How to read individual bits from an array?

Question

Lets say i have an array dynamically allocated.

int* array=new int[10]

That is 10*4=40 bytes or 10*32=320 bits. I want to read the 2nd bit of the 30th byte or 242nd bit. What is the easiest way to do so? I know I can access the 30th byte using array[30] but accessing individual bits is more tricky.

Answer 1

bool bitset(void const * data, int bitindex) {
  int byte = bitindex / 8;
  int bit = bitindex % 8;
  unsigned char const * u = (unsigned char const *) data;
  return (u[byte] & (1<<bit)) != 0;
}

Answer 2

this is working !

#define GET_BIT(p, n) ((((unsigned char *)p)[n/8] >> (n%8)) & 0x01)

int main()
{
    int myArray[2] = { 0xaaaaaaaa, 0x00ff00ff };
    for( int i =0 ; i < 2*32 ; i++ )
        printf("%d", GET_BIT(myArray, i));
    return 0;
}

ouput :

0101010101010101010101010101010111111111000000001111111100000000

Be carefull of the endiannes !

Answer 3

First, if you're doing bitwise operations, it's usually preferable to make the elements an unsigned integral type (although in this case, it really doesn't make that much difference). As for accessing the bits: to access bit i in an array of n int's:

static int const bitsPerWord = sizeof(int) * CHAR_BIT;
assert( i >= 0 && i < n * bitsPerWord );
int wordIndex = i / bitsPerWord;
int bitIndex = i % bitsPerWord;

then to read:

return (array[wordIndex] & (1 << bitIndex)) != 0;

to set:

array[wordIndex] |= 1 << bitIndex;

and to reset:

array[wordIndex] &= ~(1 << bitIndex);

Or you can use bitset, if n is constant, or vector<bool> or boost::dynamic_bitset if it's not, and let someone else do the work.