Challenge Yourself!

Karl · 09-10-2007, 10:31 AM

Just thought I'd give people a little challenge, have a try at this Zend certification practice question. It's a tricky one if you don't know how to use the mathematical and bitshift operators properly.

PHP Code:


			
$a    = 10;
$b    = 20;
$c     = 4;
$d    = 8;
$e    = 1.0;

$f = $c + $d * 2;
$g = $f % 20;
$h = $b - $a + $c + 2;
$i = $h << $c;
$j = $i * $e;

echo $j;

What does $j output?

Do not scroll down if you don't want to see peoples' answers

ok, now I've put that warning there, feel free to post your answers :)

Salathe · 09-10-2007, 10:48 AM

* Salathe raises hand, "I know, I know!"

Karl · 09-10-2007, 11:31 AM

lol, I knew you'd have it figured out in no time :)

aSAP · 09-11-2007, 03:52 AM

Not sure but is it 128?

Karl · 09-11-2007, 10:53 AM

Here's the solution

PHP Code:


			

$a    = 10;
$b    = 20;
$c     = 4;
$d    = 8;
$e    = 1.0;

$f = $c + $d * 2;
$g = $f % 20;
$h = $b - $a + $c + 2;
$i = $h << $c;
$j = $i * $e;

echo $j;

Ok, so the best way to work this out is to work from the end back to the beginning. So first we look at out outcome variable $j and work back.

So, $j is equal to $i * $e, we know $e is 1.0, so we only need to worry about $i.

We can see that $i is equal to:

PHP Code:


			
$i = $h << $c

Since $h holds the result of a previous calculation, we must first discover its value before we can find the value of $i.

PHP Code:


			
$h = $b - $a + $c + 2

Now, we know $b is 20, $a is 10 and $c is 4 (as these variables were initialised with these values) so the previous statement becomes:

PHP Code:


			
$h = 20 - 10 + 4 + 2

which equals 16, so we now know that

PHP Code:


			
$h = 16

Ok so we have our $h, we can now calculate $i:

PHP Code:


			
$i = $h << $c

or

PHP Code:


			
$i = 16 << 4

Which means shift 16 4 bits to the left (Im not gunna bog you down with bit manipulation here, I'll release an article on this soon), so we can now determine that:

PHP Code:


			
16  << 1 = 32
32  << 1 = 64
64  << 1 = 128
128 << 1 = 256

so therefor:

PHP Code:


			
$i = 256

Finally, now we know the value of $i, we can now determine the value of $j:

PHP Code:


			
$j = $i * $e

or

PHP Code:


			
$i = 256 * 1

so, the answer to this little mystery is 256!

Salathe · 09-11-2007, 11:37 AM

PHP Code:


			
$a    = 10;

$b    = 20;

$c     = 4;

$d    = 8;

$e    = 1.0;



$f = $c + $d * 2;      // 4 + (8 * 2) = 20

$g = $f % 20;          // 20 % 20 = 0 [modulus, a%b = remainder of a/b]

$h = $b - $a + $c + 2; // 20 - 10 + 4 + 2 = 16

$i = $h << $c;         // 16 << 4 = 16 * 2^4 = 16 * (2*2*2*2) = 256

$j = $i * $e;          // 256 * 1.0 = 256.0 [int * float = float]



echo $j;               // 256  [echoing float with no decimals]

Edit: oops didn't see that Karl put up the answer. Oh and remember that int * float = float, Karl's post doesn't make that clear. :)