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Computer Interview Questions Answers

Perl Interview Questions Answers

Question - 1 : - How do I print the entire contents of an array with Perl?

Answer - 1 : - 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"; }

Question - 2 : - How do I generate a list of all .html files in a directory?

Answer - 2 : - Here's a snippet of code that just prints a listing of every file in the current directory that ends with the extension .html: #!/usr/bin/perl -w opendir(DIR, "."); @files = grep(/\.html$/,readdir(DIR)); closedir(DIR); foreach $file (@files) { print "$file\n"; }

Question - 3 : - How do I do < fill-in-the-blank > for each element in a hash?

Answer - 3 : - 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"; }

Question - 4 : - Perl uses single or double quotes to surround a zero or more characters. Are the single(' ') or double quotes (" ") identical?

Answer - 4 : - 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.

Question - 5 : - What happens to objects lost in "unreachable" memory, such as the object returned by Ob->new() in `{ my $ap; $ap = [ Ob->new(), \$ap ]; }' ?  

Answer - 5 : - 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"

Question - 6 : - How to open and read data files with Perl?

Answer - 6 : - Data files are opened in Perl using the open() function. When you open a data file, all you have to do is specify (a) a file handle and (b) the name of the file you want to read from. As an example, suppose you need to read some data from a file named "checkbook.txt". Here's a simple open statement that opens the checkbook file for read access: open (CHECKBOOK, "checkbook.txt"); In this example, the name "CHECKBOOK" is the file handle that you'll use later when reading from the checkbook.txt data file. Any time you want to read data from the checkbook file, just use the file handle named "CHECKBOOK". Now that we've opened the checkbook file, we'd like to be able to read what's in it. Here's how to read one line of data from the checkbook file: $record = < CHECKBOOK > ; After this statement is executed, the variable $record contains the contents of the first line of the checkbook file. The "<>" symbol is called the line reading operator. To print every record of information from the checkbook file open (CHECKBOOK, "checkbook.txt") || die "couldn't open the file!"; while ($record = < CHECKBOOK >) { print $record; } close(CHECKBOOK);

Question - 7 : - How to concatenate strings with Perl?

Answer - 7 : - 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";

Question - 8 : - What does `$result = f() .. g()' really return?

Answer - 8 : - False so long as f() returns false, after which it returns true until g() returns true, and then starts the cycle again. This is scalar not list context, so we have the bistable flip-flop range operator famous in parsing of mail messages, as in `$in_body = /^$/ .. eof()'. Except for the first time f() returns true, g() is entirely ignored, and f() will be ignored while g() later when g() is evaluated. Double dot is the inclusive range operator, f() and g() will both be evaluated on the same record. If you don't want that to happen, the exclusive range operator, triple dots, can be used instead. For extra credit, describe this: $bingo = ( a() .. b() ) ... ( c() .. d() );

Question - 9 : - When would `local $_' in a function ruin your day?

Answer - 9 : - 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.

Question - 10 : - How do I sort a hash by the hash key?

Answer - 10 : - 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 }

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