Re[4]: [Dev-C++] using char* as bitfield

[<ca...@kr...>]

hi Per,

Tuesday, September 30, 2003, 11:25:00 AM, you wrote:
> First:
> sizeof(mybits) is just size of pointer - not size of the array in bytes or
> bits.
yes, my fault. corrected. ;-)

> mybits[bit/8] |= 1 << (bit&3);

> isn't so complicated. Each character contains 8 bits. So by dividing bit
> with 8, I get which character the bit is stored in.
that was clear.

> bit&3 will use the 3 least significant bits for a numeric range of 0-7.
understood.

> 1 << (bit&3) will then access one of the 8 bits in the character.
ok. with you.

everything seems clear to me now. but the out put of my current code,
will be:

mybits: 0000 0000 0000 0000
mybits: 0400 0000 0000 0000

this does mean that the first byte changed from

(bin) 00000000 to
(bin) 00000100

right?
this should be

(bin) 0100000
(hex) 40

since I changed the second bit of all, right?

somewhere I made a mistake, I know, but where?


char *alloc_bits(unsigned int);
void  set_bit(char*, unsigned int);
int   get_bit(const char*, unsigned int);
void  clear_bit(char*, unsigned int bit);

int main(int argc, char *argv[])
{
    char *mybits;
    
    mybits = alloc_bits(60);

    if (mybits) printf("mybits: %X%X %X%X %X%X %X%X (%s)\n", *(mybits+0), *(mybits+1), *(mybits+2), *(mybits+3), *(mybits+4), *(mybits+5), *(mybits+6), *(mybits+7), mybits);

    if (mybits) set_bit(mybits,2);

    if (mybits) printf("mybits: %X%X %X%X %X%X %X%X (%s)\n", *(mybits+0), *(mybits+1), *(mybits+2), *(mybits+3), *(mybits+4), *(mybits+5), *(mybits+6), *(mybits+7), mybits);

    system("PAUSE");
    return 0;
}

char *alloc_bits(unsigned int bits)
{
    return calloc(((bits+7)/8), sizeof(char));
}

void set_bit(char *bits,unsigned int bit)
{
    bits[bit/8] |= 1 << (bit&3);
}

int get_bit(const char *bits, unsigned int bit)
{
    return ((bits[bit/8] | (1 << (bit&3))) != 0) ? bit : 0;
}

void clear_bit(char *mybits,unsigned int bit) {
    mybits[bit/8] &= ~(1 << (bit&3));
}


 
gruss
/Christian                   mailto:ca...@kr...

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I propose that the following character sequence for joke markers:

:-)

                             19-Sep-82 11:44    Scott E  Fahlman