在 Perl 中使用 foreach 或 while 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，我想遍历文件中的所有行.以下有区别吗?

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

在大多数情况下，您可能不会注意到差异.但是，foreach 将每一行读入列表(不是数组)，然后一行一行地遍历它，而 while 在一个时间.由于 foreach 将使用更多内存并需要预先处理时间，因此通常建议使用 while 来遍历文件的行.

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) {
    ...
}

遗憾的是，Perl 没有像对范围运算符 (1..10) 那样优化这种特殊情况.

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

例如，如果我使用 for 循环和 while 循环读取/usr/share/dict/words 并让它们在完成后睡眠，我可以使用 ps 查看进程消耗了多少内存.作为一个控件，我包含了一个打开文件但不执行任何操作的程序.

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

for 程序消耗了近 32 兆的实际内存(RSS 列)来存储我的 2.4 兆/usr/share/dict/words 的内容.while 循环一次仅存储一行，仅消耗 70k 用于行缓冲.

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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