPerson with private visibility. You then need to implement an Add operator, a Count property and a default Items[] property. This could be made generic too so you only need write it once.

TTurboArray = record<T>
private
  FItems: array of T;
  //property accessors here
 public
   class operator Add(a, b: TTurboArray<T>): TTurboArray<T>;
   property Count: Integer read GetCount write SetCount;
   property Items[Index: Integer]: T read GetItem write SetItem; default;
end;

This idea can be extended into a very powerful data structure as you see fit.

回答3:

There is a quick-and-dirty way to do this. It is a terrible hack, but it should work and even take care of reference counting:

function ConcatPeople(const A, B: TPeople): TPeople;
var
  Temp: TPeople;
  ALen, BLen: Integer;
begin
  Result := Copy(A);
  BLen := Length(B);
  if BLen = 0 then
    Exit;
  ALen := Length(A);
  Temp := Copy(B);
  SetLength(Result, ALen + BLen);
  Move(Temp[0], Result[ALen], BLen * SizeOf(B[0]));
  FillChar(Temp[0], BLen * SizeOf(B[0]), 0);
end;

In effect, the data in Temp are "swapped" with the empty records in Result, so the balance is maintained and refcounting will keep on working.

Update

For what it is worth: This is aparently the same technique as used in this accepted SO answer and in, e.g. TList<T>.Insert. I had deleted this answer, but I still think it is valid, so I undeleted it again. It could do with a lock around the Move/FillChar block, so no one accesses the items when they are being "swapped". I'll add that.

回答4:

Here's how I handled it, though it required a slight (but hopefully immaterial to you) modification to your original code to use TArray:

(tested in XE2)

uses
  Generics.Collections;

type
  TArrayExt = class(TArray)
    class function Concat<T>(const First, Second: array of T): TArray<T>; overload;
  end;

class function TArrayExt.Concat<T>(const First, Second: array of T): TArray<T>;
var
  i: Integer;
  l: Integer;
begin
  l := Length(First);
  SetLength(Result, l + Length(Second));
  for i := Low(First) to High(First) do
    Result[i] := First[i];
  for i := Low(Second) to High(Second) do
    Result[l + i] := Second[i];
end;

type
  TPerson = record
    Birthday: TDate;
    Name, Surname: String;
  end;

  TPeople = TArray<TPerson>;

var
  A, B, C: TPeople;
begin
  C := TArrayExt.Concat<TPerson>(A, B);

The main difference here is that I use "TArray" rather than "array of TPerson". This can be used for arrays strings, records, etc. I find the main benefit of doing it this way is that it's truly making a copy rather than a move. And I am using the "normal" Delphi functions instead of things like bulk memory copies, which can give me the willies.

Of course, if you were doing this in a tight loop and needed the performance, this way might not be best for you. But I think this is the best for most other situations, especially in terms of maintenance and readability.

(Unless someone posts a comment about how there's some horrible hidden memory leak here. Hopefully not!)

回答5:

You code works fine in the newest version of delphi C := A+B;.

But for dynamic arrays in older versions you can use the function concat. Example:

C := Concat(A, B);

来源：https://stackoverflow.com/questions/7102887/is-any-way-to-add-2-arrays-into-one

标签

arrays

Delphi

concatenation