Bit Maipulations

Most hated area in programming. Anyways here you go few important examples.

1) given 32 bit integer, swap ith and jth positions
swap(int n,int i,int j)
{
    if( (n & 1<<i)>>i ^ (n & (i<<j))>>j) ) // if bits i and j are different
    {
        n^= 1<<i;
        n^= 1<<j;
    }
    return n;
}

2) check 9th bit ser or not
return (number & (position - 1 ) ;

3) To find the number of zeros in an integer
int zerocount(int n)
{
    int ctr  = 0 ;
    while (n>0) {
        ctr = ctr + !(n & 1 ) ;
        n = n << 1 ;
    }
    return ctr ;
}

4) ip string to integer
uint ip2int(char *str)
{
    uint x = 0 ;
    char *tok = strtok(str, ".") ;
    for (i=3; tok != NULL; i--) {
        ans |= atoi(tok) << (8*i)) ;
        tok = strtok(tok, ".") ;
    }
    return tok ;
}

5) MSB of a number
int IsMSB(int x)
{
    if (x & 0x8000)
        printf("msb = 1") ;
    else
        printf("msb = 0") ;
}

6) count bits
int count(uint x)
{
    int i ;
    while (x) {
        x = x & (x-1) ;
        i++ ;
    }
    return i ;
}

7) result = a?b:c
 result = !!a * b + !a * c;

8) #define mysize(x) ((char*) &x+1) - ((char*) &x)  ;

9) multiply by power of 2
n << 2 ;

10) set a bit
unsigned char a = a  | (1<<n) ;

11) clear a bit
unsigned char b = b & ~(1<<n) ;

12) toggle a bit
unsigned char c = c ^ (1<<n) ;

13) test a bit
unsigned d = c & (1<<n) ;

14) right most bit
uchar right = val & 0xff ;
uchar left = (val>8) & 8 ;