在Perl或Perl之间迭代文件有什么区别？ [英] What's the difference between iterating over a file with foreach or while in Perl?

问题描述

我在Perl中有一个文件句柄 FILE ，我想迭代文件中的所有行。以下之间有区别吗？

I have a filehandle FILE in Perl, and I want to iterate over all the lines in the file. Is there a difference between the following?

while (<FILE>) {
    # do something
}

和

foreach (<FILE>) {
    # do something
}

推荐答案

对于大多数用途，您可能不会注意到差异。但是， foreach 将每行读入列表（不是数组），然后逐行查看，而而一次读取一行。因为 foreach 会使用更多的内存并且需要处理时间，所以通常建议使用而来遍历一个文件。

For most purposes, you probably won't notice a difference. However, foreach reads each line into a list (not an array) before going through it line by line, whereas while reads one line at a time. As foreach will use more memory and require processing time upfront, it is generally recommended to use while to iterate through lines of a file.

编辑（通过Schwern）： foreach 循环相当于：

EDIT (via Schwern): The foreach loop is equivalent to this:

my @lines = <$fh>;
for my $line (@lines) {
    ...
}

不幸的是，Perl并没有优化这个特殊情况，因为它与范围运算符（ 1..10 ）

It's unfortunate that Perl doesn't optimize this special case as it does with the range operator (1..10).

例如，如果我在之间读取/ usr / share / dict / c $ c>循环并让它们睡眠完成后，我可以使用 ps 来查看进程占用多少内存。作为一个控件，我已经包括一个打开该文件的程序，但不做任何操作。

For example, if I read /usr/share/dict/words with a for loop and a while loop and have them sleep when they're done I can use ps to see how much memory the process is consuming. As a control I've included a program that opens the file but does nothing with it.

USER       PID %CPU %MEM      VSZ    RSS   TT  STAT STARTED      TIME COMMAND
schwern  73019   0.0  1.6   625552  33688 s000  S     2:47PM   0:00.24 perl -wle open my $fh, shift; for(<$fh>) { 1 } print "Done";  sleep 999 /usr/share/dict/words
schwern  73018   0.0  0.1   601096   1236 s000  S     2:46PM   0:00.09 perl -wle open my $fh, shift; while(<$fh>) { 1 } print "Done";  sleep 999 /usr/share/dict/words
schwern  73081   0.0  0.1   601096   1168 s000  S     2:55PM   0:00.00 perl -wle open my $fh, shift; print "Done";  sleep 999 /usr/share/dict/words

程序正在消耗几乎32兆的真实内存（ RSS 列）来存储我的2.4兆/ usr / share / dict / words的内容。 while 循环仅在一段时间内只存储一行，线路缓冲为70k。

The for program is consuming almost 32 megs of real memory (the RSS column) to store the contents of my 2.4 meg /usr/share/dict/words. The while loop only stores one line at a time consuming just 70k for line buffering.

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