How to get input from the terminal in Perl

Question

I am creating a simple chatterbot program like ELIZA.

I'm taking questions from the terminal and sending reply with dialog, but my program takes only the first input and repeats.

For example, when I run my script the output may be something like this:

[Eliza]: Hi, I'm a psychotherapist. What is your name? user Input: hello my name is adam. [Eliza]: hello adam, how are you? [Eliza]: your name is adam [Eliza]: your name is adam [Eliza]: your name is adam [Eliza]: your name is adam [Eliza]: your name is adam

And it repeats endlessly.

I don't know where I am doing wrong. How can I get my program to read the next line from the keyboard?

sub hello { print "[Eliza]: Hi, I'm a psychotherapist. What is your name? \n"; } sub getPatientName { my ($reply) = @_; my @responses = ( "my name is", "i'm", "i am", "my name's" ); foreach my $response ( @responses ) { if ( lc($reply) =~ /$response/ ) { return "$'"; } } return lc($reply); } sub makeQuestion { my ($patient) = @_; my %reflections = ( "am" => "are", "was" => "were", "i" => "you", "i'd" => "you would", "i've" => "you have", "i'll" => "you will", "my" => "your", "are" => "am", "you've"=> "I have", "you'll"=> "I will", "your" => "my", "yours" => "mine", "you" => "me", "me" => "you" ); if ( $count == 0 ) { $patientName = getPatientName($patient); $count += 1; print "Hello $patientName , How are you? \n"; } my @toBes = keys %reflections; foreach my $toBe (@toBes) { if ($patient =~/$toBe/) { $patient=~ s/$toBe/$reflections{$toBe}/i; print "$patient? \n"; } } } sub eliza { hello(); my $answer = <STDIN>; while ($answer) { chomp $answer; #remove . ! ; $answer =~ s/[.!,;]/ /; makeQuestion($answer); } } eliza();

It seems that all you need is while (my $answer = <STDIN>). What you have "loops over" $answer ... which is always true (so it keeps going), and which never again reads new input (so it keeps printing the same). — zdim
– zdim, Commented Jan 31, 2017 at 6:03
@zdim: I think "looping over" generally refers to a list, like a for statement. — Borodin
– Borodin, Commented Jan 31, 2017 at 6:43

zdim · Accepted Answer · 2023-12-18 18:59:22Z

Your while loop never reads input. The $answer got assigned STDIN before the loop and presumably has a string, that evaluates true in the while condition. The regex in the loop could in principle change $answer to false (by removing everything from it, turning it into an empty string)^† but it doesn't fetch STDIN, nor does anything else in the loop body.

Thus not only is no new input assigned to $answer, but after the first iteration nothing at all changes in the loop.^† So it keeps running forever, printing the question based on the same $answer.

You need

while (my $answer = <STDIN>) { chomp $answer; # ... }

instead.

Every time the condition of while (...) is evaluated the new input is read via <STDIN> and is assigned to $answer. Then each new question uses the new $answer. Note how a variable may be declared inside the while condition so to exist only inside the loop body (and in the condition after its declaration). This is a nice way to keep its scope restricted to where it is needed, inside the loop.

The filehandle read <...> returns undef when it gets EOF (or on error) and the loop terminates. See I/O Operators in perlop. A user at the terminal can normally achieve this by Ctrl-d.

^† For example, a regex like $answer =~ s/.//; removes a character each time it runs (except a newline, which . doesn't match without the /s modifier), so if applied repeatedly eventually $answer winds up being an empty string. Of course, the regex in the question doesn't do that but it changes the string only the first time it runs on it.

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