c programming stupid question of the day

blackzone · 01-23-2005, 08:44 PM

I have a byte.

is a function that let me set the 3rd bit of the byte? instead of manual adding 2^(3-1)?

aluser · 01-23-2005, 09:23 PM

how about

Code:

if (byte & (1 << 4))
    /* byte is set */

that's as good as it gets, unless you write the function/macro yourself.

qwijibow · 01-23-2005, 09:24 PM

no that i know of.

i usually write a function to decode the byte into a bool array,
and a function to convert a bool array into a byte.
manupilate individial bits between the 2 functions.

tamoneya · 01-23-2005, 09:44 PM

i am less familiar with c but i think it is still possible to do. You could take your byte and cast it into a boolean and the you should AND it with the number 4 also casted as a boolean.

here is the code in java.

Code:

(boolean)mybyte && (boolean)4

CroMagnon · 01-23-2005, 09:45 PM

Quote:

instead of manual adding 2^(3-1)?

Adding a value like that will only set the bit if the bit is currently unset. If it's already set, you will get extremely unwelcome side effects.

What you want to do is:
byte = byte | (1 << n)
where n is the (zero based) index of the bit.

tamoneya · 01-23-2005, 09:52 PM

oops instead of anding (boolean)4 you should actually AND (boolean)251 which is 11111011 in binary. this will turn the third bit to zero.

aluser · 01-23-2005, 09:52 PM

hah, I read "set" as "test". oh well. Do what CroMagnon said.

CroMagnon · 01-23-2005, 10:15 PM

My C is rusty, and my compiler doesn't recognise 'byte' or 'BYTE' (I assume I need to include something, but damned if I can remember), but char works just as well. I'm sure you can modify these to work with bytes easily enough.

Code:

void setbit( char *value, int indx ) {
   *value = *value | (1 << indx);
}

void clearbit( char *value, int indx ) {
   *value = *value & (255 - (1 << indx));
}

int main(void) {
   char b = 40;
   setbit( &b, 2 );           // Sets b to 44
   clearbit( &b, 5 );        // Sets b to 12
   return 0;
}

deiussum · 01-24-2005, 08:34 AM

Another way to clear a bit that I haven't seen mentioned yet is:

value &= ~BIT_TO_UNSET;

The easiest way to set a bit, as mentioned already is:

value |= BIT_TO_SET;

Hex literals are pretty nice to use as the values for the bits you need because they are easy to work with.
0x01
0x02
0x04
0x08
0x10
0x20
0x40
0x80
etc...

So when you say the third bit, if you mean the bit that is set in 0x04 (As opposed to 0x08 if you meant 3rd bit using a typical 0-based index) just use:

value |= 0x04; // set
value &= ~0x04; // unset

Matir · 01-25-2005, 12:47 PM

Quote:

Originally posted by tamoneya
i am less familiar with c but i think it is still possible to do. You could take your byte and cast it into a boolean and the you should AND it with the number 4 also casted as a boolean.

here is the code in java.

Code:

(boolean)mybyte && (boolean)4

Umm... that code just type casts 4 and mbyte to be boolean, and then does a boolean and... the result would be TRUE or FALSE. And I don't think you can do that casting in java at all, since java's understanding of booleans does not include integer values.