($x, $y) = ($y, $x);

#include <stdio.h>
#include <limits.h>

#define HEXIT ( CHAR_BIT / 2 )

unsigned char swap_nibble ( unsigned char x )
{
  return x << HEXIT >> HEXIT << HEXIT | x >> HEXIT;
}

int main ( void )
{
  unsigned char x = 0xEA;

  printf ( "Before: 0x%X\n", x );
  printf ( "After:  0x%X\n", swap_nibble ( x ) );

  return 0;
}

#include <stdio.h>
#include <limits.h>

#define HEXIT ( CHAR_BIT / 2 )

unsigned char swap_nibble ( unsigned char x )
{
  return x << HEXIT | x >> HEXIT;
}

int main ( void )
{
  unsigned char x = 0xEA;

  printf ( "Before: 0x%X\n", x );
  printf ( "After:  0x%X\n", swap_nibble ( x ) );

  return 0;
}

/* my output
Before: 0xEA
After:  0xAE
*/

#include <stdio.h>
#include <limits.h>

#define HEXIT ( CHAR_BIT / 2 )

unsigned char swap_nibble ( unsigned char x )
{
  return x << HEXIT >> HEXIT << HEXIT >> HEXIT << HEXIT >> HEXIT << HEXIT
    | x >> HEXIT << HEXIT >> HEXIT << HEXIT >> HEXIT << HEXIT >> HEXIT;
}

int main ( void )
{
  unsigned char x = 0xEA;

  printf ( "Before: 0x%X\n", x );
  printf ( "After:  0x%X\n", swap_nibble ( x ) );

  return 0;
}

>>> a = 5
>>> b = 4
>>> a, b = b, a
>>> a
4
>>> b
5
>>> a = 'text1'
>>> b = 'text2'
>>> a, b = b, a
>>> a
'text2'
>>> b
'text1'
>>>

<xsl:template name="blah">
    <xsl:param name="x" select="2"/>
    <xsl:param name="y" select="5"/>
    <xsl:param name="swapped" select="0"/>

    ...

    <xsl:choose>
        <xsl:when test="$swapped = 0">
            <!-- First we run this code. -->

            <!-- And say we want to swap x and y! -->

            <!-- Oh no, we can't change their values! Curse!  Recurse!!-->
            <xsl:call-template name="blah">
                <xsl:with-param name="x" select="$y"/>
                <xsl:with-param name="y" select="$x"/>
                <xsl:with-param name="swapped" select="1"/>
            </xsl:call-template>
        </xsl:when>

        <!-- In the spirit of XML, they picked the most verbose name
        out of the choices 'else', 'default', and 'otherwise.' -->
        <xsl:otherwise>
            <!-- Now x and y are swapped! -->

            ...
        </xsl:otherwise>
    </xsl:choose>
</xsl:template>

#include <iostream>
#include <string>

using namespace std;

string printBinary(unsigned char aByte) {  //  prints aByte in binary.
  int moder=128;
  string ret;
  for (int i=0; i<8; i++) {
    if (aByte >= moder) {
      ret+='1';
      aByte-=moder;
    } else
      ret+='0';
    moder /= 2;
  }
  return ret;
}

unsigned char getFromBinary(string buffer) {
  unsigned char ret=0, moder=1;
  for (int i=buffer.length()-1; i>=0; i--) {
    if ((char)buffer[i] == '1')
      ret+=moder;
    moder*=2;
  }
  return ret;
}

unsigned char swapBit(unsigned char byte, int a, int b) {
  string str = printBinary(byte);
  char temp = str[a];
  str[a] = str[b];
  str[b] = temp;

  return getFromBinary(str);
}

int main(int argc, char **argv) {
  unsigned char a = getFromBinary("11101010");
  cout << "A is " <<  (int)a << "\t" << printBinary(a) << endl;

  a = swapBit(a, 4, 5);
  cout << "A is " << (int)a << "\t" << printBinary(a) << endl;

  a = swapBit(a, 2, 3);
  cout << "A is " << (int)a << "\t" << printBinary(a) << endl;

  a = swapBit(a, 3, 4);
  cout << "A is " << (int)a << "\t" << printBinary(a) << endl;

  a = swapBit(a, 1, 2);
  cout << "A is " << (int)a << "\t" << printBinary(a) << endl;

  return 0;
}