blocks|key|1188310|text|%2B%2B$i递增$i，但计算结果为$i%2B1的值$i%2B%2B递增$i，但计算结果为$i的旧值。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1188311|下面是一个例子：|1188312|$i+=+10;
$a+=+$i%2B%2B;
//+Now+$a+is+10,+and+$i+is+11

$i+=+10;
$a+=+%2B%2B$i;
//+Now+$a+is+11,+and+$i+is+11|code-block|syntax|javascript|1188313|使用$i%2B%2B有时会有轻微的性能成本。你看，当你做一些像这样的事情|1188314|$a+=+$i%2B%2B;|1188315|你真的这么做了：|1188316|$temporary_variable+=+$i;
$i=$i%2B1;
$a=$temporary_variable;|1188317|entityMap^0|0|4|6|2|F|4|L|4|R|2|10|2|0|0|0|2|4|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]|$9|15|A|16|B|C]|$9|17|A|18|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|19|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|1A|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|1B|8|@$9|1C|A|1D|B|C]]|D|@]|E|$]]|$1|O|3|P|5|J|7|1E|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|1F|8|@]|D|@]|E|$]]|$1|S|3|T|5|J|7|1G|8|@]|D|@]|E|$K|L]]|$1|U|3|-4|5|6|7|1H|8|@]|D|@]|E|$]]]|V|$]]

<code>++$i</code> increments <code>$i</code>, but evaluates to the value of <code>$i+1</code>
<code>$i++</code> increments <code>$i</code>, but evaluates to the old value of <code>$i</code>.

Here's an example:

<pre><code>$i = 10;
$a = $i++;
// Now $a is 10, and $i is 11

$i = 10;
$a = ++$i;
// Now $a is 11, and $i is 11
</code></pre>

There is sometimes a slight preformance cost for using <code>$i++</code>. See, when you do something like

<pre><code>$a = $i++;
</code></pre>

You're really doing this:

<pre><code>$temporary_variable = $i;
$i=$i+1;
$a=$temporary_variable;
</code></pre>

blocks|key|1403519|text|%2B%2B$i+//first+increment+$i+then+run+line
$i%2B%2B+//first+run+line+then+increment+$i+|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1403520|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>++$i //first increment $i then run line
$i++ //first run line then increment $i 
</code></pre>

blocks|key|1403741|text|在这种情况下，没有区别：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1403742|for($i+=+0;$i<3;%2B%2B$i)var_dump+$i;
/*
int(0)
int(1)
int(2)
*/
for($i+=+0;$i<3;$i%2B%2B)var_dump+$i;
/*
int(0)
int(1)
int(2)
*/|code-block|syntax|javascript|1403743|但是：|1403744|for($i+=+0;$i<3;+$j+=+%2B%2B$i+)var_dump($j);
/*
NULL
int(1)
int(2)
*/
for($i+=+0;$i<3;+$j+=+$i%2B%2B+)var_dump($j);
/*
NULL
int(0)
int(1)
*/|1403745|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

in this case there is no difference:

<pre><code>for($i = 0;$i&lt;3;++$i)var_dump $i;
/*
int(0)
int(1)
int(2)
*/
for($i = 0;$i&lt;3;$i++)var_dump $i;
/*
int(0)
int(1)
int(2)
*/
</code></pre>

but:

<pre><code>for($i = 0;$i&lt;3; $j = ++$i )var_dump($j);
/*
NULL
int(1)
int(2)
*/
for($i = 0;$i&lt;3; $j = $i++ )var_dump($j);
/*
NULL
int(0)
int(1)
*/
</code></pre>

blocks|key|1188496|text|这个例子简单地解释了|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1188497|<?php+

$x+=+10;++

echo+$x%2B%2B.+'+'.$x;++//+the+result+is+10+and+11

echo+' ';

$y+=+10;

echo+%2B%2B$y.+'+'+.$y;+//+the+result+is+11+and+11

//+so+the++$x%2B%2B+is+not+showing+%2B1+at+first+but+the+next+time
//+and+the+%2B%2By+is+showing+%2B1+first+time+but+not+increasing+next|code-block|syntax|javascript|1188498|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

this example elplains simply

<pre class="lang-php prettyprint-override"><code>&lt;?php 

$x = 10; 

echo $x++. ' '.$x; // the result is 10 and 11

echo '&lt;br&gt;';

$y = 10;

echo ++$y. ' ' .$y; // the result is 11 and 11

// so the $x++ is not showing +1 at first but the next time
// and the ++y is showing +1 first time but not increasing next
</code></pre>

blocks|key|1403567|text|为了解释jldupont的观点：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1403568|$i+=+1;
$x+=+$i%2B%2B;
echo+$x;+//+prints+1
$x+=+%2B%2B$i;
echo+$x;+//+prints+3|code-block|syntax|javascript|1403569|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

To explain jldupont's point:

<pre><code>$i = 1;
$x = $i++;
echo $x; // prints 1
$x = ++$i;
echo $x; // prints 3
</code></pre>

blocks|key|1403910|text|$i%2B%2B被称为后增量。只有在先将$i的原始值赋给$j之后，它才会递增$i的值。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1403911|%2B%2B$i被称为预增量。在将值赋给$j之前，它会递增$i的值，因此更新后的$i值将被赋给$j。|1403912|因此，|1403913|$i+=+4;
$j+=+$i%2B%2B;
//+Now,+$i+=+5+and+$j+=+4

$i+=+4;
$j+=+%2B%2B$i;
//+Now,+$i+=+5+and+$j+=+5|code-block|syntax|javascript|1403914|这些理论也以类似的方式适用于递减。|1403915|希望这能有所帮助！|1403916|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|S|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|T|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|V|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

$i++ is known as post-increment. It increments the value of $i only after assigning the original value of $i to $j first.

++$i is known as pre-increment. It increments the value of $i before assigning the value to $j, so the updated value of $i will be assigned to $j.

Hence,

<pre><code>$i = 4;
$j = $i++;
// Now, $i = 5 and $j = 4

$i = 4;
$j = ++$i;
// Now, $i = 5 and $j = 5
</code></pre>

These theories apply in a similar manner for decrementing as well.

Hope this helps!

blocks|key|1403679|text|查看前增量和后增量的另一种方式是，它是组合2个语句的简写。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1403680|预增|1403681|//+long+form
$y+=+$y+%2B+1;
$x+=+$y;+//+any+statement+using+$y

//+shorthand
$x+=+%2B%2B$y;+//+the+same+statement+using+$y|code-block|syntax|javascript|1403682|递增后|1403683|//+long+form
$x+=+$y;+//+any+statement+using+$y
$y+=+$y+%2B+1;

//+shorthand
$x+=+$y%2B%2B;+//+the+same+statement+using+$y|1403684|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

Another way of looking at pre and post incrementing is that it's shorthand for combining 2 statements.

Pre-incrementing

<pre><code>// long form
$y = $y + 1;
$x = $y; // any statement using $y

// shorthand
$x = ++$y; // the same statement using $y
</code></pre>

Post-incrementing

<pre><code>// long form
$x = $y; // any statement using $y
$y = $y + 1;

// shorthand
$x = $y++; // the same statement using $y
</code></pre>

blocks|key|1188426|text|我运行了以下代码来测试%2B%2B$i是否比$i%2B%2B快10%25。我承认，代码没有稳定的结果，但即使这样，我至少应该看到一些接近10%25的数字。我得到的最高约为4-4.5%25。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1188427|<?php

$randomFloat+=+rand(0,+10)+/+10;

$before1+=+microtime(true);

for($i=0;+$i+<1000000;+%2B%2B$i){
++++$rand+=+(rand(0,+10)+/+10)+*+(rand(0,+10)+/+10);
}

$after1+=+microtime(true);
echo+'it+took+'.($after1-$before1)+.+'+seconds+fot+%2B%2B$i<br+/>';

$before2+=+microtime(true);

for($i=0;+$i+<1000000;+$i%2B%2B){
++++$rand+=+(rand(0,+10)+/+10)+*+(rand(0,+10)+/+10);
}

$after2+=+microtime(true);
echo+'it+took+'.($after2-$before2)+.+'+seconds+fot+$i%2B%2B<br+/><br+/>';

echo+'%2B%2B$i+is+'.((($after1-$before1)*100)/($after2-$before2)-100).'%25+faster+than+$i%2B%2B';|code-block|syntax|javascript|1188428|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I ran the following code to test if ++$i is 10% faster than $i++. I admit, the code does not have a stable outcome but even then I should at least have seen some numbers near the 10%. The highest I got was 4-4.5% approximately. 

<pre><code>&lt;?php

$randomFloat = rand(0, 10) / 10;

$before1 = microtime(true);

for($i=0; $i &lt;1000000; ++$i){
 $rand = (rand(0, 10) / 10) * (rand(0, 10) / 10);
}

$after1 = microtime(true);
echo 'it took '.($after1-$before1) . ' seconds fot ++$i&lt;br /&gt;';

$before2 = microtime(true);

for($i=0; $i &lt;1000000; $i++){
 $rand = (rand(0, 10) / 10) * (rand(0, 10) / 10);
}

$after2 = microtime(true);
echo 'it took '.($after2-$before2) . ' seconds fot $i++&lt;br /&gt;&lt;br /&gt;';

echo '++$i is '.((($after1-$before1)*100)/($after2-$before2)-100).'% faster than $i++';
</code></pre>

What's the difference between <code>++$i</code> and <code>$i++</code> in PHP?

问在PHP中++$i和$i++有什么不同？
EN

回答 8

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问在PHP中++$i和$i++有什么不同？EN

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问在PHP中++$i和$i++有什么不同？
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