Perl中的标量vs列表上下文 [英] Scalar vs list context in Perl

问题描述

我在O'Reilly的书中发现了一个奇怪的例子:

I found an example in an O'Reilly book a little weird:

@backwards = reverse qw/ yabba dabba doo /;
print "list context: @backwards\n";
$backward = reverse qw/ yabba dabba doo /;
print "scalar1 context: $backward\n";
$notbackward = qw/ yabba dabba doo /;
print "scalar2 context: $notbackward\n";
print "print context: ",reverse qw/ yabba dabba doo /;
print "\n";

输出为:

list context: doo dabba yabba
scalar1 context: oodabbadabbay
scalar2 context: doo
print context: doodabbayabba

我不理解的是scalar1上下文:

这本书说逆向"给出了一个列表上下文，所以我猜qw/ yabba dabba doo /被视为一个列表，而reverse qw/ yabba dabba doo /被视为("doo"，"dabba"，"yabba").

The book says 'reverse something' gives a list context, so I guess qw/ yabba dabba doo / is seen as a list and reverse qw/ yabba dabba doo / as ('doo', 'dabba', 'yabba').

$backward = something来了，它暗示某事是一个标量，所以我期待的结果是'doo dabba yabba'，但这是不同的:'oodabbadabbay'.

So comes the $backward = something which implies something is a scalar, so I was expecting the result 'doo dabba yabba', but it is différent: 'oodabbadabbay'.

我认为，原因是因为无法直接将列表设置为标量.因此，我进行了scalar2测试:仅打印列表中的最新项目.为什么?为什么不参加scalar1测试?

I thought, the reason was because one cannot set a list to a scalar directly. So I made the scalar2 test: only the latest item in the list is printed. Why? Why not in the scalar1 test?

标量测试输出如何工作?

How do the scalar tests output work?

推荐答案

对于该行:

$backward = reverse qw/ yabba dabba doo /;

您正在从反方向请求标量.反向Perldoc表示:

You are requesting a scalar here from reverse. The perldoc for reverse says:

在标量上下文中，将 LIST的元素并返回一个字符串 值中的所有字符 相反的顺序.

In scalar context, concatenates the elements of LIST and returns a string value with all characters in the opposite order.

因此它返回每个颠倒的字母.

So it returns each of the letters reversed.

对于 $ notbackward = qw/yabba dabba doo/; qw//的perldoc表示:

For $notbackward = qw/ yabba dabba doo /; the perldoc for qw// says:

求出单词列表 使用以下方法从STRING中提取 嵌入空格作为单词 定界符.可以理解为 大致等同于:

Evaluates to a list of the words extracted out of STRING, using embedded whitespace as the word delimiters. It can be understood as being roughly equivalent to:

               split(’ ’, q/STRING/);

区别在于它在以下位置生成真实列表 编译时间，并在标量上下文中 返回列表中的最后一个元素.

the differences being that it generates a real list at compile time, and in scalar context it returns the last element in the list.

因此，请求标量仅返回列表中的最后一项.

So requesting the scalar only returns the last item in the list.

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