Parsing out text from string - perl

Question

I have a tab-delimited file1:
20 50 80 110
520 590 700 770
410 440 20 50
300 340 410 440
read and put them into an array:
while(<INPUT>)
{
chomp;
push #inputarray, $_;
}
Now I'm looping through another file2:
20, 410, 700
80, 520
300
foreach number of each line in file2, I want to search the #inputarray for the number. If it exists, I want to grab the corresponding number that follows. For instance, for number 20, I want to grab the number 50. I assume that they are still separated by a tab in the string that exists as an array element in #inputarray.
while(my $line = <INPUT2>)
{
chomp $line;
my #linearray = split("\t", $line);
foreach my $start (#linearray)
{
if (grep ($start, #inputarray))
{
#want to grab the corresponding number
}
}
}
Once grep finds it, i don't know how to grab that array element to find the position of the number to extract the corresponding number using perhaps the substr function. How do i grab the array element that grep found?
A desired output would be:
line1:
20 50
410 440
700 770
line2:
80 110
520 590
line3:
300 340

Answer 1

IMHO, it would be best to store the numbers from file1 in a hash. Referring to the example clontent of file1 as you provided above you can have something like below
{
'20' => '50',
'80' => '110',
'520'=> '590',
'700'=> '770',
'410'=> '440',
'20' => '50',
'300'=> '340',
'410' => '440'
}
A sample piece of code will be like
my %inputarray;
while(<INPUT>)
{
my #numbers = split $_;
my $length = scalar $numbers;
# For $i = 0 to $i < $length;
# $inputarray{$numbers[$i]} = $numbers[$i+1];
# $i+=2;
}
An demonstration of the above loop
index: 0 1 2 3
numbers: 20 50 80 110
first iteration: $i=0
$inputarray{$numbers[0]} = $numbers[1];
$i = 2; #$i += 2;
second iteration: $i=2
$inputarray{$numbers[2]} = $numbers[3];
And then while parsing file2, you just need to treat the number as the key of %inputarray.

Answer 2

I believe this gets you close to what you want.
#!/usr/bin/perl -w
my %follows;
open my $file1, "<", $ARGV[0] or die "could not open $ARGV[0]: $!\n";
while (<$file1>)
{
chomp;
my $prev = undef;
foreach my $curr ( split /\s+/ )
{
$follows{$prev} = $curr if ($prev);
$prev = $curr;
}
}
close $file1;
open my $file2, "<", $ARGV[1] or die "could not open $ARGV[1]: $!\n";
my $lineno = 1;
while (<$file2>)
{
chomp;
print "line $lineno\n";
$lineno++;
foreach my $val ( split /,\s+/, $_ )
{
print $val, " ", ($follows{$val} // "no match"), "\n";
}
print "\n";
}
If you only want to consider numbers from file1 in pairs, as opposed to seeing which numbers follow what other numbers without taking pair boundaries into account, then you need to change the logic in the first while loop slightly.
#!/usr/bin/perl -w
my %follows;
open my $file1, "<", $ARGV[0] or die "could not open $ARGV[0]: $!\n";
while (<$file1>)
{
chomp;
my $line = $_;
while ( $line =~ s/(\S+)\s+(\S+)\s*// )
{
$follows{$1} = $2;
}
}
close $file1;
open my $file2, "<", $ARGV[1] or die "could not open $ARGV[1]: $!\n";
my $lineno = 1;
while (<$file2>)
{
chomp;
print "line $lineno\n";
$lineno++;
foreach my $val ( split /,\s+/, $_ )
{
print $val, " ", ($follows{$val} // "no match"), "\n";
}
print "\n";
}

Answer 3

If you want to read the input once but check for numbers a lot, you might be better off to split the input line into individual numbers. Then add each each number as key into a hash with the following number as value. That makes reading slow and takes more memory but the second part, where you want to check for following numbers will be a breeze thanks to exist and the nature of hashes.
If i understood your question correct, you could use just one big hash. That is of course assuming that every number is always followed by the same number.