We need not to do anything if a number is positive. We want to change only negative numbers. Since negative numbers are stored in 2’s complement form, to get the absolute value of a negative number we have to toggle bits of the number and add 1 to the result.

For example -2 in a 8 bit system is stored as follows 1 1 1 1 1 1 1 0 where leftmost bit is the sign bit. To get the absolute value of a negative number, we have to toggle all bits and add 1 to the toggled number i.e, 0 0 0 0 0 0 0 1 + 1 will give the absolute value of 1 1 1 1 1 1 1 0. Also remember, we need to do these operations only if the number is negative (sign bit is set).

Method 1
1) Set the mask as right shift of integer by 31 (assuming integers are stored using 32 bits).

mask = n>>31

For negative numbers, above step sets mask as 1 1 1 1 1 1 1 1 and 0 0 0 0 0 0 0 0 for positive numbers. Add the mask to the given number.

 mask + n

3) XOR of mask +n and mask gives the absolute value.

 (mask + n)^mask

Implementation:

#include <stdio.h>
#define CHAR_BIT 8
 
/* This function will return absoulte value of n*/
unsigned int getAbs(int n)
{
  int const mask = n >> (sizeof(int) * CHAR_BIT - 1);
  return ((n + mask) ^ mask);
}
 
/* Driver program to test above function */
int main()
{
  int n = -6;
  printf("Absoute value of %d is %u", n, getAbs(n));
 
  getchar();
  return 0;
}
Run on IDE

Method 2:
1) Set the mask as right shift of integer by 31 (assuming integers are stored using 32 bits).

 mask = n>>31

2) XOR the mask with number

 mask ^ n

3) Subtract mask from result of step 2 and return the result.

(mask^n) - mask

Implementation:

/* This function will return absoulte value of n*/
unsigned int getAbs(int n)
{
  int const mask = n >> (sizeof(int) * CHAR_BIT - 1);
  return ((n ^ mask) - mask);
}

On machines where branching is expensive, the above expression can be faster than the obvious approach, r = (v < 0) ? -(unsigned)v : v, even though the number of operations is the same.