Why after the while loop I am only getting last row value?

Question

This is the files I am reading,

#Log1
Time    Src_id  Des_id  Address
0   34  56  x9870
2   36  58  x9872
4   38  60  x9874
6   40  62  x9876
8   42  64  x9878


        

Answer 1

Because @fields* gets overwritten during each loop. You need this:

while(my $line = <IN1>){
    my @tmp = split(" ", $line);
    push(@fields1, \@tmp);
}
foreach $item (@fields1){
    print("@{$item}\n");
}

Then @fields1 contains references pointing to the splited array.

The final @fields1 looks like:

@fields1 = (
  <ref> ----> ["0", "34", "56", "x9870"]
  <ref> ----> ["2", "36", "58", "x9872"]
  ...
)

The print will print:

Time Src_id Des_id Address
0 34 56 x9870
2 36 58 x9872
4 38 60 x9874
6 40 62 x9876
8 42 64 x9878

And I guess it would be better if you do chomp($line).

But I'd like to simply do push(@fields1, $line). And split each array item when in comparison stage.

To compare the content of 2 files, I personally would use 2 while loops to read into 2 arrays just like what you have done. Then do the comparison in one for or foreach.

Answer 2

Following code demonstrates how to read and print log files (OP does not specify why he splits lines into fields)

use strict;
use warnings;
use feature 'say';

my $fname1  = 'log1.txt';
my $fname2  = 'log2.txt';
my $div     = "\t";

my $file1   = read_file($fname1);
my $file2   = read_file($fname2);

print_file($file1,$div);
print_file($file2,$div);

sub read_file {
    my $fname = shift;
    
    my @data;
    
    open my $fh, '<', $fname
        or die "Couldn't read $fname";
        
    while( <$fh> ) {
        chomp;
        next if /^#Log/;
        push @data, [split];
    }
        
    close $fh;
    
    return \@data;
}

sub print_file {
    my $data = shift;
    my $div  = shift;
    
    say join($div,@{$_}) for @{$data};
}

Output

Time    Src_id  Des_id  Address
0       34      56      x9870
2       36      58      x9872
4       38      60      x9874
6       40      62      x9876
8       42      64      x9878
Time    Src_id  Des_id  Address
1       35      57      x9871
3       37      59      x9873
5       39      61      x9875
7       41      63      x9877
9       43      65      x9879

Let's assume that OP wants to merge two files into one with sorted lines on Time field

• read files into %data hash with Time field as key
• print header (@fields)
• print hash values sorted on Time key

use strict;
use warnings;
use feature 'say';

my(@fields,%data);

my $fname1  = 'log1.txt';
my $fname2  = 'log2.txt';

read_data($fname1);
read_data($fname2);

say join("\t",@fields);
say join("\t",@{$data{$_}}) for sort { $a <=> $b } keys %data;

sub read_data {
    my $fname = shift;
    
    open my $fh, '<', $fname
        or die "Couldn't open $fname";
        
    while( <$fh> ) {
        next if /^#Log/;
        if( /^Time/ ) {
            @fields = split;
        } else {
            my @line = split;
            $data{$line[0]} = \@line;
        }
    }
        
    close $fh;
}

Output

Time    Src_id  Des_id  Address
0       34      56      x9870
1       35      57      x9871
2       36      58      x9872
3       37      59      x9873
4       38      60      x9874
5       39      61      x9875
6       40      62      x9876
7       41      63      x9877
8       42      64      x9878
9       43      65      x9879

Answer 3

You can merge the log files using paste, and read the resulting merged file one line at a time. This is more elegant and saves RAM. Here is an example of a possible comparison of time1 and time2, writing STDOUT and STDERR into separate files. The example prints into STDOUT all the input fields if time1 < time2 and time1 < 4, otherwise prints a warning into STDERR:

cat > log1.log <<EOF
Time    Src_id  Des_id  Address
0   34  56  x9870
2   36  58  x9872
4   38  60  x9874
6   40  62  x9876
8   42  64  x9878
EOF


cat > log2.log <<EOF
Time    Src_id  Des_id  Address
1   35  57  x9871
3   37  59  x9873
5   39  61  x9875
7   41  63  x9877
9   43  65  x9879
EOF


# Paste files side by side, skip header, read data lines together, compare and print:

paste log1.log log2.log | \
    tail -n +2 | \
    perl -lane '
BEGIN {
    for $file_num (1, 2)  { push @col_names, map { "$_$file_num" } qw( time src_id des_id address ) }
}
my %val;
@val{ @col_names } = @F;
if ( $val{time1} < $val{time2} and $val{time1} < 4) {
    print join "\t", @val{ @col_names};
} else {
    warn "not found: @val{ qw( time1 time2 ) }";
}
' 1>out.tsv 2>out.log

Output:

% cat out.tsv
0       34      56      x9870   1       35      57      x9871
2       36      58      x9872   3       37      59      x9873
% cat out.log
not found: 4 5 at -e line 10, <> line 3.
not found: 6 7 at -e line 10, <> line 4.
not found: 8 9 at -e line 10, <> line 5.

The Perl one-liner uses these command line flags:
-e : Tells Perl to look for code in-line, instead of in a file.
-n : Loop over the input one line at a time, assigning it to $_ by default.
-l : Strip the input line separator ("\n" on *NIX by default) before executing the code in-line, and append it when printing.
-a : Split $_ into array @F on whitespace or on the regex specified in -F option.

SEE ALSO:
perldoc perlrun: how to execute the Perl interpreter: command line switches