Author Topic: Getting the bits out of a number  (Read 1976 times)

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Offline Spyro543

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Getting the bits out of a number
« 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?

Offline Juju

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Re: Getting the bits out of a number
« Reply #1 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.
« Last Edit: January 18, 2013, 02:28:04 pm by Juju »

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Offline ruler501

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Re: Getting the bits out of a number
« Reply #2 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);
}
« Last Edit: January 18, 2013, 02:37:36 pm by ruler501 »
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Offline Juju

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Re: Getting the bits out of a number
« Reply #3 on: January 18, 2013, 02:37:50 pm »
Actually, this could work:

Code: [Select]
#define getBit(x,n) x&(1<<n)

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Offline Vogtinator

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Re: Getting the bits out of a number
« Reply #4 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)

Offline Juju

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Re: Getting the bits out of a number
« Reply #5 on: January 18, 2013, 02:50:18 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.

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Offline calc84maniac

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Re: Getting the bits out of a number
« Reply #6 on: January 18, 2013, 02:51:46 pm »
Or even
Code: [Select]
#define getBit(x,n) ((x)>>(n)&1)
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