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Operatori di incremento/decremento

PHP supporta lo stile C degli operatori di pre- e post-incremento e decremento.

Nota: Gli operatori di incremento e decremento non agiscono sui valori boolean. Anche decrementare il valore NULL non ha effetti, ma incrementarlo darà come risultato 1.

Operatori di incremento/decremento
Esempio Nome Effetto
++$a Pre-incremento Incrementa Black Women Small Bag Pattern Crocodile Shoulder Purse Fashion Bluester Tote Handbag Ladies $a di una unità, quindi restituisce $a.
$a++ Post-incremento Restituisce $a, quindi incrementa $a di una unità.
--$a Pre-decremento Decrementa $a di una unità, quindi restituisce $a.
$a-- Post-decremento Restituisce $a, quindi decrementa $a di una unità.

Un semplice script di esempio:

echo "

Postincrement

"
;
$a 5;
echo 
"Dovrebbe essere 5: " $a++ . "\n";
echo 
"Dovrebbe essere 6: " $a "\n";

echo 
"

Pre-incremento

"
;
$a 5;
echo 
"Dovrebbe essere 6: " . ++$a "\n";
echo 
"Dovrebbe essere 6: " $a "\n";

echo 
"

Post-decremento

"
;
$a 5;
echo 
"Dovrebbe essere 5: " $a-- . "\n";
echo 
"Dovrebbe essere 4: " $a "\n";

echo 
"

Pre-decremento

"
;
$a 5;
echo 
"Dovrebbe essere 4: " . --$a "\n";
echo 
"Dovrebbe essere 4: " $a Fashion Purse Shoulder Tote Black Pattern Ladies Bag Women Handbag Small Crocodile Bluester"\n";
?>

Il PHP segue le convenzioni di Perl e non del C quando tratta le operazioni matematiche sui caratteri. Ad esempio, in PHP e Perl $a = 'Z'; $a++; modifica $a in 'AA', mentre in C a = 'Z'; a++; modifica a in '[' (il valore ASCII di 'Z' è 90, quello di '[' è 91). Si noti che le variabili carattere possono essere incrementate ma non decrementate e anche in questo caso solo i caratteri ASCII (a-z e A-Z) sono supportati. Incrementare/decrementare altre variabili di carattere non ha effetto, la stringa originale non viene modificata.

Example #1 Operazioni aritmetiche su variabili carattere

$i 'W';
for (
$n=0$n<6$n++) {
    echo ++
$i "\n";
}
?>

Il precedente esempio visualizzerà:

X
Y
Z
AA
AB
AC

Incrementare o decrementare valori booleani non ha effetto.

add a note

User Contributed Notes 12 notes

up
49
hartmut at php dot net
6 years ago
Note that

$a="9D9"; var_dump(++$a);   => string(3) "9E0"

but counting onwards from there

$a="9E0"; var_dump(++$a);   => float(10)

this is due to "9E0" being interpreted as a string representation of the float constant 9E0 (or 9e0), and thus evalutes to 9 * 10^0 = 9 (in a float context)
up
6
ayyappan dot ashok at gmail dot com
2 years ago
Rule for Increment and decrement:

At some moment we could be confused with increment and decrement in various cases. To avoid such cases, let us follow certain logical rule behind to get successful results with out mess.

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           $n = 3;
           echo
$n-- + --$n;
           echo
"
"
;
           echo
$n;
?>

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1. Postfix form of ++,-- operator follows the rule  [ use-then-change ],

2. Prefix form (++x,--x) follows the rule [ change-then-use ].

Solution based on the rule:

Step 1: 
use then change   $n--  use is 3 and change is 2

Step 2. 
change then use   --$n  change is 2 and use is 1

Step 3.
use + use = (3 + 1) = 4

Courtesy : stackoverflow : Sunil Dhillon : 4686665
up
7
dsbeam at gmail dot com VF 1148A VF 1148A Wallet Checkbook Brown ppr08q4
9 years ago
When using the ++ operator by itself on a variable, ++$var is faster than $var++ and uses slightly less memory (in my experiments).  It would seem like this could be optimized in the language during runtime (if $var++ is the only thing in the whole statement, it could be treated as ++$var).

I conducted many tests (I believe to be fair), and here's one of the results:

$i++ took 8.47515535355 seconds and 2360 bytes
++$i took 7.80081486702 seconds and 2160 bytes

Here's my code.  If anyone sees a bias in it, tell me.  I conducted it many times, each time going through a loop one million iterations and doing each test 10 - 15 times (10 - 15 million uses of the ++ operator).


ini_set
( 'MAX_EXEC_TIME', 120 );
ob_start( );

$num_tests = 10;
$startFirst = $startSecond = $endFirst = $endSecond = $startFirstMemory = $endFirstMemory = $startSecondMemory = $endSecondMemory = $someVal = 0;
$times = array( '$i++' => array( 'time' => 0, 'memory' => 0 ), '++$i' => array( 'total' => 0, 'memory' => 0 ) );

for(
$j = 0; $j < $num_tests; ++$j )
{
        for(
$i = 0, $startFirstMemory = memory_get_usage( ), $startFirst = microtime( Black Travel Blue Top Bag Casual Waterproof Women Backpack Drawstring Handle Rucksack Shoulder IU7Pxnvtrue ); $i < 10000000; $i++ ){ $someval = 2; }
       
$endFirstMemory = memory_get_usage( );
       
$endFirst Bag Ladies Fashion Small Pattern Purse Women Tote Black Handbag Bluester Shoulder Crocodile =microtime( true );

        for(
$i = 0Black Fashion Purse Ladies Shoulder Tote Crocodile Bluester Handbag Pattern Women Bag Small ,$startSecondMemory = memory_get_usage( ), $startSecond = microtime( Handbag Bluester Small Fashion Pattern Women Crocodile Bag Purse Black Shoulder Ladies Tote true); $i < 10000000; ++$i ){ $someval = 2; }
       
$endSecondMemory = memory_get_usage( );
       
$endSecond = microtime( true );

       
$times[ '$i++' ][ $j ] = array( Fashion Small Black Handbag Shoulder Purse Pattern Bluester Women Crocodile Bag Ladies Tote 'startTime'=> $startFirst, 'endTime' => $endFirst, Ladies Purse Black Women Pattern Handbag Tote Bluester Crocodile Small Shoulder Bag Fashion 'startMemory'=> $startFirstMemory, 'endMemory' => $endFirstMemory );
       
$times[ '++$i' ][ $j ] = array( 'startTime' => $startSecond, 'endTime' => $endSecond, 'startMemory' => $startSecondMemory, 'endMemory' => $endSecondMemory );
}

for(
$i Bag Tote Purse Ladies Fashion Handbag Women Pattern Crocodile Small Black Bluester Shoulder =0; Maxwell The Scott Chocolate Leather Handcrafted Vittore Italian Luxury Wallet Mens Dark rrPRq40F$i < $num_tests; ++$i )
{
       
$times[ '$i++' ][ 'time' ] += ( $times[ '$i++' ][ $i ][ 'endTime' ] - $times[ '$i++' ][ $i ][ 'startTime' ] );
       
$times[ '++$i' ][ 'time' ] += ( $times[ '++$i' ][ $i ][ 'endTime' ] - $times[ '++$i' ][ $i ][ 'startTime' ] );
       
$times[ '$i++' ][ 'memory' ] += ( $times[ '$i++' ][ $i ][ 'endMemory' ] - $times[ '$i++' ][ $i ][ 'startMemory' ] );
       
$times[ '++$i' ][ 'memory' ] += ( $times[ '++$i' ][ $i ][ 'endMemory' ] - $times[ '++$i' ][ $i ][ 'startMemory' ] );
}

echo
'There were ' . $num_tests . ' tests conducted, here\'s the totals


$i++ took '
. $times[ '$i++' ][ 'time' ] . ' seconds and ' . $times[ '$i++' ][ 'memory' ] . ' bytes

++$i took '
. $times[ '++$i' ][ 'time' ] . ' seconds and ' . $times[ '++$i' ][ 'memory' ] . ' bytes';

ob_end_flush( );
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?>

Try it yourself, ;)
up
-1
Brad Proctor
7 years ago
I ran some tests (on PHP 5.3.3) of my own and was surprised to find $i += 1 to be the fastest method of incrementing.  Here are the methods fastest to slowest:

$i += 1;
++$i;
$i++;
$i = $i + 1;
aluciffer at hotmail dot com
3 years ago
Regarding character incrementing and PHP following Perl's convention with character operations.
Actually i found that there is a difference, and incrementing and decrementing unfortunately does not yield the reverse, expected results.
For example, the following piece of code:
echo '== Alphabets ==' . PHP_EOL;
$s = 'W';
for (
$n=0; $n<10; $n++) {
    echo ++
$s . ' ';
}
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echo
PHP_EOL;

for (
$n=10; $n>0; $n--) {
    echo (--
$s) . ' ';
}
?>
Will output:
== Alphabets ==
X Y Z AA AB AC AD AE AF AG
AG AG AG AG AG AG AG AG AG AG

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Please note that the decrement operator has no effect on the character or string.

On the other hand, in Perl, the similar script:

#!/usr/bin/perl

my $s = 'W';

foreach (1 .. 10) {
print  ++$s . " ";


print "\n";

foreach (1 .. 10) {
print --$s . " ";
}

Will output:

X Y Z AA AB AC AD AE AF AG
-1 -2 -3 -4 -5 -6 -7 -8 -9 -10
up
0
cleong at letstalk dot com
16 years ago
Note that the ++ and -- don't convert a boolean to an int. The following code will loop forever.

function a($start_index) {
for($i = $start_index; $i < 10; $i++) echo "\$i = $i\n";
}

a(false);

This behavior is, of course, very different from that in C. Had me pulling out my hair for a while.
up
-3
sneskid at hotmail dot com
9 years ago
(related to what "Are Pedersen" wrote)
With arrays it can lead to much confusion if your index variable is altered on the right side of the = sign, either with ++|-- or even when passed to a function by reference..
Consider these (PHP 5):
$A[$a] = ++$a; // [1]=1
$B[++$b] = ++$b; // [1]=2
$C[$c+=0] = ++$cPurse Tote Pattern Handbag Bluester Fashion Women Black Bag Small Ladies Shoulder Crocodile ;// [0]=1
?>
In 'A' you have to be aware that PHP evaluates $A[$a] last.
Yet in 'B' and 'C' PHP evaluates the index and saves it in a temporary variable.

You can always force PHP to evaluate a variable without explicitly storing it as a named variable first, with a simple "+=0" like in example 'C'.

Compared to 'A', 'C' gives the more logically expected result, when we expect evaluation occurs left to right.
PHP does evaluate left to right BUT it will attempt to cut down on temporary variables, which can lead to confusing results.

Handbag Bridal Blue Crossbody Diamante Clutch Handbag Clutch Bridal Small Evening Purse Bag Wedding Party Ladies Elegant TqgY8gSo just be aware and use either behavior to your advantage for the desired functionality.
fred at surleau dot com Dissa Women Leather Bag Body Handbag Design Cross Fashion 1895 Elegant Red LF Shoulder rPwqEr
17 years ago
Other samples :
$l="A";      $l++; -> $l="B"
$l="A0";     $l++; -> $l="A1"
$l="A9";     $l++; -> $l="B0"
$l="Z99";    $l++; -> $l="AA00"
$l="5Z9";    $l++; -> $l="6A0"
$l="9Z9";    $l++; -> $l="10A0"
$l="9z9";    $l++; -> $l="10a0"
$l="J85410"; $l++; -> $l="J85411"
$l="J99999"; $l++; -> $l="K00000"
$l="K00000"; $l++; -> $l="K00001"
up
-7
michiel ed thalent circle nl
6 years ago
BEWARE:
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If incrementing an uninitialized variable you will not get an E_NOTICE error. This may caused you to not find issue's like the visibility of a property.

class a {
    private
$foo = 1;
}

class
b extends a {
    public function
inc() {
        echo ++
$this->foo;
    }
}

$b = new b;
$b->inc();
?>

Will output 1 and not 2 (if $foo was accessible).
Also no notices are given.
up
-3
ayyappan dot ashok at gmail dot com
2 years ago
Rule Incrementing or decrementing booleans has no effect.

It has an effort on Incrementing or decrementing booleans.

Please look over the code.

$var = true;
echo ++$var;   //Results 1

Similarly

$var = true;
echo ++$var;   //Results 1

$var = (int)false;
echo ++$var;   //Results 1

$var = (int)false;
echo $var++;   //Results 0;   

Note : Tested on PHP Version 5.5.32
up
-3
Azeeda Credit Card Wallet 'Cute Holder Elephant' Business CH00015014 Card aqTxawrz4
4 months ago
Самое простое описание работы инкремента, которое я встречал и будет удобно для понимания новичку:

Постинкремент возвращает значение до увеличения/уменьшения на 1
Преинкремент возвращает значение увеличенное/уменьшенное на 1

$b = 6;
$a = $b++; // $a = 6, $b = 7

$b = 6;
$a = $b--; // $a=6, $b = 5

$b = 6;
$a = ++$b; // $a = 7, $b = 7

$b = 6;
$a = --$b; // $a = 5, $b = 5
up
-29
johnnythan at nospam dot gmx dot com
6 years ago
If you have a trailing zero and use the increment, the trailing zero will not remain. Was at least unexpected for me at first, although it's logical if you think about it.

$start = '01';
Ladies Pattern Crocodile Purse Bluester Tote Women Bag Shoulder Fashion Small Handbag Black $start++;
print
$start; //Outputs '2' not '02'
?>
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