Question

I have 2 or more dynamic string array that fill with some huge data , i want to merge this 2 array to one array , i know i can do it with a for loop like this :

var
  Arr1, Arr2, MergedArr: Array of string;
  I: Integer;
begin
  // Arr1:= 5000000 records
  // Arr2:= 5000000 records

  // Fill MergedArr by Arr1
  MergedArr:= Arr1;

  // Set length of MergedArr to length of ( Arra1 + Arr2 )+ 2
  SetLength(MergedArr, High(Arr1)+ High(Arr2)+2);

  // Add Arr2 to MergedArr
  for I := Low(Arr2)+1 to High(Arr2)+1 do
    MergedArr[High(Arr1)+ i]:= Arr2[i-1];
end;

but it is slow on huge data , is there faster way like copy array memory data ?

Answer 1

You can use built-in Move function which moves a block of memory to another location. Parameters are source and target memory blocks and size of data to be moved.

Because you are copying strings, source arrays must be destroyed after the merging by filling them with zeroes. Otherwise refcounts for strings will be all wrong causing havoc and destruction later in the program.

var
  Arr1, Arr2, MergedArr: Array of string;
  I: Integer;
begin
  SetLength(Arr1, 5000000);
  for I := Low(Arr1) to High(Arr1) do
    Arr1[I] := IntToStr(I);

  SetLength(Arr2, 5000000);
  for I := Low(Arr2) to High(Arr2) do
    Arr2[I] := IntToStr(I);

  // Set length of MergedArr to length of ( Arra1 + Arr2 )+ 2
  SetLength(MergedArr, High(Arr1)+ High(Arr2)+2);

  // Add Arr1 to MergedArr
  Move(Arr1[Low(Arr1)], MergedArr[Low(MergedArr)], Length(Arr1)*SizeOf(Arr1[0]));

  // Add Arr2 to MergedArr
  Move(Arr2[Low(Arr2)], MergedArr[High(Arr1)+1], Length(Arr2)*SizeOf(Arr2[0]));

  // Cleanup Arr1 and Arr2 without touching string refcount.
  FillChar(Arr1[Low(Arr1)], Length(Arr1)*SizeOf(Arr1[0]), 0);
  FillChar(Arr2[Low(Arr2)], Length(Arr2)*SizeOf(Arr2[0]), 0);

  // Test
  for I := Low(Arr1) to High(Arr1) do begin
    Assert(MergedArr[I] = IntToStr(I));
    Assert(MergedArr[I] = MergedArr[Length(Arr1) + I]);
  end;

  // Clear the array to see if something is wrong with refcounts
  for I := Low(MergedArr) to High(MergedArr) do
    MergedArr[I] := '';
end;