[RESOLVED] Binary Functions

[RossC0]

Hello,

I've returned a binary in decimal form from mysql and what to know if there are any nifty functions so I can query the binary value match my decimal binary value to its real value.

Heres what I mean:

I've returned 17 which is in Binary: 10001

Now that equates to:

| apple | pear | banana | orange | plum |   

|   1   |   0  |    0   |    0   |   1  | 

Now I want be able to do something like: if (apple) { do something }

So I suppose the question is whats the easiest / cleanest / most efficient way to query if apple has been selected...

Any ideas ?

[laserlight]

You're basically looking to test for bits. In PHP, an example would be:

$fruits = array('plum', 'orange', 'banana', 'pear', 'apple');
$num = 17;
for ($i = 0; $i < count($fruits); ++$i) {
	$bit = ($num & (1 << $i)) ? 1 : 0;
	echo $fruits[$i] . ': ' . $bit . '<br />';
}

[RossC0]

Very Nice,

Can you explain the logic / whats going on this line:

laserlight wrote:

$bit = ($num & (1 << $i)) ? 1 : 0;

I'm ok on ternary operators.

[laserlight]

(1 << $i) just generates the sequence 1, 2, 4, 8, 16, ...
In binary (ignoring signs) this is 1, 10, 100, 1000, 10000, ...

The bitwise and operator, &, compares bits from its operands. If both corresponding bits are set, that bit of the result will be set.

17 in binary is 10001
10001 & 1 = 1
10001 & 10 = 0
10001 & 100 = 0
10001 & 1000 = 0
10001 & 10000 = 10000

10000 in binary is 16 in base 10, which is a true value, hence $bit = 1 for this case.

Now, while this may work for you, if you are designing the database schema from scratch, you might want to give it another look - this is a space efficient way of storing data, but it isnt necessarily the best way, especially when you are using a relational database.

[RossC0]

Thanks LaserLight,

I eventually found the bitwise shift in the manual - hard to search these terms are! 🙂