Omnimaga

General Discussion => Technology and Development => Computer Programming => Topic started by: Spyro543 on January 18, 2013, 02:21:54 pm

Title: Getting the bits out of a number
Post by: Spyro543 on January 18, 2013, 02:21:54 pm

Let's say I have a variable that is char type and is equal to 'A' which is 65 in decimal and 01000001 in binary. How can I access the individual bits in that variable? For example, accessing bit 0 would result in 1. How can I do this in C?

Title: Re: Getting the bits out of a number
Post by: Juju on January 18, 2013, 02:26:46 pm

You would want to use the & operator. Say char = 65 and you want bit 6 , 2⁶ = 64, so you would do something like char & 64 == 64. If it returns true, the bit is set, false, the bit isn't set.

Title: Re: Getting the bits out of a number
Post by: ruler501 on January 18, 2013, 02:35:48 pm

I would do it like this in C++

Code: [Select]

template <class T>
bool retrievebit(T source, int bit)
{
if(bit > (sizeof(T)*8)) throw "Too large";
return ((source >> (bit-1)) & 1);
}
I haven't tested that and it will only work for types that have >> defined as the left shift, but  otherwise it should be good.

EDIT: I just saw this was in C I'll rewrite this real quick to be C code

In c for just char it would be like this

Code: [Select]

int retrievebit(char source, int bit)
{
if(bit > 8)) return -1;
return ((source >> (bit-1)) & 1);
}

Title: Re: Getting the bits out of a number
Post by: Juju on January 18, 2013, 02:37:50 pm

Actually, this could work:

Code: [Select]

#define getBit(x,n) x&(1<<n)

Title: Re: Getting the bits out of a number
Post by: Vogtinator on January 18, 2013, 02:47:29 pm

Quote

Actually, this could work:

Code: [Select]

#define getBit(x,n) x&(1<<n)

Nope :P

Code: [Select]

#define getBit(x,n) ((x&(1<<n))>>n)

Title: Re: Getting the bits out of a number
Post by: Juju on January 18, 2013, 02:50:18 pm

Quote from: Vogtinator on January 18, 2013, 02:47:29 pm

Quote

Actually, this could work:

Code: [Select]

#define getBit(x,n) x&(1<<n)

Nope :P

Code: [Select]

#define getBit(x,n) ((x&(1<<n))>>n)

Oh right, I forgot making it return 1 or 0 and not (1<<n) or 0.

Title: Re: Getting the bits out of a number
Post by: calc84maniac on January 18, 2013, 02:51:46 pm

Or even

Code: [Select]

#define getBit(x,n) ((x)>>(n)&1)