Questions every good Perl developer should be able to answer

Question

Inspired by the original thread and the up and coming clones, here's one for the Perl community.

What are questions a good Perl programmer should be able to respond to?

Answer 1

Why is use strict helpful?

Answer 2

My bellweather question is What's the difference between a list and an array?.

I also tend to like asking people to show me as many ways as they can to define a scope. There's one that people almost always forget, and another that most people think provides a scope but doesn't.

Answer 3

What is the difference between

if ($foo) { ... }

and

if (defined $foo) { ... }

and when should you use one over the other?

Answer 4

Questions

• What is a reference?
• How does Perl implement object orientation?
• How does Perl's object orientation differ from other languages like C# and Java?
• Traditional object orientation in core Perl has largely been superseded by what?
  • Why?
• What is the difference between a package and a module?
• What features were implemented in 5.10?
• What is a Schwartzian transform?

• Explain the difference between these lines of code and the values of the variables.

  my $a = (4, 5, 6);
  my @a = (4, 5, 6);
  my $b = 4, 5, 6;
  my $c = @a;
  

• What are some of Perl's greatest strengths?

• What are some of Perl's greatest weaknesses?
• Name several hallmarks of the "modern Perl" movement.
• What does the binding operator do?
• What does the flip-flop operator do?
• What is the difference between for and foreach?
• What makes Perl difficult to parse?
• What are prototypes?
• What is AUTOLOAD?
• What is the Perl motto?
  • Why is this a problem?
• What does use strict; do? Why is it useful?

• What does the following block of code do?

  print (3 + 4) * 2;
  

Tests

• Implement grep using map.
• Implement and use a dispatch table.
• Given a block of text, replace a word in that block with the return value of a function that takes that word as an argument.
• Implement a module, including documentation compatible with perldoc.
• Slurp a file.
• Draw a table that illustrates Perl's concept of truthiness.

Answer 5

• What is the difference between my and our?
• What is the difference between my and local?
• For the above, when is it appropriate to use one over the other?

Answer 6

• What are list context and scalar context?
• What is the difference between my $x = ... and my($x) = ...?
• What does my($x,undef,$z) = ... do?
• Why is my(@a,@b) = (@list1, @list2) likely a bug?
• How can a user-defined sub know whether it was called in list or scalar context? Give an example of when it makes sense for the same sub to return different values in one context or the other.

Answer 7

What is the difference between /a(.*)b/ and /a(.*?)b/?

Answer 8

What's wrong with this code?

my @array = qw/a b c d e f g h/;

for ( @array ) {
    my $val = shift @array;
    print $val, "\n";
}

Answer 9

my $a = 1;

if($a) {
    my $a = 2;
}

print $a;

What is the value of $a at the end?

Answer 10

What's wrong with using a variable as a variable name?

Study guide: Part 1, Part 2, and Part 3.

Answer 11

I think brian d foy's approach is an ingenious tactic to test knowledge, understanding, and partiality about the language and the programming craft in general: What are five things you hate about your favorite language?. If they can't name 5 they probably aren't great with the language, or are totally inept at other approaches.

He applies this to people trying to a push a language: I would extend that and say it is just as applicable here. I would expect every good Perl programmer to be able to name five things they don't like. And, I would expect those five things to have some degree of merit.

Answer 12

Write code that builds a moderately complex data structure, say an array of hashes of arrays. How would you access a particular leaf? How would you traverse the entire structure?

Answer 13

For each of the following problems, how would you solve it using hashes?

• Compute set relationships, e.g., union, intersection, mutual exclusion.
• Find unique elements of a list.
• Write a dispatch table.

Answer 14

My favourite question. What is following code missing:

open(my $fh, "<", "file.txt");
while (<$fh>) {
    print $_;
}
close($fh);

This question should open discussion about error handling in perl. It also can be adopted to other languages too.

Answer 15

Some months ago, chromatic—author of Modern Perl—wrote a similar nice article “How to Identify a Good Perl Programmer,” which contains a list of questions that every good Perl programmer should be able to answer effectively.

Some of those nice questions are given below:

1. What’s the difference between accessing an array element with $items[$index] and @items[$index]?

2. What’s the difference between == and eq?

3. How do you load and import symbols from a Perl 5 module?

4. What is the difference, on the caller side, between return; and return undef;?

5. What is the difference between reading a file with for and with while?

For complete details read How to Identify a Good Perl Programmer.

Answer 16

How is $foo->{bar}[$baz]($quux) evaluated?

Answer 17

What is a lexical closure? When are closures useful? (Please, no counter-creators!)

Answer 18

What is the difference between list context and scalar context. How do you access each? Is there such a thing as Hash context? Maybe a little bit?

Answer 19

How to swap the values of two variables without using a temporary variable?

Answer 20

• What does this one-liner print and why :

  perl -pe '}{$_ = $.' file
  

  _{answer: number of lines in the file, similar to wc -l.}

• What's wrong with this code :

  my $i;
  print ++$i + ++$i;
  

  _{answer: modifying a variable twice in the same statement leads to undefined behaviour.}

• Simple one: will the if block run :

  my @arr = undef;
  if (@arr) { ... }
  

  _{answer: yes}

• How would you code a reverse() perl builtin yourself ? You can use other perl functions.

  _{answer: many ways. A short one: sub my_reverse {sort {1} @_})}

Answer 21

I would also probably dig on regex, as I expect every good Perl programmer to master regex (but not just that). Some possible questions:

• what are lookahead and lookbehind assertion /modifierq?
• how do you check that two individual parts of a regex are identical ?
• what means greedy ?
• what are Posix character classes ?
• what does \b match ?
• what is the use of \c modifier ?
• how do you precompile a regex ?

Answer 22

What is the correct way to initialize a empty string?

 my $str = q{};

or

 my $str = "";

or

 my $str = '';