Perl interview faqs,perl interview question and answers

Perl interview question and answers
When would `local $_' in a function ruin your day?
When your caller was in the middle for a while(m//g) loop
The /g state on a global variable is not protected by running local on it. That'll teach you to stop using locals. Too bad $_ can't be the target of a my() -- yet.

What happens to objects lost in "unreachable" memory, such as the object returned by Ob->new() in `{ my $ap; $ap = [ Ob->new(), \$ap ]; }' ?
Their destructors are called when that interpreter thread shuts down.
When the interpreter exits, it first does an exhaustive search looking for anything that it allocated. This allows Perl to be used in embedded and multithreaded applications safely, and furthermore guarantees correctness of object code.

Assume that $ref refers to a scalar, an array, a hash or to some nested data structure. Explain the following statements:
$$ref; # returns a scalar
$$ref[0]; # returns the first element of that array
$ref- > [0]; # returns the first element of that array
@$ref; # returns the contents of that array, or number of elements, in scalar context
$&$ref; # returns the last index in that array
$ref- > [0][5]; # returns the sixth element in the first row
@{$ref- > {key}} # returns the contents of the array that is the value of the key "key"


How do you match one letter in the current locale?
/[^\W_\d]/
We don't have full POSIX regexps, so you can't get at the isalpha() macro save indirectly. You ask for one byte which is neither a non-alphanumunder, nor an under, nor a numeric. That leaves just the alphas, which is what you want.

How do I print the entire contents of an array with Perl?
To answer this question, we first need a sample array. Let's assume that you have an array that contains the name of baseball teams, like this:
@teams = ('cubs', 'reds', 'yankees', 'dodgers');
If you just want to print the array with the array members separated by blank spaces, you can just print the array like this:
@teams = ('cubs', 'reds', 'yankees', 'dodgers');
print "@teams\n";
But that's not usually the case. More often, you want each element printed on a separate line. To achieve this, you can use this code:
@teams = ('cubs', 'reds', 'yankees', 'dodgers');
foreach (@teams) {
print "$_\n";
}

Perl uses single or double quotes to surround a zero or more characters. Are the single(' ') or double quotes (" ") identical?
They are not identical. There are several differences between using single quotes and double quotes for strings.
1. The double-quoted string will perform variable interpolation on its contents. That is, any variable references inside the quotes will be replaced by the actual values.
2. The single-quoted string will print just like it is. It doesn't care the dollar signs.
3. The double-quoted string can contain the escape characters like newline, tab, carraige return, etc.
4. The single-quoted string can contain the escape sequences, like single quote, backward slash, etc.

How many ways can we express string in Perl?
Many. For example 'this is a string' can be expressed in:
"this is a string"
qq/this is a string like double-quoted string/
qq^this is a string like double-quoted string^
q/this is a string/
q&this is a string&
q(this is a string)

How do you give functions private variables that retain their values between calls?
Create a scope surrounding that sub that contains lexicals.
Only lexical variables are truly private, and they will persist even when their block exits if something still cares about them. Thus:
{ my $i = 0; sub next_i { $i++ } sub last_i { --$i } }
creates two functions that share a private variable. The $i variable will not be deallocated when its block goes away because next_i and last_i need to be able to access it.

How do I read command-line arguments with Perl?

How do I read command-line arguments with Perl?
With Perl, command-line arguments are stored in the array named @ARGV.
$ARGV[0] contains the first argument, $ARGV[1] contains the second argument, etc.
$#ARGV is the subscript of the last element of the @ARGV array, so the number of arguments on the command line is $#ARGV + 1.
Here's a simple program:
#!/usr/bin/perl
$numArgs = $#ARGV + 1;
print "thanks, you gave me $numArgs command-line arguments.\n";
foreach $argnum (0 .. $#ARGV) {
print "$ARGV[$argnum]\n";
}

How to concatenate strings with Perl?

How to concatenate strings with Perl?
Method #1 - using Perl's dot operator:
$name = 'checkbook';
$filename = "/tmp/" . $name . ".tmp";

Method #2 - using Perl's join function
$name = "checkbook";
$filename = join "", "/tmp/", $name, ".tmp";

Method #3 - usual way of concatenating strings
$filename = "/tmp/${name}.tmp";

What is the easiest way to download the contents of a URL with Perl?

What is the easiest way to download the contents of a URL with Perl?

Once you have the libwww-perl library, LWP.pm installed, the code is this:
#!/usr/bin/perl
use LWP::Simple;
$url = get 'http://www.websitename.com/';

How do I replace every character in a file with a comma in perl?-Perl faqs

How do I replace every character in a file with a comma in perl?-Perl faqs
perl -pi.bak -e 's/\t/,/g' myfile.txt

What does length(%HASH) produce if you have thirty-seven random keys in a newly created hash in perl?

What does length(%HASH) produce if you have thirty-seven random keys in a newly created hash?
5
length() is a built-in prototyped as sub length($), and a scalar prototype silently changes aggregates into radically different forms. The scalar sense of a hash is false (0) if it's empty, otherwise it's a string representing the fullness of the buckets, like "18/32" or "39/64". The length of that string is likely to be 5. Likewise, `length(@a)' would be 2 if there were 37 elements in @a.

How to dereference a reference in perl-Perl faqs

How to dereference a reference in perl-Perl faqs
There are a number of ways to dereference a reference.
Using two dollar signs to dereference a scalar.
$original = $$strref;
Using @ sign to dereference an array.
@list = @$arrayref;
Similar for hashes.

Does Perl have reference type?

Does Perl have reference type?
Yes. Perl can make a scalar or hash type reference by using backslash operator.
For example
$str = "here we go"; # a scalar variable
$strref = \$str; # a reference to a scalar

@array = (1..10); # an array
$arrayref = \@array; # a reference to an array
Note that the reference itself is a scalar.

What's the difference between /^Foo/s and /^Foo/ in perl -Perl faqs

What's the difference between /^Foo/s and /^Foo/ in perl -Perl faqs
The second would match Foo other than at the start of the record if $* were set.
The deprecated $* flag does double duty, filling the roles of both /s and /m. By using /s, you suppress any settings of that spooky variable, and force your carets and dollars to match only at the ends of the string and not at ends of line as well -- just as they would if $* weren't set at all.

What value is returned by a lone `return;' statement in perl-Perl Faqs

What value is returned by a lone `return;' statement in perl-Perl Faqs
The undefined value in scalar context, and the empty list value () in list context.
This way functions that wish to return failure can just use a simple return without worrying about the context in which they were called.

How to find the length of an array in perl?-Perl interview

How to find the length of an array in perl?-Perl interview

$@array

How to read file into hash array in perl?-Perl interview

How to read file into hash array in perl?-Perl interview
open (IN, ""(filename in quotes))
or die "Couldn't open file for processing: $!";
while () {
chomp;
$hash_table{$_} = 0;
}
close IN;

print "$_ = $hash_table{$_}\n" foreach keys %hash_table;

How do I sort a hash by the hash value in perl?

How do I sort a hash by the hash value in perl?
Here's a program that prints the contents
of the grades hash, sorted numerically by the hash value:

#!/usr/bin/perl -w
# Help sort a hash by the hash 'value', not the 'key'.
to highest).
sub hashValueAscendingNum {
$grades{$a} <=> $grades{$b};
}

# Help sort a hash by the hash 'value', not the 'key'.
# Values are returned in descending numeric order
# (highest to lowest).
sub hashValueDescendingNum {
$grades{$b} <=> $grades{$a};
}

%grades = (
student1 => 90,
student2 => 75,
student3 => 96,
student4 => 55,
student5 => 76,
);
print "\n\tGRADES IN ASCENDING NUMERIC ORDER:\n";
foreach $key (sort hashValueAscendingNum (keys(%grades))) {
print "\t\t$grades{$key} \t\t $key\n";
}
print "\n\tGRADES IN DESCENDING NUMERIC ORDER:\n";
foreach $key (sort hashValueDescendingNum (keys(%grades))) {
print "\t\t$grades{$key} \t\t $key\n";
}

What does read() return at end of file in perl?-Perl interview

What does read() return at end of file in perl?
0
A defined (but false) 0 value is the proper indication of the end of file for read() and sysread().

Why does Perl not have overloaded functions? -Perl interview

Why does Perl not have overloaded functions?
Because you can inspect the argument count, return context, and object types all by yourself.
In Perl, the number of arguments is trivially available to a function via the scalar sense of @_, the return context via wantarray(), and the types of the arguments via ref() if they're references and simple pattern matching like /^\d+$/ otherwise. In languages like C++ where you can't do this, you simply must resort to overloading of functions.

How to read from a pipeline with Perl- Perl interview

How to read from a pipeline with Perl

Example 1:
To run the date command from a Perl program, and read the output
of the command, all you need are a few lines of code like this:
open(DATE, "date|");
$theDate = ;
close(DATE);
The open() function runs the external date command, then opens
a file handle DATE to the output of the date command.
Next, the output of the date command is read into
the variable $theDate through the file handle DATE.
Example 2:

The following code runs the "ps -f" command, and reads the output:
open(PS_F, "ps -f|");
while () {
($uid,$pid,$ppid,$restOfLine) = split;
# do whatever I want with the variables here ...
}
close(PS_F);

Perl interview faqs

Perl interview Faqs
Why should I use the -w argument with my Perl programs?
Many Perl developers use the -w option of the interpreter, especially during the development stages of an application. This warning option turns on many warning messages that can help you understand and debug your applications.
To use this option on Unix systems, just include it on the first line of the program, like this:
#!/usr/bin/perl -w
If you develop Perl apps on a DOS/Windows computer, and you're creating a program named myApp.pl, you can turn on the warning messages when you run your program like this:
perl -w myApp.pl

Assuming $_ contains HTML, which of the following substitutions will remove all tags in it?
1.s/<.*>//g;
2.s/<.*?>//gs;
3.s/<\/?[A-Z]\w*(?:\s+[A-Z]\w*(?:\s*=\s*(?:(["']).*?\1|[\w-.]+))?)*\s*>//gsix;

You can't do that.
If it weren't for HTML comments, improperly formatted HTML, and tags with interesting data like < SCRIPT >, you could do this. Alas, you cannot. It takes a lot more smarts, and quite frankly, a real parser.

I want users send data by formmail but when they send nothing or call it from web site they will see error.
codes in PHP like this:
if (isset($HTTP_POST_VARS)){
..........
}
else{
echo ("error lalalalal")
}
How it will look in perl?

In php it will be like
if (isset($HTTP_POST_VARS)){
....
}
In perl, tried this.
if ($ENV{'REQUEST_METHOD'} eq 'POST'){
.....
}

What is the output of the following Perl program?
1 $p1 = "prog1.java";
2 $p1 =~ s/(.*)\.java/$1.cpp/;
3 print "$p1\n";

prog1.cpp

Why aren't Perl's patterns regular expressions?
Because Perl patterns have backreferences.
A regular expression by definition must be able to determine the next state in the finite automaton without requiring any extra memory to keep around previous state. A pattern /([ab]+)c\1/ requires the state machine to remember old states, and thus disqualifies such patterns as being regular expressions in the classic sense of the term.

What does Perl do if you try to exploit the execve(2) race involving setuid scripts?
Sends mail to root and exits.
It has been said that all programs advance to the point of being able to automatically read mail. While not quite at that point (well, without having a module loaded), Perl does at least automatically send it.

How do I do < fill-in-the-blank > for each element in a hash?
Here's a simple technique to process each element in a hash:

#!/usr/bin/perl -w

%days = (
'Sun' =>'Sunday',
'Mon' => 'Monday',
'Tue' => 'Tuesday',
'Wed' => 'Wednesday',
'Thu' => 'Thursday',
'Fri' => 'Friday',
'Sat' => 'Saturday' );

foreach $key (sort keys %days) {
print "The long name for $key is $days{$key}.\n";
}

How do I sort a hash by the hash key?
Suppose we have a class of five students.
Their names are kim, al, rocky, chrisy, and jane.

Here's a test program that prints the contents
of the grades hash, sorted by student name:

#!/usr/bin/perl -w

%grades = (
kim => 96,
al => 63,
rocky => 87,
chrisy => 96,
jane => 79,
);

print "\n\tGRADES SORTED BY STUDENT NAME:\n";
foreach $key (sort (keys(%grades))) {
print "\t\t$key \t\t$grades{$key}\n";
}

The output of this program looks like this:

GRADES SORTED BY STUDENT NAME:
al 63
chrisy 96
jane 79
kim 96
rocky 87

}

How do you print out the next line from a filehandle with all its bytes reversed?
print scalar reverse scalar
Surprisingly enough, you have to put both the reverse and the into scalar context separately for this to work.

How do I send e-mail from a Perl/CGI program on a Unix system?
Sending e-mail from a Perl/CGI program on a Unix computer system is usually pretty simple. Most Perl programs directly invoke the Unix sendmail program. We'll go through a quick example here.
Assuming that you've already have e-mail information you need, such as the send-to address and subject, you can use these next steps to generate and send the e-mail message:
# the rest of your program is up here ...
open(MAIL, "|/usr/lib/sendmail -t");
print MAIL "To: $sendToAddress\n";
print MAIL "From: $myEmailAddress\n";
print MAIL "Subject: $subject\n";
print MAIL "This is the message body.\n";
print MAIL "Put your message here in the body.\n";
close (MAIL);

CGI Perl interview questions

CGI Perl interview questions
1)what is the meaning of rigging?
2)What is the difference between having a parenthesis after module name and without parenthsis after module name?? i.e Package::Module(); and Package::Module;
3)I have one question regarding to eval function. I know eval function is use for error checking but I am not able to understand below line. eval \'exec perl -S $0 ${1+\"$@\"}\' if 0; $0 for script name $@ set if error occur
4)write a Perl script to find a particular word in a paragraph???
5)how to create a flat file database as shown below s.no name age city phone 0 hema 22 Calcutta 4312542 1 hema 21 Bangalore 2344345 2 ganesh 25 delhi 2445454 3 kartik 45 pune 4312121 4 santosh 25 Hyderabad 2254231 5 kumar 25 mysore 2344567 6 gita 34 mangalore 6532123 7 gita 32 pune 2213456 Q1.print the details of the person who r from bangalore q2.Replace the city name managlore to pune q3.prints no of person having name gita and hema q4.print how many are of age 25

6)write a perl script to find whether a given line of text is starting and ending with same word or not ???
7)How to disable the mod_perl from apache_server as i have used perlfect search on the site and its pagination is not working and the remedy is to disable the mod_perl.
8)What is caller function in perl?
9)how to extract pin_code,phone_number,year from text file using regular expressions in perl
10)what are steps to do to lock the sony ericsson mobile with password?
11)How to make the following assignment, as arrayreference assignment ? my $arr_ref='[1,2,3,4,4,'elem']';
12)Difference between Perl and Mod_perl?
13)Why we use "use lib $path"?
14)write a script to generate n prime no.s?
15)write a script to display mirror image of a entered value and also check whether Palindrome