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bigdilliams

Bitwise AND expanation

11 posts in this topic

Could someone expain me the bitwise and operation (&) in C++?

For example:
What happens here?
[CODE]
if(63 & 1)
{
......
}
[/CODE] Edited by bigdilliams
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Have you read [url="http://en.wikipedia.org/wiki/Bitwise_operation"]http://en.wikipedia.org/wiki/Bitwise_operation[/url]? Anything in particular you don't understand?
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The bitwise AND operator applies the AND operation to each bit of the operands individually.

The AND operations does the following: If both operands are 1, the result is 1, in any other case it's 0.
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Wikipedias explanation is quite good:
http://en.wikipedia.org/wiki/Bitwise_operation#AND

[quote]A bitwise AND takes two binary representations of equal length and performs the logical AND operation on each pair of corresponding bits. The result in each position is 1 if the first bit is 1 and the second bit is 1; otherwise, the result is 0. In this, we perform the multiplication of two bits; i.e., 1 × 0 = 0 and 1 × 1 = 1[/quote]

For instance, you have a = 100 (01100100) and b = 50 (00110010), and if you write c = a & b (100 & 50 == 32) the result is:
01100100 (100)
00110010 (50)
------------
00100000 (32)

Since the 6th column (from the right) is the only one with a 1 in both a and b.

EDIT: Way too slow, it seems :-) Edited by nife87
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Yes, but I made a small programm wich results in:
[CODE]
int Zahl = (54 & 63);
std::cout << Zahl;
[/CODE]
63= 111111
54= 110110
so Zahl = 54
What is the sense behind that?
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[quote name='bigdilliams' timestamp='1353248315' post='5002038']
Yes, but I made a small programm wich results in:
[CODE]
int Zahl = (54 &amp; 63);
std::cout << Zahl;
[/CODE]
63= 111111
54= 110110
so Zahl = 54
What is the sense behind that?
[/quote]

Because if you multiply the numbers, bit for bit, and write it out (0x0=0, 1x0=0, 0x1=0, 1x1=1), that is what you get. Edited by nife87
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What if I make this:
[CODE]
if(63 & 1)
{
...
}
[/CODE]

When will the if-loop start?
Is the condition everytime true, and is it possible to be false with an other number?
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[quote name='bigdilliams' timestamp='1353248721' post='5002041']
When will the if-loop start?
Is the condition everytime true, and is it possible to be false with an other number?
[/quote]

In this case always, since the result > 0, 1 to be precise. It's possible to be false with other numbers, e.g. 2 & 1 = 0.
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[quote name='bigdilliams' timestamp='1353248721' post='5002041']
What if I make this:
[CODE]
if(63 &amp; 1)
{
...
}
[/CODE]

When will the if-loop start?
Is the condition everytime true, and is it possible to be false with an other number?
[/quote]

Looks like this question is more related with how the if-[b]statement[/b] work.

The if-statement divides the flow of your program in 2 branches:
- Branch1 will be activated only if the value inside the if-statement evaluates to zero. This branch ignores the contents of the if-statement and starts immediately after its end or inside an else-statement associated with this if.
- Branch2 will be activated when the first one didn't. So, for every value different than zero, the if-statement will be executed. Edited by kuramayoko10
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[quote name='bigdilliams' timestamp='1353248721' post='5002041']
What if I make this:
[CODE]
if(63 & 1)
{
...
}
[/CODE]

When will the if-loop start?
Is the condition everytime true, and is it possible to be false with an other number?
[/quote]

You are mistaken. an "if" is not a loop, because it only runs once. If you want to loop, you have to use the while loop, for loop, or do while; loop. If you put plain numbers in the if brackets, the result will always be the same, either always true or always false.

In the case of 63 & 1:
63[sub]10[/sub] = 00111111[sub]2[/sub]
1[sub]10[/sub] = 00000001[sub]2[/sub]

00111111
[u]00000001[/u] &
00000001

00000001[sub]2[/sub] = 1[sub]10[/sub]
so
[CODE]if(63 & 1)[/CODE]
is equal to
[CODE]if(1)[/CODE]

That means what comes after the if will always be executed.
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