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

[Per W. <pw...@ia...>]

Sorry, I must have got brain-fever

It should be bit&7 not bit&3

Don't know where the 3 came from in the first example, but then I managed
to continue with it for the test_bit() and clear_bit() functions too...

Am I bad or just a bit tired :-)

The code I sent defined bit 0 as least significant bit of first char, and
bit 7 as most significant bit of first char.
If you want to reverse order of bits within a character, you can use

0x80 >> (bit&7)

Then you will allocate bits from high bit to low in each character.


Also, the bits are zero-relative. If you do set_bit(mybits,2) the relevant
character should contain binary value 00000100 (4 decimal) if you use the
bit order of my example, or 00100000 (32 decimal) if you use the reverse
bit order of (0x80 >> (bit&7)).

/Per W

On Tue, 30 Sep 2003, Christian Sch=E4fer wrote:

> 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] |=3D 1 << (bit&3);
>
> > isn't so complicated. Each character contains 8 bits. So by dividing bi=
t
> > 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 =3D 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] |=3D 1 << (bit&3);
> }
>
> int get_bit(const char *bits, unsigned int bit)
> {
>     return ((bits[bit/8] | (1 << (bit&3))) !=3D 0) ? bit : 0;
> }
>
> void clear_bit(char *mybits,unsigned int bit) {
>     mybits[bit/8] &=3D ~(1 << (bit&3));
> }
>
>
>
> gruss
> /Christian                   mailto:ca...@kr...
>
> ---
>
>
> I propose that the following character sequence for joke markers:
>
> :-)
>
>                              19-Sep-82 11:44    Scott E  Fahlman
>
>
>
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