how does this work

[windandwaves]

Hi Folk

I came accross this very simple function, but I have no idea how it works
(the ampersands). Can you explain it.

function is_even($num){
return (is_numeric($nu m)&(!($num&1))) ;
}

TIA

Nicolaas

[milahu]

Re: how does this work

Heh, nice solution! :)

The 1st statement checks, if $num is a number.

The 2nd one checks, if the the binary "one" is set, as it is with every
odd number.
Alternatively, you could use modulo:
!($num % 2)

IMO, the most odd thing about this is the single "&" between the two
statements; I would use "&&" or "and" here. ;)

Cheers..

[Tim Martin]

Re: how does this work

milahu wrote:[color=blue]
> Heh, nice solution! :)
>
> The 1st statement checks, if $num is a number.
>
> The 2nd one checks, if the the binary "one" is set, as it is with every
> odd number.
> Alternatively, you could use modulo:
> !($num % 2)
>
> IMO, the most odd thing about this is the single "&" between the two
> statements; I would use "&&" or "and" here. ;)[/color]

Why would you do that? '&&' and '&' are completely different operators,
and give different results when evaluated in boolean context on two numbers.

Tim

[milahu]

Re: how does this work

Tim Martin wrote:[color=blue]
> milahu wrote:[color=green]
> > IMO, the most odd thing about this is the single "&" between the two
> > statements; I would use "&&" or "and" here. ;)[/color]
>
> Why would you do that? '&&' and '&' are completely different operators,
> and give different results when evaluated in boolean context on two numbers.[/color]

Because I prefer comparing boolean values with logical instead of byte
operators.

[Tim Martin]

Re: how does this work

milahu wrote:[color=blue]
> Tim Martin wrote:[color=green]
>> milahu wrote:[color=darkred]
>>> IMO, the most odd thing about this is the single "&" between the two
>>> statements; I would use "&&" or "and" here. ;)[/color]
>> Why would you do that? '&&' and '&' are completely different operators,
>> and give different results when evaluated in boolean context on two numbers.[/color]
>
> Because I prefer comparing boolean values with logical instead of byte
> operators.
>[/color]

Sorry, I misread you. I thought you were referring to replacing the
second '&', not the first one. I didn't even spot that the two boolean
expressions were being combined with a bitwise operator (which I agree
is wrong).

For reference, the function in question was:

function is_even($num){
return (is_numeric($nu m)&(!($num&1))) ;
}


Tim

[windandwaves]

Re: how does this work

windandwaves wrote:
Thanks folk

It makes more sense now. I gather that the single ampersand test for a
particular bit in a byte, while the double ampersand is the AND operator.

Thanks a million.

Nicolaas

[Steve]

Re: how does this work

On Fri, 21 Apr 2006 11:20:27 +0100, Tim Martin wrote:
[color=blue]
> milahu wrote:[color=green]
>> Tim Martin wrote:[color=darkred]
>>> milahu wrote:
>>>> IMO, the most odd thing about this is the single "&" between the two
>>>> statements; I would use "&&" or "and" here. ;)
>>> Why would you do that? '&&' and '&' are completely different operators,
>>> and give different results when evaluated in boolean context on two numbers.[/color]
>>
>> Because I prefer comparing boolean values with logical instead of byte
>> operators.
>>[/color]
>
> Sorry, I misread you. I thought you were referring to replacing the
> second '&', not the first one. I didn't even spot that the two boolean
> expressions were being combined with a bitwise operator (which I agree
> is wrong).
>
> For reference, the function in question was:
>
> function is_even($num){
> return (is_numeric($nu m)&(!($num&1))) ;
> }
>
>
> Tim[/color]

Why is it wrong? you're anding 0 or 1 with 0 or 1, and that'll return 1
only if both are 1. It might even be faster - probably not though.

[Tim Martin]

Re: how does this work

Steve wrote:[color=blue]
> On Fri, 21 Apr 2006 11:20:27 +0100, Tim Martin wrote:[color=green]
>> Sorry, I misread you. I thought you were referring to replacing the
>> second '&', not the first one. I didn't even spot that the two boolean
>> expressions were being combined with a bitwise operator (which I agree
>> is wrong).
>>
>> For reference, the function in question was:
>>
>> function is_even($num){
>> return (is_numeric($nu m)&(!($num&1))) ;
>> }[/color]
>
> Why is it wrong? you're anding 0 or 1 with 0 or 1, and that'll return 1
> only if both are 1. It might even be faster - probably not though.[/color]

It's wrong in the sense of being a bad habit, not in the sense of
returning the wrong result. Doing this sort of thing regularly can lead
to brittle code that breaks due to seemingly innocuous changes.

Tim