Quiz 6

1.  What type of loop would you suggest for printing each element of an array? For counting to 100 by 3s? For waiting until the user correctly types in a secret password?

a.  for, while, foreach

b.  while, foreach, for

c.  foreach, for, while

d.  foreach, while, for

2.  What criticism would you make about the following program, which is supposed to print the integers from ten to one?

#!/usr/bin/perl
$num = 10;
while ($num > 0) {
    if ($num == 0) { print “\n”; }
    else           { print $num, ‘ ’; }
    $num++;
}

a.  $num isn’t modified, so the loop never exits.

b.  == is used instead of =.

c.  There’s a syntax error in the if statement.

d.  $num++ should be $num -- instead.

3.  What’s printed by the following loop?

for ($i = -3; $i < 3; $i++) {
    print “$i ”;
    $i++;
}

a.  -3 -2 -1 0 1 2

b.  -3 -2 -1 0 1 2 3

c.  -3 -1 1 3

d.  -3 -1 1

4.  What’s printed by the following code?

$num = 20;
while ($num < 30) {
   $num++;
    next if ($num % 3) == 0;
    last if $num == 19;
    if (($num % 2) == 0) { $num += 3; } else { $num -= 3; }
    print $num;
}

a.  Nothing

b.  25 29

c.  18

d.  19 17

Exercise 6

Difficulty: Easy

Pretend you’re collaborating with your boss on a Perl program. The program should print out every multiple of 5 from 0 to 100. You know that you could easily do this in one line with a for loop.

for ($i = 0; $i <= 100; $i += 5) { print $i, ‘ ’; }

Unfortunately, your boss didn’t read this book, so his loop is a bit flawed. He gives you the following.

#!/usr/bin/perl
$num = 0;
while ($num >= 0) {
                       # <-----------------------your code goes here.
     print “$num ”;
     $num++;
}

Your boss prides himself on his programming skill. Don’t offend him by changing his code. Instead, save your job by judiciously using next and last to make the program work.

Session 7
The Default Variable $_

“The President said that the President will sign any bill that arrives on the President’s desk.”

“The President said that she’ll sign any bill that arrives on her desk.”

Pronouns such as “hers” shorten and simplify our sentences, relying upon listeners to put the right value in its place. Wouldn’t it be nice if programming languages could do that? Unfortunately, most programming languages can’t: If a function expects two arguments, you can’t just say, “Here’s the first argument; use the obvious choice for the second.”

Perl has something not found in many other programming languages: the default variable $_ (Figure 1-11).

Figure 1-11  $_

Some functions and operators use $_ in place of an argument. If so, you can omit that argument and Perl will use the contents of $_ instead. That might sound horrible; after all, won’t that lead to surprising behavior sometimes? Not often, and statements with fewer arguments are easier to read. You’ll find $_ quite handy..

Anyway, let’s see what $_ can do. Consider the following foreach loop.

foreach $movie (@movies) { print $movie }

Thanks to $_, you can eliminate the somewhat redundant variable $movie.

foreach (@movies) { print }

“Huh?” you ask? “Where’d $movie go? And how does print() know what to print?”

Here’s how it works. When a foreach loop has no iteration variable (the $movie in foreach $movie (@movies) ), it uses $_ instead. And if print() is given nothing to print, it uses the contents of $_. So you don’t need $movie at all.

Several of the functions you’ve seen use $_.

chomp;

is the same as

chomp($_);

and

print;

is the same as

print($_);

and

int;

is the same as

int($_);

Whenever a while loop is given <> as its condition, it stores each line of user input in $_. That’s demonstrated in parrot (Listing 1-18), which echoes everything the user types.

Listing 1-18 parrot: Evaluating <> inside a while loop

#!/usr/bin/perl -w
while (<>) {         # Stuff whatever the user types into $_
    print;           # ...and print it!
}

Look ma, no variables!

        % parrot
RESULT: This is line one.
        This is line one.
        Hello?
        Hello?
        Polly wanna cracker?
        Polly wanna cracker?
        <CTRL-D>

[CTRL]-[D] terminates standard input, so the while loop exits. (Use [CTRL]-[Z] followed by a [RETURN] on MS-DOS computers.)

You can also assign values to $_, just like any other variable.

$_ = “Poughkeepsie”;
print;

prints Poughkeepsie.