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POHow to swap two rows of a two-dimensional array, and why would it work?
 

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  1. POHow to swap two rows of a two-dimensional array, and why would it work?
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    <p><strong>Summarization:</strong></p> <p>Please check the comments below from David, Uwe, and other experts.</p> <p>================================================================================</p> <p>The following code swaps two rows in a two-dimensional, dynamic array of double values. I am wondering: (1) whether the following code is a best practice of swapping two rows of a two-dimensional array? If not, then what is the best practice to do this kind of job? (2) why would the following code work? I mean, isn't two-dimensional array a <strike>continuous</strike> contiguous section of memory? Does the following code work only by luck? Any suggestion is appreciated! </p> <pre><code> unit Unit5; interface uses Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms, Dialogs; type TAADouble = array of array of Double; TForm5 = class(TForm) procedure FormShow(Sender: TObject); private { Private declarations } public { Public declarations } end; var Form5: TForm5; procedure SwapRows(arr: TAADouble; row0, row1: Integer); implementation {$R *.dfm} procedure SwapRows(arr: TAADouble; row0, row1: Integer); var Tmp: Integer; begin {$IFDEF FPC} Tmp := PtrUInt(arr[row0]); PtrUInt(arr[row0]) := PtrUInt(arr[row1]); PtrUInt(arr[row1]) := Tmp; {$ELSE} Tmp := Integer(arr[row0]); Integer(arr[row0]) := Integer(arr[row1]); Integer(arr[row1]) := Tmp; {$ENDIF} end; procedure TForm5.FormShow(Sender: TObject); var tmpArray: TAADouble; I, J: Integer; rowStr: string; begin SetLength(tmpArray, 10, 10); rowStr := ''; for I := 0 to 9 do for J := 0 to 9 do tmpArray[I][J] := I * J; for I := 0 to 9 do begin rowStr := ''; for J := 0 to 9 do rowStr := rowStr + FloatToStr(tmpArray[I][J]) + ' '; OutputDebugString(PWideChar(rowStr)); end; SwapRows(tmpArray, 3, 4); for I := 0 to 9 do begin rowStr := ''; for J := 0 to 9 do rowStr := rowStr + FloatToStr(tmpArray[I][J]) + ' '; OutputDebugString(PWideChar(rowStr)); end; end; end. </code></pre>
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    1. COWhile the above works (at the moment), casting a pointer to an Integer is not safe - this will fail miserably when the 64-bit version arrives... if you are using a recent Delphi, use the NativeInt type, otherwise define your own that you can set to match the pointer size yourself.
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      1. POHow to swap two rows of a two-dimensional array, and why would it work?
      1. USAlistair Ward
 

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