Bit Array in C++

Question

When working with Project Euler problems I often need large (> 10**7) bit array\'s.

My normal approach is one of:

bool* sieve = new bool[N];

bool sieve[         


        

Answer 1

A 'bool' type isn't stored using only 1 bit. From your comment about the size, it seems to use 1 entire byte for each bool.

A C like way of doing this would be:

uint8_t sieve[N/8]; //array of N/8 bytes

element of array is:

result = sieve[index / 8] || (1 << (index % 8)); 

or

result = sieve[index >> 3] || (1 << (index & 7));

set 1 in array:

sieve[index >> 3] |= 1 << (index & 7);