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)
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 agiscono solo su numeri e stringhe. Array, oggetti, booleani e risorse non ne sono interessati. Anche decrementare il valore
nullnon ha effetti, ma incrementarlo darà come risultato1.
| Esempio | Nome | Effetto |
|---|---|---|
| ++$a | Pre-incremento | Incrementa $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:
<?php
echo "<h3>Postincrement</h3>";
$a = 5;
echo "Dovrebbe essere 5: " . $a++ . "<br />\n";
echo "Dovrebbe essere 6: " . $a . "<br />\n";
echo "<h3>Pre-incremento</h3>";
$a = 5;
echo "Dovrebbe essere 6: " . ++$a . "<br />\n";
echo "Dovrebbe essere 6: " . $a . "<br />\n";
echo "<h3>Post-decremento</h3>";
$a = 5;
echo "Dovrebbe essere 5: " . $a-- . "<br />\n";
echo "Dovrebbe essere 4: " . $a . "<br />\n";
echo "<h3>Pre-decremento</h3>";
$a = 5;
echo "Dovrebbe essere 4: " . --$a . "<br />\n";
echo "Dovrebbe essere 4: " . $a . "<br />\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 e i numeri ASCII (a-z, A-Z e 0-9) sono supportati.
Incrementare/decrementare altre variabili di carattere non ha effetto, la
stringa originale non viene modificata.
Example #1 Operazioni aritmetiche su variabili carattere
<?php
echo '== Alphabets ==' . PHP_EOL;
$s = 'W';
for ($n=0; $n<6; $n++) {
echo ++$s . PHP_EOL;
}
// I caratteri numerici si comportano diversamente
echo '== Digits ==' . PHP_EOL;
$d = 'A8';
for ($n=0; $n<6; $n++) {
echo ++$d . PHP_EOL;
}
$d = 'A08';
for ($n=0; $n<6; $n++) {
echo ++$d . PHP_EOL;
}
?>Il precedente esempio visualizzerà:
== Characters == X Y Z AA AB AC == Digits == A9 B0 B1 B2 B3 B4 A09 A10 A11 A12 A13 A14
Incrementare o decrementare valori booleani non ha effetto.
add a note
User Contributed Notes 10 notes
hartmut at php dot net ¶
11 years ago
ayyappan dot ashok at gmail dot com ¶
8 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.
<?php
$n = 3;
echo $n-- + --$n;
echo "<br/>";
echo $n;
?>
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
aluciffer at hotmail dot com ¶
9 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:
<?php
echo '== Alphabets ==' . PHP_EOL;
$s = 'W';
for ($n=0; $n<10; $n++) {
echo ++$s . ' ';
}
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
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 --$s . " ";
}
Will output:
X Y Z AA AB AC AD AE AF AG
-1 -2 -3 -4 -5 -6 -7 -8 -9 -10
cleong at letstalk dot com ¶
22 years ago
Note that the ++ and -- don't convert a boolean to an int. The following code will loop forever.
function a($start_index) {
for($i = $start_index; $i < 10; $i++) echo "\$i = $i\n";
}
a(false);
This behavior is, of course, very different from that in C. Had me pulling out my hair for a while.
dsbeam at gmail dot com ¶
14 years ago
When using the ++ operator by itself on a variable, ++$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).
<?php
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( true ); $i < 10000000; $i++ ){ $someval = 2; }
$endFirstMemory = memory_get_usage( );
$endFirst = microtime( true );
for( $i = 0, $startSecondMemory = memory_get_usage( ), $startSecond = microtime( true ); $i < 10000000; ++$i ){ $someval = 2; }
$endSecondMemory = memory_get_usage( );
$endSecond = microtime( true );
$times[ '$i++' ][ $j ] = array( 'startTime' => $startFirst, 'endTime' => $endFirst, 'startMemory' => $startFirstMemory, 'endMemory' => $endFirstMemory );
$times[ '++$i' ][ $j ] = array( 'startTime' => $startSecond, 'endTime' => $endSecond, 'startMemory' => $startSecondMemory, 'endMemory' => $endSecondMemory );
}
for( $i = 0; $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<br /><br />
$i++ took ' . $times[ '$i++' ][ 'time' ] . ' seconds and ' . $times[ '$i++' ][ 'memory' ] . ' bytes<br />
++$i took ' . $times[ '++$i' ][ 'time' ] . ' seconds and ' . $times[ '++$i' ][ 'memory' ] . ' bytes';
ob_end_flush( );
?>
Try it yourself, ;)
Brad Proctor ¶
13 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;
ayyappan dot ashok at gmail dot com ¶
8 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
leo zandvliet ¶
5 years ago
Please note the difference between post-incrementing in a for-loop and recursive function calls (don't use it in the latter!).
<?php
$increment = $preIncrement = $postIncrement = 1;
echo $increment.' - '.$preIncrement.' - '.$postIncrement;
echo '<br>';
echo ($increment+1).' - '.(++$preIncrement).' - '.($postIncrement++);
echo '<br>';
echo ($increment+1).' - '.(++$preIncrement).' - '.($postIncrement++);
echo '<br>';
echo ($increment+1).' - '.(++$preIncrement).' - '.($postIncrement++);
?>
Outputs:
1 - 1 - 1
2 - 2 - 1
2 - 3 - 2
2 - 4 - 3
The for-loop:
<?php
for($i=0; $i<4; $i++)
{
echo $i.'<br>';
}
?>
Outputs:
0
1
2
3
And the 'headache' mistake, post-increment as parameter of a recursive call:
<?php
testFunctionNesting(3, 1, 1, 1);
function testFunctionNesting($max, $increment, $preIncrement, $postIncrement)
{
echo $increment.' - '.$preIncrement.' - '.$postIncrement;
echo '<br>';
if($increment>=$max)
{
$inc = $increment;
$pre = $preIncrement;
$post = $postIncrement;
return;
}
testFunctionNesting($max, ($increment+1), (++$preIncrement), ($postIncrement++));
}
?>
Output shows that the postIncremented value is never really available as incremented value:
1 - 1 - 1
2 - 2 - 1
3 - 3 - 1
sneskid at hotmail dot com ¶
14 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):
<?php
$A[$a] = ++$a; // [1]=1
$B[++$b] = ++$b; // [1]=2
$C[$c+=0] = ++$c; // [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.
So just be aware and use either behavior to your advantage for the desired functionality.
fred at surleau dot com ¶
22 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"
