Question

How to swap all odd and even bits in Pascal

Answer 1

program BitManipulation;

// Prints the 8-bit binary representation of an integer 
procedure PrintBinary(value: LongInt);
var
  i: Integer;
begin
  for i := 7 downto 0 do
    Write((value shr i) and 1);
  Writeln;
end;

// Swaps odd and even bits in a 32-bit integer 
function SwapOddAndEvenBits(n: LongInt): LongInt;
var
  oddBits, evenBits: LongInt;
begin
  // (binary: 101010...) to isolate odd bits 
  oddBits := n and $AAAAAAAA;

  // (binary: 010101...) to isolate even bits 
  evenBits := n and $55555555;

  Write('oddBits:  ');
  PrintBinary(oddBits);

  Write('evenBits: ');
  PrintBinary(evenBits);

  // Right-shift odd bits by 1 to move them to even positions 
  oddBits := oddBits shr 1;

  // Left-shift even bits by 1 to move them to odd positions 
  evenBits := evenBits shl 1;

  Write('oddBits Right-shift: ');
  PrintBinary(oddBits);

  Write('evenBits Left-shift: ');
  PrintBinary(evenBits);

  // Combine shifted bits 
  SwapOddAndEvenBits := oddBits or evenBits;
end;

var
  n: Integer;
  result: LongInt;

begin
  n := 90;

  // Original number in binary 
  PrintBinary(n);

  result := SwapOddAndEvenBits(n);

  // Final result in binary 
  PrintBinary(result);
end.



(*
run:

01011010
oddBits:  00001010
evenBits: 01010000
oddBits Right-shift: 00000101
evenBits Left-shift: 10100000
10100101

*)

 

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