Conditional Logic: What is the Meaning of This !

[ATS16805]

seriously-- the character [ ! ]

i realize it basically makes things "the opposite" of what you might normally expect it to be... the opposite of true, for example, in [$apple !== $orange] or, man != woman, or the opposite of the very state of being-- in other words i guess it generally could be read as "Not"
and yes, i realize there is the section on comparison operators.

my problem is that i've encountered some code written by another author of course-- in which he or she has used such logic as:

    if ($variable) { // no arguments . just a the 'if' and then the var-- as if it wants to be instead: if (isset($variable)) {...
// and we continue similarly...
    $happy = $value;

}

print_r($happy);

if(!$value) { // where again there are no arguments, yet there exists what seems 
// to be valid, conditional logic based on the presense (or lack thereof), simply, of ' ! '

$value = (1+($xyz));
break; // and we do something else
}

though i've never seen (that i recall) specific mention, or recommendation to use this as an abrreviation for empty(), it looks to me as if that is what is happening -- both visually, and in execution.

wishing to have a more proper reference-- do you know of a page in the manual, or elsewhere-- which would explain this use; this abbreviation??

thank you!

[laserlight]

[man]isset/man, [man]empty/man and an implict cast to boolean are not equivalent.

if ($value) {
    // ...
}

In the above example, $value should exist. If the value concerned is not a boolean, an implicit cast to boolean is done for it to be evaluated as the condition.

if (!$value) {
    // ...
}

Now, if $value is not a boolean, the same implicit cast is performed, and then the resulting value is inverted (i.e., true -> false, false -> true). Read up on the Logical Operators in the PHP Manual for this.

[NogDog]

To elaborate a bit:

When converting to boolean, the following values are considered FALSE:

• the boolean FALSE itself

• the integer 0 (zero)

• the float 0.0 (zero)

• the empty string, and the string "0"

• an array with zero elements

• an object with zero member variables (PHP 4 only)

• the special type NULL (including unset variables)

• SimpleXML objects created from empty tags

Every other value is considered TRUE (including any resource).

(From http://www.php.net/manual/en/language.types.boolean.php)

Therefore, if the value of the variable in a if($variable) expression evaluates to any of the above "false" values, then the if expression is evaluated as false, otherwise it is evaluated as true. (And if you prepend the variable with a "!", then it inverts the logic, and the if expression would then evaluate as true if the variable evaluates as false. :queasy: )

Now, for your final test, will the following if condition evaluate as true or false?

$var1 = "0";
$var2 = NULL;
$var3 = "Hello, World!";
$var3 = FALSE;
if(!(!$var1 or !$var2) and (!($var3 or $var4))) {

😉

[ATS16805]

hey there, Laserlight! long time, no see...
(wow-- i was distracted, and i see there has been another reply... assume i haven't read it yet cause i've gotta finish this... doh!)

thanks for the info... but as i do all too often-- i think i danced around what i'm really trying to ask here. i also don't quite get what you're telling me (what do you mean by "implicit cast to the boolean?).

forgive me for presenting a few more questions about this-- but as usual, the research leads to only more questions-- so as i took a look around in the manual for finding something specific to this usage; a specific manual entry (other than its natural placement under the logical operators), i am more curious about this programmer's use of it-- as it doesn't seem quite prevalent in Control Structures, and a cursory glance through Ch. 16 turned up no sign of an identical use (though i may have missed it).

I'm thinking that the answer to my question (whether fumbled here in my text or not) probably lies in a better understanding of Control Structures, if not a more "everyday use of..." to where i would be inclined to abbreviate in such a way (assuming this is perhaps a Control Structure abbreviation). after all, i've only lately begun to use switch when i see it is appropriate.

what i'm asking might even be just a lot more simple than all of this blabbering...
let me try again...

"Why would one want to use the code, as shown above [vs. some construct / function which might be more commonly used]?" (i.e. Do You use this? if so, as you're coding, and you think "hmm.... i will use the if(!$var) structure here...", why vs perhaps something else?
"In what situation is this the preferred syntax?"
"Would we identify this as an "alternative construct", or is it in and of itself something unique when used as shown here? (as in-- we have also &$references, for example )
(i.e. if it is NOT interchangeable with empty() function... then where is our specific manual entry?)

it's looking like the answer is-- "it is an element of (or stylistic approach to) a Control Structure" -- but this is why i bring it up-- i want to hear what any respondents might have to say about it-- to help me understand.

thanks for your patience here!!
😉

[ATS16805]

NogDog wrote:

if the value of the variable in a if($variable) expression evaluates to any of the above "false" values, then the if expression is evaluated as false, otherwise it is evaluated as true.

ah ha! i see what's going on here now!
while i'm getting all hung-up on, as ND explains:

NogDog wrote:

if you prepend the variable with a "!", then it inverts the logic

-- this is in fact NOT the true focus of this technique. so-- the whole "!" is no more important than what i already know of it-- that it means "Not", quite simply.
and now i believe i might have learned something (but not yet-- gotta play w/ it a bit-- maybe examine ND's query, etc.). so-- indeed, this is a "Control Structures" issue. definitely a little more complex than what i'm used to:
if it's an apple and not an orange, then it's an apple. if it's an apple and a fruit, then .... etc.
now i see that perhaps it's time to go back and re-examine some of this theory. maybe i'm ready for a second look at the basic elements.
cool, man! i dig stuff like this-- especially if i actually "get something out of it" that i might begin to apply in my own stuff.

thanks very much for entertaining my query!!! i'm glad both of you were able to see i didn't just mean "What does '!' do?"
thanks!

EDIT:
but, NogDog, your query is an expression, no? with arguments?. it's more of the common type of conditional structure i'm used to.
i'm asking "Why can we use an if($var) { }"? which i think Laserlight was trying to explain, i just got a little lost on it is all...
am i wrong, or are the two of you coming at this from different directions?
(sorry!!)

laserlight wrote:

[man]isset/man, [man]empty/man and an implict cast to boolean are not equivalent.

-- but i just didn't follow quite the rest... of the explanation-- but i definitely was asking that part. (which is why i say: if it's not like empty(), then what are the differences)

[NogDog]

The syntax of an if construct is:

if ([i]expression[/i])
{ 
  [i]code to execute if [b]expression[/b] evaluates as TRUE[/i]
}
else
{
  [i]code to execute if [b]expression[/b] evaluates as FALSE[/i]
}

The important thing to note here is that any expression will be evaluated as either true or false, based on the rules in the quoted section of my prior reply. It does not necessarily have to be a comparison that results in an explicit boolean TRUE or FALSE, as any expression will be cast as necessary to a boolean TRUE or FALSE as per those rules.

Then, if you should negate that expression with the "!" operator, you toggle whatever boolean value to which the expression would normally evaluate to its opposite boolean value: TRUE becomes FALSE, and vice versa. So....

if(TRUE)
{
   echo "This would be echoed.";
}
if(1)
{
   echo "This would be echoed.";
}
$var = TRUE;
if($var)
{
   echo "This would be echoed.";
}
if(!$var)
{
   echo "This would NOT be echoed.";
}
if(!TRUE)
{
   echo "This would NOT be echoed.";
}
if(!FALSE)
{
   echo "This WOULD be echoed.";
}
$var = "0";
if($var)
{
   echo "This would NOT be echoed, as the string '0' is cast to FALSE.";
}

Any clearer?