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Checking if a bit is set in a byte


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#1 DarkRonin   Members   -  Reputation: 613

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Posted 02 June 2014 - 06:42 PM

Hi Guys,

Just wondering if there is an easy way to check if a bit is set in a byte.

So, if I wanted to see it bit 5 was set for example?

Thanks in advance smile.png

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#2 Hodgman   Moderators   -  Reputation: 30964

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Posted 02 June 2014 - 06:44 PM

bool IsBitSet( unsigned char byte, int index )
{
  int mask = 1<<index;
  return (byte & mask) != 0;
}

Edited by Hodgman, 02 June 2014 - 08:33 PM.
Thanks, Chris_F


#3 Servant of the Lord   Crossbones+   -  Reputation: 20296

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Posted 02 June 2014 - 07:42 PM

When you AND two values together, every bit that wasn't 1 in *both* values is set to zero. So if you want to check if a certain bit is set, you AND that one bit to a value, and then see if the result is 0 (the bit wasn't present), or if the result is the same what you AND'd.

 

if( (10110 & 10000) == 10000 )

if( (value & mask) == mask )

 

You can drop the " == mask " part because, in C++, non-zero integers automaticly convert to 'true':

if(value & mask) //If the result is 00000, then the statement evaluates to false, otherwise it evaluates to true.

 

 

Hodgeman's func does the same thing, but also uses an index to create the mask by shifting the value 00000001 over N places to become (for example) 00010000.


Edited by Servant of the Lord, 02 June 2014 - 07:45 PM.

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#4 DarkRonin   Members   -  Reputation: 613

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Posted 02 June 2014 - 08:18 PM

Thanks guys!

I'll give this a try very shortly smile.png

#5 Chris_F   Members   -  Reputation: 2436

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Posted 02 June 2014 - 08:31 PM

bool IsBitSet( unsigned char byte, int index )
{
  int mask = 1<<index;
  return (byte & index) != 0;
}

 

I think you meant to use mask and not index in your return statement.



#6 Álvaro   Crossbones+   -  Reputation: 13651

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Posted 02 June 2014 - 08:34 PM

Alternatively:

bool is_bit_set(unsigned char byte, int index) {
  return (byte >> index) & 1;
}


#7 Chris_F   Members   -  Reputation: 2436

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Posted 02 June 2014 - 08:53 PM

Here is the results I get with Clang 3.3 with -O3:

 

IsBitSet(unsigned char, int):                          # @IsBitSet(unsigned char, int)
    btl    %esi, %edi
    setb    %al
    ret

is_bit_set(unsigned char, int):                       # @is_bit_set(unsigned char, int)
    btl    %esi, %edi
    setb    %al
    ret

 

and with GCC 4.9 with -O3:

 

IsBitSet(unsigned char, int):
    mov    ecx, esi
    mov    eax, 1
    movzx    edi, dil
    sal    eax, cl
    test    eax, edi
    setne    al
    ret
is_bit_set(unsigned char, int):
    movzx    eax, dil
    mov    ecx, esi
    sar    eax, cl
    and    eax, 1
    ret


#8 DarkRonin   Members   -  Reputation: 613

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Posted 02 June 2014 - 09:14 PM

When you AND two values together, every bit that wasn't 1 in *both* values is set to zero. So if you want to check if a certain bit is set, you AND that one bit to a value, and then see if the result is 0 (the bit wasn't present), or if the result is the same what you AND'd.
 
if( (10110 & 10000) == 10000 )
if( (value & mask) == mask )
 
You can drop the " == mask " part because, in C++, non-zero integers automaticly convert to 'true':
if(value & mask) //If the result is 00000, then the statement evaluates to false, otherwise it evaluates to true.
 
 
Hodgeman's func does the same thing, but also uses an index to create the mask by shifting the value 00000001 over N places to become (for example) 00010000.


This actually seems to work very well. Easy to understand too smile.png

#9 Pink Horror   Members   -  Reputation: 1211

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Posted 04 June 2014 - 08:11 AM

if( (10110 & 10000) == 10000 )
if( (value & mask) == mask )


I wonder, why do some people write "== mask" instead of "!= 0"? I suppose it doesn't matter much when the variable is "mask" and/or you use a function, but I've seen this pattern with huge blocks of hard-coded if checks with duplicated giant enum names everywhere.

#10 Brother Bob   Moderators   -  Reputation: 8430

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Posted 04 June 2014 - 08:21 AM

 

if( (10110 & 10000) == 10000 )
if( (value & mask) == mask )


I wonder, why do some people write "== mask" instead of "!= 0"? I suppose it doesn't matter much when the variable is "mask" and/or you use a function, but I've seen this pattern with huge blocks of hard-coded if checks with duplicated giant enum names everywhere.

 

Consider what happens if mask contains more than one bit set. Comparing the AND-result against the mask checks if all bits are set, while comparing against 0 checks if any bit is set.

 

In that case it is not a matter of relative style or convention, but a matter of absolute correctness; one is correct and the other one is wrong.



#11 aregee   Members   -  Reputation: 1026

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Posted 04 June 2014 - 02:29 PM

 

 

if( (10110 & 10000) == 10000 )
if( (value & mask) == mask )


I wonder, why do some people write "== mask" instead of "!= 0"? I suppose it doesn't matter much when the variable is "mask" and/or you use a function, but I've seen this pattern with huge blocks of hard-coded if checks with duplicated giant enum names everywhere.

 

Consider what happens if mask contains more than one bit set. Comparing the AND-result against the mask checks if all bits are set, while comparing against 0 checks if any bit is set.

 

In that case it is not a matter of relative style or convention, but a matter of absolute correctness; one is correct and the other one is wrong.

 

 

To me it is also a question about clarity.  When you read the code, it is immediately clear what the intention is when you test against 'mask', rather than wondering for a few seconds why you are testing against 'not equal to zero'.



#12 L. Spiro   Crossbones+   -  Reputation: 13968

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Posted 04 June 2014 - 03:23 PM

To me it is also a question about clarity.  When you read the code, it is immediately clear what the intention is when you test against 'mask', rather than wondering for a few seconds why you are testing against 'not equal to zero'.

It’s not about clarity. Brother Bob is exactly correct. It is about correct functionality of the program.
 

ui32Mask = 0xF;
if ( (ui32Value & ui32Mask) != 0 ) // Are ANY of the lower 4 bits set?
if ( (ui32Value & ui32Mask) == ui32Mask ) // Are ALL of the lower 4 bits set?

There is nothing about style or clarity—they absolutely do not do the same thing (unless only 1 bit is set in ui32Mask).
It depends purely on whether you want to test for any bits or all bits, and that is essential for the correctness of any algorithm/code/program.


L. Spiro


Edited by L. Spiro, 04 June 2014 - 03:26 PM.

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#13 aregee   Members   -  Reputation: 1026

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Posted 05 June 2014 - 12:52 PM

 

To me it is also a question about clarity.  When you read the code, it is immediately clear what the intention is when you test against 'mask', rather than wondering for a few seconds why you are testing against 'not equal to zero'.

It’s not about clarity. Brother Bob is exactly correct. It is about correct functionality of the program.
 

ui32Mask = 0xF;
if ( (ui32Value & ui32Mask) != 0 ) // Are ANY of the lower 4 bits set?
if ( (ui32Value & ui32Mask) == ui32Mask ) // Are ALL of the lower 4 bits set?

There is nothing about style or clarity—they absolutely do not do the same thing (unless only 1 bit is set in ui32Mask).
It depends purely on whether you want to test for any bits or all bits, and that is essential for the correctness of any algorithm/code/program.


L. Spiro

 

 

Where do I say that Brother Bob is wrong?  He is spot on.  I just said that for me it is ALSO about clarity.  Notice the word 'also'.  In the case where you are just testing for one bit, both cases are right, but then you should probably not call it 'mask', but rather call it 'bit'.  OP asked how to test for a bit, but the case with a mask is more generic.

 

Edit: if it was not clear; I meant that using 'mask' was more clear than using '!= 0'.


Edited by aregee, 05 June 2014 - 12:54 PM.


#14 Joshhua5   Members   -  Reputation: 454

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Posted 05 June 2014 - 06:57 PM

I do it like this, but I'd have the enum in it's own header to be reused

enum struct BIT{
	_1 = 0x1,
	_2 = 0x2,
	_3 = 0x4,
	_4 = 0x8,
	_5 = 0x10,
	_6 = 0x20,
	_7 = 0x40,
	_8 = 0x80,
};

int eg = 20;

if(eg & BIT::_5){

}


#15 L. Spiro   Crossbones+   -  Reputation: 13968

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Posted 05 June 2014 - 07:27 PM

I do it like this, but I'd have the enum in it's own header to be reused

enum struct BIT{
	_1 = 0x1,
	_2 = 0x2,
	_3 = 0x4,
	_4 = 0x8,
	_5 = 0x10,
	_6 = 0x20,
	_7 = 0x40,
	_8 = 0x80,
};

int eg = 20;

if(eg & BIT::_5){

}

You’re still falling short of optimal.
 
enum BIT{   // “struct” does not belong.
	_1 = 1 << 0,
	_2 = 1 << 1,
	_3 = 1 << 2,
	_4 = 1 << 3,
	_5 = 1 << 4,
	_6 = 1 << 5,
	_7 = 1 << 6,
	_8 = 1 << 7,
};
 

L. Spiro

Edited by L. Spiro, 05 June 2014 - 07:28 PM.

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I went to my local Subway once to find some guy yelling at the staff. When someone finally came to take my order and asked, “May I help you?”, I replied, “Yeah, I’ll have one asshole to go.”
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#16 Ectara   Crossbones+   -  Reputation: 3015

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Posted 06 June 2014 - 02:07 AM

 

I do it like this, but I'd have the enum in it's own header to be reused

enum struct BIT{
	_1 = 0x1,
	_2 = 0x2,
	_3 = 0x4,
	_4 = 0x8,
	_5 = 0x10,
	_6 = 0x20,
	_7 = 0x40,
	_8 = 0x80,
};

int eg = 20;

if(eg & BIT::_5){

}

That forces people to be one-indexed for bit enumeration; sometimes, being zero-indexed is easier, when you use that index directly as the number of positions to shift. Or, put another way, if the index is the exponent of 2 for the value represented by the bit at that index.



#17 Joshhua5   Members   -  Reputation: 454

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Posted 06 June 2014 - 08:26 AM

The strict does belong, it means you must access it with scope operator (BIT::) instead of making everything global.

#18 Kian   Members   -  Reputation: 238

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Posted 10 June 2014 - 10:30 AM

If you want to use the scope operator (which I endorse), it would be better to do:
 
struct BIT
{
    enum {
	_1 = 1 << 0,
	_2 = 1 << 1,
	_3 = 1 << 2,
	_4 = 1 << 3,
	_5 = 1 << 4,
	_6 = 1 << 5,
	_7 = 1 << 6,
	_8 = 1 << 7,
    };
};


#19 Kian   Members   -  Reputation: 238

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Posted 10 June 2014 - 10:31 AM

Huh, misread the first bit. Also, I can't edit my answer?

#20 Kian   Members   -  Reputation: 238

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Posted 10 June 2014 - 10:39 AM

I'm not sure you can use scoped enums (enum class/enum struct) to do bitwise operations, since you can't convert them to int without an explicit cast. If you want to use it as a mask you need to stick a regular enum in a class or struct.




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