2a) Write a script that asks someone to input their first name, last name 
and phone number. If the user does not type at least some characters for 
each of these, print "Do not leave any fields empty" otherwise print "Thank 
you". 

#!/usr/local/bin/perl -w

print "Please, type in your first name: ";
$fname = <STDIN> ;
chomp $fname;
print "Please, type in your second name: ";
$lname = <STDIN> ;
chomp $lname;
print "Please, type in your phone number: ";
$phone = <STDIN> ;
chomp $phone;
if (not ($fname and $lname and $phone)) {
     print "Do not leave any fields empty\n";
}
else {
     print "Thank you!\n";
}  

--------------------------------------------------------

2b) Change the script so that the script prints "Thank you" if either the 
first name or the last name or the phone number is supplied. Print "Do not 
leave all fields empty" otherwise. 

if ($fname or $lname or $phone) {
     print "Thank you!\n";
}
else {
     print "Do not leave all fields empty\n";
}

--------------------------------------------------------

2c) Change the script so that only first name and last name are required. 
The phone number is optional. 

if ($fname and $lname) {
     print "Thank you!\n";
}
else {
     print "Do not leave any fields empty\n";
}

--------------------------------------------------------

3) Write a program that asks a user to input a color.  If the color is
black or white, output "The color was black or white". If it starts
with a letter that comes after "k" in the alphabet, output "The color
starts with a letter that comes after "k" in the alphabet".


#!/usr/local/bin/perl -w
#
# if statement
#
print "Please enter a color: ";
$answer = <STDIN> ;
chomp $answer;
if (($answer eq "black") or ($answer eq "white"))
        {print "The color was black or white.\n";}
elsif ($answer gt "k")
	{print "The color starts with a letter that comes after \"k\" in the alphabet.\n";}

--------------------------------------------------------

4) Optional Material:

a) if ($a++ and $b++) {}

Observation: If the first variable is 0 then only the first variable is 
increased by one, otherwise both variables are increased by one.

Explanation: Perl looks at the expression from left to right. If the first part is 
zero, Perl does not even look at the second part because an expression
with "and" is always false if the first part is false. Therefore only $a++ is
executed, not $b++.

b) if ($a++ or $b++) {}

Observation: If the first variable is 1 then only the first variable is
increased by one, otherwise both variables are increased by one.

Explanation: Perl looks at the expression from left to right. If the first part is 
not zero, Perl does not even look at the second part because an expression
with "or" is true if the first part is true. Therefore only $a++ is
executed, not $b++.

--------------------------------------------------------

5) Modify the program so that it answers "That is great!" if the answer
was "yes", "That is disappointing" if the answer was "no" and "That is not
an answer to my question." otherwise.

#!/usr/local/bin/perl -w
#
# if statement
#
print "Do you like Perl? ";
$answer = <STDIN> ;
chomp $answer;
if ($answer eq "yes")
        {print "That is great!\n";}
elsif ($answer eq "no")
        {print "That is disappointing!\n";}
else
        {print "That is not an answer to my question.\n";}


--------------------------------------------------------


6) Modify the program from above so that it asks users to "guess the lucky 
number". If the correct number is guessed the program stops, otherwise it 
continues forever. 

#!/usr/local/bin/perl -w
#
# guess the lucky number
#
$number = 0;
while ($number ne 5){
   print "Please, guess the lucky number!\n";
   $number = <STDIN>;
   chomp $number;
   if ($number eq 5) {
      print "Great guess!\n";
   } else {
     print "That is not the right number! Try again.\n";
   }
}  

Here is an alternative version:

#!/usr/local/bin/perl -w
#
# guess the lucky number
#
print "Guess the lucky number: ";
$number = <STDIN> ;
chomp $number;
while ($number ne "5"){
	print "That is not the right number! Try again.\n";
	$number = <STDIN> ;
	chomp $number;
}     

--------------------------------------------------------

7) Modify the program so that it asks users whether they want to guess again 
each time. The program stops if users answer "no" or if the correct number 
is guessed. 

#!/usr/local/bin/perl -w
#
# while statement
#
$number = 0;
$answer = "yes";
while ($number ne "5" and $answer ne "no"){
	print "Guess the lucky number: ";
	$number = <STDIN> ;
	chomp $number;
	if ($number ne "5") {
		print "That is not the right answer\n";
		print "Would you like to try again?";
		$answer = <STDIN>;
		chomp $answer;
	}
}    

--------------------------------------------------------

8) Write a program that asks five times to guess the lucky number. 
If the correct number is guessed, the program outputs "Good guess!\n" 
otherwise it outputs "Try again!\n". After the fifth guess it stops and 
prints "Game over.\n". 

#!/usr/local/bin/perl -w
#
# while statement
#
$counter = 1;
while ($counter <= 5) {
    print "Guess the $counter number\n";
    $answer = <STDIN> ;
    chomp $answer;
    if ($answer ne "5"){
        print "Try again.\n";
    } else {
        print "Good guess!\n";
    }
    $counter++;
}   
print "Game over!\n";

--------------------------------------------------------

9) In the previous example, insert "last;" after the "Good guess!\n" print 
statement. "last" will terminate the while loop so that users do not have 
to continue guessing after they found the number. 

#!/usr/local/bin/perl -w
#
# while statement
#
$counter = 1;
while ($counter <= 5) {
    print "Guess the $counter number\n";
    $answer = <STDIN> ;
    chomp $answer;
    if ($answer ne "5"){
        print "Try again.\n";
    } else {
        print "Good guess!\n";
        last;
    }
    $counter++;
}

--------------------------------------------------------
   
10) Let users again guess five numbers but this time do not tell them 
immediately whether they were successful or not. After the while loop is 
finished print "The correct number was guessed\n" if the number was among 
the five guesses and "The correct number was not among them\n" otherwise. 

#!/usr/local/bin/perl -w
#
# while statement
#
$counter = 1;
$flag = 0;
while ($counter <= 5) {
    print "Guess the $counter number\n";
    $answer = <STDIN> ;
    chomp $answer;
    if ($answer eq "5"){
        $flag = 1;
    }
    $counter++;
}
if ($flag == 1) {
    print "The correct number was guessed\n";
} else {
    print "The correct number was not among them\n";
}   

--------------------------------------------------------

11) Modify the counter program from above using a for loop so that it asks the 
user for five guesses and then stops. Use "last" to terminate the for loop 
as soon as the correct number is guessed. 

#!/usr/local/bin/perl -w
#
# while statement
#
for ($counter = 1; $counter <= 5; $counter++) {
    print "Guess the $counter number\n";
    $answer = <STDIN> ;
    chomp $answer;
    if ($answer ne "5"){
        print "Wrong number.\n";
    } else {
        print "Good guess!\n";
        last;
    }
}