blocks|key|3069048|text|Perl有许多这种近乎奇怪的语法，它们极大地简化了常见的任务(比如迭代列表和以某种方式更改内容)，但如果您没有意识到它们，就会被绊倒。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3069049|$a别名为数组中的值-这允许您在循环中修改数组。如果你不想这样做，就不要修改$a。|offset|length|style|CODE|3069050|entityMap^0|0|0|2|12|2|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@$D|L|E|M|F|G]|$D|N|E|O|F|G]]|9|@]|A|$]]|$1|H|3|-4|5|6|7|P|8|@]|9|@]|A|$]]]|I|$]]

Perl has lots of these almost-odd syntax things which greatly simplify common tasks (like iterating over a list and changing the contents in some way), but can trip you up if you're not aware of them.

<code>$a</code> is aliased to the value in the array - this allows you to modify the array inside the loop. If you don't want to do that, don't modify <code>$a</code>.

blocks|key|3069060|text|请参阅perldoc+perlsyn|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3069061|如果LIST的任何元素是一个左值，你可以通过在循环中修改|3069062|来修改它。相反，如果LIST的任何元素不是左值，则任何修改该元素的尝试都将失败。换句话说，foreach循环索引变量是您循环遍历的列表中每个项目的隐式别名。|blockquote|3069063|3069064|3069065|文档中的语言特性并不奇怪，尽管我确实发现有多少人在遇到他们不理解的行为时拒绝检查文档。|style|BOLD|3069066|entityMap|0|LINK|mutability|MUTABLE|url|http://perldoc.perl.org/perlsyn.html#Foreach-Loops^0|3|F|0|0|0|0|0|0|0|4|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@$A|X|B|Y|1|Z]]|C|$]]|$1|D|3|E|5|6|7|10|8|@]|9|@]|C|$]]|$1|F|3|G|5|H|7|11|8|@]|9|@]|C|$]]|$1|I|3|-4|5|6|7|12|8|@]|9|@]|C|$]]|$1|J|3|-4|5|6|7|13|8|@]|9|@]|C|$]]|$1|K|3|L|5|6|7|14|8|@$A|15|B|16|M|N]]|9|@]|C|$]]|$1|O|3|-4|5|6|7|17|8|@]|9|@]|C|$]]]|P|$Q|$5|R|S|T|C|$U|V]]]]

See <a href="http://perldoc.perl.org/perlsyn.html#Foreach-Loops" rel="noreferrer">perldoc perlsyn</a>:

<blockquote>
 If any element of LIST is an lvalue, you can modify it by modifying VAR inside the loop. Conversely, if any element of LIST is NOT an lvalue, any attempt to modify that element will fail. In other words, the foreach loop index variable is an implicit alias for each item in the list that you're looping over. 
</blockquote>

There is nothing weird or odd about a documented language feature although I do find it odd how many people refuse check the docs upon encountering behavior they do not understand.

blocks|key|3069024|text|在本例中，$a是数组元素的别名。只要你的代码中没有$a+=，你就不会修改数组。:-)|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3069025|如果我没记错的话，map、grep等都有相同的别名行为。|3069026|entityMap^0|5|2|P|4|0|9|3|D|4|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|I|$]]

<code>$a</code> in this case is an alias to the array element. Just don't have <code>$a =</code> in your code and you won't modify the array. :-)

If I remember correctly, <code>map</code>, <code>grep</code>, etc. all have the same aliasing behaviour.

blocks|key|3282143|text|正如其他人所说，这是有文档记录的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3282144|我的理解是，@_、for、map和grep的别名行为提供了速度和内存优化，也为创造性的开发提供了有趣的可能性。本质上，所发生的是对构造块的按引用传递调用。这样可以避免不必要的数据拷贝，从而节省时间和内存。|offset|length|style|CODE|3282145|use+strict;
use+warnings;

use+List::MoreUtils+qw(apply);

my+@array+=+qw(+cat+dog+horse+kanagaroo+);

foo(@array);


print+join+"\n",+'',+'foo()',+@array;

my+@mapped+=+map+{+s/oo/ee/g+}+@array;

print+join+"\n",+'',+'map-array',+@array;
print+join+"\n",+'',+'map-mapped',+@mapped;

my+@applied+=+apply+{+s/fee//g+}+@array;

print+join+"\n",+'',+'apply-array',+@array;
print+join+"\n",+'',+'apply-applied',+@applied;


sub+foo+{
+++$_+.=+'foo'+for+@_;
}|code-block|syntax|javascript|3282146|注意List::MoreUtils+apply函数的用法。它的工作原理类似于map，但它复制了topic变量，而不是使用引用。如果你讨厌像这样写代码：|3282147|+my+@foo+=+map+{+my+$f+=+$_;+$f+=~+s/foo/bar/+}+@bar;|3282148|你会喜欢apply的，它会变成：|3282149|+my+@foo+=+apply+{+s/foo/bar/+}+@bar;|3282150|需要注意的是:如果您将只读值传递到其中一个修改其输入值的构造中，您将得到一个“修改尝试的只读值”错误。|3282151|perl+-e+'$_%2B%2B+for+"o"'|3282152|entityMap|0|LINK|mutability|MUTABLE|url|http://search.cpan.org/perldoc?List::MoreUtils^0|0|6|2|9|3|D|3|H|4|0|0|I|5|12|3|2|F|0|0|0|4|5|0|0|0|0^^$0|@$1|2|3|4|5|6|7|16|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|17|8|@$D|18|E|19|F|G]|$D|1A|E|1B|F|G]|$D|1C|E|1D|F|G]|$D|1E|E|1F|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|1G|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|1H|8|@$D|1I|E|1J|F|G]|$D|1K|E|1L|F|G]]|9|@$D|1M|E|1N|1|1O]]|A|$]]|$1|O|3|P|5|J|7|1P|8|@]|9|@]|A|$K|L]]|$1|Q|3|R|5|6|7|1Q|8|@$D|1R|E|1S|F|G]]|9|@]|A|$]]|$1|S|3|T|5|J|7|1T|8|@]|9|@]|A|$K|L]]|$1|U|3|V|5|6|7|1U|8|@]|9|@]|A|$]]|$1|W|3|X|5|J|7|1V|8|@]|9|@]|A|$K|L]]|$1|Y|3|-4|5|6|7|1W|8|@]|9|@]|A|$]]]|Z|$10|$5|11|12|13|A|$14|15]]]]

As others have said, this is documented.

My understanding is that the aliasing behavior of <code>@_</code>, <code>for</code>, <code>map</code> and <code>grep</code> provides a speed and memory optimization as well as providing interesting possibilities for the creative. What happens is essentially, a pass-by-reference invocation of the construct's block. This saves time and memory by avoiding unnecessary data copying.

<pre><code>use strict;
use warnings;

use List::MoreUtils qw(apply);

my @array = qw( cat dog horse kanagaroo );

foo(@array);


print join "\n", '', 'foo()', @array;

my @mapped = map { s/oo/ee/g } @array;

print join "\n", '', 'map-array', @array;
print join "\n", '', 'map-mapped', @mapped;

my @applied = apply { s/fee//g } @array;

print join "\n", '', 'apply-array', @array;
print join "\n", '', 'apply-applied', @applied;


sub foo {
 $_ .= 'foo' for @_;
}
</code></pre>

Note the use of <a href="http://search.cpan.org/perldoc?List::MoreUtils" rel="noreferrer">List::MoreUtils</a> <code>apply</code> function. It works like <code>map</code> but makes a copy of the topic variable, rather than using a reference. If you hate writing code like:

<pre><code> my @foo = map { my $f = $_; $f =~ s/foo/bar/ } @bar;
</code></pre>

you'll love <code>apply</code>, which makes it into:

<pre><code> my @foo = apply { s/foo/bar/ } @bar;
</code></pre>

Something to watch out for: if you pass read only values into one of these constructs that modifies its input values, you will get a "Modification of a read-only value attempted" error.

<pre><code>perl -e '$_++ for "o"'
</code></pre>

blocks|key|320994|text|这里重要的区别是，当您在for循环的初始化部分中声明一个my变量时，它似乎共享局部变量和词典的一些属性(更了解内部结构的人愿意澄清一下吗？)|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|320995|my+@src+=+1+..+10;

for+my+$x+(@src)+{
++++#+$x+is+an+alias+to+elements+of+@src
}

for+(@src)+{
++++my+$x+=+$_;
++++#+$_+is+an+alias+but+$x+is+not+an+alias
}|code-block|syntax|javascript|320996|有趣的副作用是，在第一种情况下，在for循环中定义的sub{}是$x别名列表元素周围的闭包。了解了这一点，可以(尽管有点奇怪)接近一个别名值，它甚至可能是一个全局值，我认为这在任何其他构造中都是不可能的。|320997|our+@global+=+1+..+10;
my+@subs;
for+my+$x+(@global)+{+
++++push+@subs,+sub+{%2B%2B$x}
}

$subs[5]();+#+modifies+the+@global+array|320998|entityMap^0|C|3|S|2|0|0|Q|5|W|2|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|V|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|P|$]]

the important distinction here is that when you declare a <code>my</code> variable in the initialization section of a <code>for</code> loop, it seems to share some properties of both locals and lexicals (someone with more knowledge of the internals care to clarify?)

<pre><code>my @src = 1 .. 10;

for my $x (@src) {
 # $x is an alias to elements of @src
}

for (@src) {
 my $x = $_;
 # $_ is an alias but $x is not an alias
}
</code></pre>

the interesting side effect of this is that in the first case, a <code>sub{}</code> defined within the for loop is a closure around whatever element of the list <code>$x</code> was aliased to. knowing this, it is possible (although a bit odd) to close around an aliased value which could even be a global, which I don't think is possible with any other construct.

<pre><code>our @global = 1 .. 10;
my @subs;
for my $x (@global) { 
 push @subs, sub {++$x}
}

$subs[5](); # modifies the @global array
</code></pre>

blocks|key|3069092|text|当您循环遍历列表中的每个元素时，您的$a只是用作该元素的别名。它正在被用来代替$_。可以看出$a不是局部变量，因为它是在块的外部声明的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3069093|如果您将其视为$_的替身(它就是这样)，那么将其赋值给$a就会更明显地改变列表的内容。事实上，如果您像这样定义自己的迭代器，$_就不存在。|3069094|foreach+my+$a+(1..10)
++++print+$_;+#+error
}|code-block|syntax|javascript|3069095|如果你想知道重点是什么，考虑一下这个案例：|3069096|my+@row+=+(1..10);
my+@col+=+(1..10);

foreach+(@row){
++++print+$_;
++++foreach(@col){
++++++++print+$_;
++++}
}|3069097|在这种情况下，为$_提供一个更友好的名称更具可读性|3069098|foreach+my+$x+(@row){
++++print+$x;
++++foreach+my+$y+(@col){
++++++++print+$y;
++++}
}|3069099|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|V|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|W|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|X|8|@]|9|@]|A|$]]|$1|O|3|P|5|F|7|Y|8|@]|9|@]|A|$G|H]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

Your $a is simply being used as an alias for each element of the list as you loop over it. It's being used in place of $_. You can tell that $a is not a local variable because it is declared outside of the block.

It's more obvious why assigning to $a changes the contents of the list if you think about it as being a stand in for $_ (which is what it is). In fact, $_ doesn't exist if you define your own iterator like that.

<pre><code>foreach my $a (1..10)
 print $_; # error
}
</code></pre>

If you're wondering what the point is, consider the case:

<pre><code>my @row = (1..10);
my @col = (1..10);

foreach (@row){
 print $_;
 foreach(@col){
 print $_;
 }
}
</code></pre>

In this case it is more readable to provide a friendlier name for $_

<pre><code>foreach my $x (@row){
 print $x;
 foreach my $y (@col){
 print $y;
 }
}
</code></pre>

blocks|key|3069110|text|试一试|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3069111|foreach+my+$a+(@_+=+@ar)|code-block|syntax|javascript|3069112|现在修改$a不会修改@ar。适用于我的v5.20.2|3069113|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Try

<pre><code>foreach my $a (@_ = @ar)
</code></pre>

now modifying $a does not modify @ar.
Works for me on v5.20.2

OK, I have the following code:

<pre><code>use strict;
my @ar = (1, 2, 3);
foreach my $a (@ar)
{
 $a = $a + 1;
}

print join ", ", @ar;
</code></pre>

and the output?

<blockquote>
 2, 3, 4
</blockquote>

What the heck? Why does it do that? Will this always happen? is $a not really a local variable? What where they thinking?

Why is Perl foreach variable assignment modifying the values in the array?

Perl

好的，我有以下代码：use strict;my @ar = (1, 2, 3);foreach my $a (@ar){  $a = $a + 1;}print join ", ", @ar;那么输出结果呢？ 2、3、4什么鬼东西？它为什么要这么做？这种情况会一直发生吗？$a不是真正的局部变量吗？他们在想什么？

问为什么Perl foreach变量赋值要修改数组中的值？
EN

回答 7

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问为什么Perl foreach变量赋值要修改数组中的值？
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