How can I determine if an element exists in an array (perl)

Question

I'm looping through an array, and I want to test if an element is found in another array.

In pseudo-code, what I'm trying to do is this:

foreach $term (@array1) {
    if ($term is found in @array2) { 
        #do something here
    }
}

I've got the "foreach" and the "do something here" parts down-pat ... but everything I've tried for the "if term is found in array" test does NOT work ...

I've tried grep:

if grep {/$term/} @array2 { #do something }
# this test always succeeds for values of $term that ARE NOT in @array2

if (grep(/$term/, @array2)) { #do something }
# this test likewise succeeds for values NOT IN the array

I've tried a couple different flavors of "converting the array to a hash" which many previous posts have indicated are so simple and easy ... and none of them have worked.

I am a long-time low-level user of perl, I understand just the basics of perl, do not understand all the fancy obfuscated code that comprises 99% of the solutions I read on the interwebs ... I would really, truly, honestly appreciate any answers that are explicit in the code and provide a step-by-step explanation of what the code is doing ...

... I seriously don't grok $_ and any other kind or type of hidden, understood, or implied value, variable, or function. I would really appreciate it if any examples or samples have all variables and functions named with clear terms ($term as opposed to $_) ... and describe with comments what the code is doing so I, in all my mentally deficient glory, may hope to possibly understand it some day. Please. :-)

...

I have an existing script which uses 'grep' somewhat succesfully:

$rc=grep(/$term/, @array);
if ($rc eq 0) { #something happens here }

but I applied that EXACT same code to my new script and it simply does NOT succeed properly ... i.e., it "succeeds" (rc = zero) when it tests a value of $term that I know is NOT present in the array being tested. I just don't get it.

The ONLY difference in my 'grep' approach between 'old' script and 'new' script is how I built the array ... in old script, I built array by reading in from a file:

  @array=`cat file`;

whereas in new script I put the array inside the script itself (coz it's small) ... like this:

  @array=("element1","element2","element3","element4");

How can that result in different output of the grep function? They're both bog-standard arrays! I don't get it!!!! :-(

########################################################################

addendum ... some clarifications or examples of my actual code:

########################################################################

The term I'm trying to match/find/grep is a word element, for example "word123".

This exercise was just intended to be a quick-n-dirty script to find some important info from a file full of junk, so I skip all the niceties (use strict, warnings, modules, subroutines) by choice ... this doesn't have to be elegant, just simple.

The term I'm searching for is stored in a variable which is instantiated via split:

foreach $line(@array1) {
  chomp($line);  # habit

  # every line has multiple elements that I want to capture
  ($term1,$term2,$term3,$term4)=split(/\t/,$line);  

  # if a particular one of those terms is found in my other array 'array2'
  if (grep(/$term2/, @array2) { 
    # then I'm storing a different element from the line into a 3rd array which eventually will be outputted
    push(@known, $term1) unless $seen{$term1}++;
  }
}

see that grep up there? It ain't workin right ... it is succeeding for all values of $term2 even if it is definitely NOT in array2 ... array1 is a file of a couple thousand lines. The element I'm calling $term2 here is a discrete term that may be in multiple lines, but is never repeated (or part of a larger string) within any given line. Array2 is about a couple dozen elements that I need to "filter in" for my output.

...

I just tried one of the below suggestions:

if (grep $_ eq $term2, @array2) 

And this grep failed for all values of $term2 ... I'm getting an all or nothing response from grep ... so I guess I need to stop using grep. Try one of those hash solutions ... but I really could use more explanation and clarification on those.

Answer 1

This is in perlfaq. A quick way to do it is

my %seen;
$seen{$_}++ for @array1;
for my $item (@array2) {
    if ($seen{$item}) {
        # item is in array2, do something
    }
}

If letter case is not important, you can set the keys with $seen{ lc($_) } and check with if ($seen{ lc($item) }).

ETA:

With the changed question: If the task is to match single words in @array2 against whole lines in @array1, the task is more complicated. Trying to split the lines and match against hash keys will likely be unsafe, because of punctuation and other such things. So, a regex solution will likely be the safest.

Unless @array2 is very large, you might do something like this:

my $rx = join "|", @array2;
for my $line (@array1) {
    if ($line =~ /\b$rx\b/) {  # use word boundary to avoid partial matches
        # do something
    }
}

If @array2 contains meta characters, such as *?+|, you have to make sure they are escaped, in which case you'd do something like:

my $rx = join "|", map quotemeta, @array2;
# etc