在Perl中，如何通过菱形运算符对命令行中指定的所有文件进行两次传递？

Question

If i have a text file and i want to run two types of operations, but each operation must read each line of the text separately from the other. The only way i know how to do it is

open out,(">>out.txt");
while (<>){
    #operation one
}
while (<>){
    #operation two
}
close out;

but this will run only on the first while , in which the operation runs fine, but the second one will not be complete because the second while(<>) does not actually re-read the file but tries to continue from where the first while left. Which is at the end of the file. So is there another way? Or is there a way to tell the second while to start again at the beginning?

Answer 1

Given you mention in a comment:

 perl example.pl text.txt 

The answer is - don't use <> and instead open a filehandle.

my ( $filename ) = @ARVG;

open ( my $input, "<", $filename ) or die $!;

while ( <$input> ) { 
    print; 
}

seek ( $input, 0, 0 ); 

while (  <$input> ) { 
    #something else
}

Alternatively, you can - assuming test.txt isn't particularly large - just read the whole thing into an array.

my @input_lines = <$input>;
foreach ( @input_lines ) { 
   #something
}

If you want to specify multiple files on the command line, you can wrap the whole thing in a foreach loop:

foreach my $filename ( @ARVG ) { 
    ## open; while; seek; while etc. 
}

Answer 2

If the data fits into memory:

my @lines = <>;

for ( @lines ){
  # operation one
}

for ( @lines ){
  # operation two
}

Answer 3

Couldn't you simply use the following?

while (<>) {
   operation1($_);
   operation2($_);
}

If not, then I'm assuming you need to process the content of all the files using one operation before it's process by the other.

<> reads from the files listed in @ARGV , removing them as it opens them, so the simplest solution is to backup @ARGV and repopulate it.

my @argv = @ARGV;
while (<>) { operation1($_); }
@ARGV = @argv;
while (<>) { operation2($_); }

Of course, it will fail if <> reads from something other than a plain file or a symlink to a plain file. (Same goes for any solution using seek .) The only to make that work would be to load the entire file into temporary storage (eg memory or a temporary file). The following is the simplest example of that:

my @lines = <>;
for (@lines) { operation1($_); }
for (@lines) { operation2($_); }

Answer 4

You can localize @ARGV before the first pass.

#!/usr/bin/env perl

use strict;
use warnings;


{
    local @ARGV = @ARGV;
    while (<>){
        print "Pass 1: $_";
    }
}

while (<>){
    print "Pass 2: $_";
}

Answer 5

If no file handle is used with the diamond operator, Perl will examine the @ARGV special variable. If @ARGV has no elements, then the diamond operator will read from STDIN .

This is other way of achieve your requirements:

my @stdin=<>;

foreach my $item( @stdin ) {
   # ...
}

foreach my $item( @stdin ) {
   # ...
}

Answer 6

If you need to run the operation line by line, why not try something like this

sub operation_1 {
   my $line = shift;
   #processing for operation 1
}

sub operation_2 {
   my $line = shift;
   #processing for operation 2
} 

while(<>) {
   my $line = $_;
   chomp($line);
   operation_1($line);
   operation_2($line); 
}

Answer 7

If you were reading from an actual file, you could use

seek FILEHANDLE,0,0;

However, you are using stdin and I don't think that it's possible to rewind stdin and start over.