Conditionals



Of course Perl also allows if/then/else statements. These are of the following form: 

if ($a)
{
        print "The string is not empty\n";
}
else
{
        print "The string is empty\n";
}

For this, remember that an empty string is considered to be false. It will also give an "empty" result
if $a is the string 0. 

It is also possible to include more alternatives in a conditional statement: 

if (!$a)                        # The ! is the not operator
{
        print "The string is empty\n";
}
elsif (length($a) == 1)         # If above fails, try this
{
        print "The string has one character\n";
}
elsif (length($a) == 2)         # If that fails, try this
{
        print "The string has two characters\n";
}
else                            # Now, everything has failed
{
        print "The string has lots of characters\n";
}

In this, it is important to notice that the elsif statement really does have an "e" missing. 



Exercise

Find a fairly large file that contains some text and some blank lines. The file
~webk/WWW/Perl/Misc/electricity.txt is pretty good because it's funny apart from anything else. It
was originally posted to our local news system by David O'Brien. 

From the previous exercise you should have a program which prints out the password file with line
numbers. Change it so that works with the text file. Now alter the program so that line numbers
aren't printed or counted with blank lines, but every line is still printed, including the blank ones.
Remember that when a line of the file is read in it will still include its newline character at the end. 



       Control structures



More interesting possiblities arise when we introduce control structures and looping. Perl supports
lots of different kinds of control structures which tend to be like those in C, but are very similar to
Pascal, too. Here we discuss a few of them. 



foreach

To go through each line of an array or other list-like structure (such as lines in a file) Perl uses the
foreach structure. This has the form 

foreach $morsel (@food)         # Visit each item in turn
                                # and call it $morsel
{
        print "$morsel\n";      # Print the item
        print "Yum yum\n";      # That was nice
}

The actions to be performed each time are enclosed in a block of curly braces. The first time through
the block $morsel is assigned the value of the first item in the array @food. Next time it is assigned
the value of the second item, and so until the end. If @food is empty to start with then the block of
statements is never executed. 



Testing

The next few structures rely on a test being true or false. In Perl any non-zero number and
non-empty string is counted as true. The number zero, zero by itself in a string, and the empty string
are counted as false. Here are some tests on numbers and strings. 

$a == $b                # Is $a numerically equal to $b?
                        # Beware: Don't use the = operator.
$a != $b                # Is $a numerically unequal to $b?
$a eq $b                # Is $a string-equal to $b?
$a ne $b                # Is $a string-unequal to $b?

You can also use logical and, or and not: 

($a && $b)              # Is $a and $b true?
($a || $b)              # Is either $a or $b true?
!($a)                   # is $a false?



for

Perl has a for structure that mimics that of C. It has the form 

for (initialise; test; inc)
{
         first_action;
         second_action;
         etc
}

First of all the statement initialise is executed. Then while test is true the block of actions is
executed. After each time the block is executed inc takes place. Here is an example for loop to print
out the numbers 0 to 9. 

for ($i = 0; $i < 10; ++$i)     # Start with $i = 1
                                # Do it while $i < 10
                                # Increment $i before repeating
{
        print "$i\n";
}



while and until

Here is a program that reads some input from the keyboard and won't continue until it is the correct
password 

#!/usr/local/bin/perl
print "Password? ";             # Ask for input
$a = <STDIN>;                   # Get input
chop $a;                        # Remove the newline at end
while ($a ne "fred")            # While input is wrong...
{
    print "sorry. Again? ";     # Ask again
    $a = <STDIN>;               # Get input again
    chop $a;                    # Chop off newline again
}

The curly-braced block of code is executed while the input does not equal the password. The while
structure should be fairly clear, but this is the opportunity to notice several things. First, we can we
read from the standard input (the keyboard) without opening the file first. Second, when the
password is entered $a is given that value including the newline character at the end. The chop
function removes the last character of a string which in this case is the newline. 

To test the opposite thing we can use the until statement in just the same way. This executes the
block repeatedly until the expression is true, not while it is true. 

Another useful technique is putting the while or until check at the end of the statement block rather
than at the beginning. This will require the presence of the do operator to mark the beginning of the
block and the test at the end. If we forgo the sorry. Again message in the above password program
then it could be written like this. 

#!/usr/local/bin/perl
do
{
        "Password? ";           # Ask for input
        $a = <STDIN>;           # Get input
        chop $a;                # Chop off newline
}
while ($a ne "fred")            # Redo while wrong input



Exercise

Modify the program from the previous exercise so that each line of the file is read in one by one and
is output with a line number at the beginning. You should get something like: 

1 root:oYpYXm/qRO6N2:0:0:Super-User:/:/bin/csh
2 sysadm:*:0:0:System V Administration:/usr/admin:/bin/sh
3 diag:*:0:996:Hardware Diagnostics:/usr/diags:/bin/csh
etc

You may find it useful to use the structure 

while ($line = <INFO>)
{
        ...
}

When you have done this see if you can alter it so that line numbers are printed as 001, 002, ..., 009,
010, 011, 012, etc. To do this you should only need to change one line by inserting an extra four
characters. Perl's clever like that. 

In general variable names consists of numbers, letters and underscores, but they should not start with
a number and the variable $_ is special, as we'll see later. Also, Perl is case sensitive, so $a and $A
are different. 



Operations and Assignment

Perl uses all the usual C arithmetic operators: 

$a = 1 + 2;     # Add 1 and 2 and store in $a
$a = 3 - 4;     # Subtract 4 from 3 and store in $a
$a = 5 * 6;     # Multiply 5 and 6
$a = 7 / 8;     # Divide 7 by 8 to give 0.875
$a = 9 ** 10;   # Nine to the power of 10
$a = 5 % 2;     # Remainder of 5 divided by 2
++$a;           # Increment $a and then return it
$a++;           # Return $a and then increment it
--$a;           # Decrement $a and then return it
$a--;           # Return $a and then decrement it

and for strings Perl has the following among others: 

$a = $b . $c;   # Concatenate $b and $c
$a = $b x $c;   # $b repeated $c times

To assign values Perl includes 

$a = $b;        # Assign $b to $a
$a += $b;       # Add $b to $a
$a -= $b;       # Subtract $b from $a
$a .= $b;       # Append $b onto $a

Note that when Perl assigns a value with $a = $b it makes a copy of $b and then assigns that to $a.
Therefore the next time you change $b it will not alter $a. 

Other operators can be found on the perlop manual page. Type man perlop at the prompt. 



Interpolation

The following code prints apples and pears using concatenation: 

$a = 'apples';
$b = 'pears';
print $a.' and '.$b;

It would be nicer to include only one string in the final print statement, but the line 

print '$a and $b';

prints literally $a and $b which isn't very helpful. Instead we can use the double quotes in place of
the single quotes: 

print "$a and $b";

The double quotes force interpolation of any codes, including interpreting variables. This is a much
nicer than our original statement. Other codes that are interpolated include special characters such as
newline and tab. The code \n is a newline and \t is a tab. 



Exercise

This exercise is to rewrite the Hello world program so that (a) the string is assigned to a variable and
(b) this variable is then printed with a newline character. Use the double quotes and don't use the
concatenation operator. Make sure you can get this to work before proceeding. 

 Array variables



A slightly more interesting kind of variable is the array variable which is a list of scalars (ie
numbers and strings). Array variables have the same format as scalar variables except that they are
prefixed by an @ symbol. The statement 

@food  = ("apples", "pears", "eels");
@music = ("whistle", "flute");

assigns a three element list to the array variable @food and a two element list to the array variable
@music. 

The array is accessed by using indices starting from 0, and square brackets are used to specify the
index. The expression 

$food[2]

returns eels. Notice that the @ has changed to a $ because eels is a scalar. 



Array assignments

As in all of Perl, the same expression in a different context can produce a different result. The first
assignment below explodes the @music variable so that it is equivalent to the second assignment. 

@moremusic = ("organ", @music, "harp");
@moremusic = ("organ", "whistle", "flute", "harp");

This should suggest a way of adding elements to an array. A neater way of adding elements is to use
the statement 

push(@food, "eggs");

which pushes eggs onto the end of the array @food. To push two or more items onto the array use
one of the following forms: 

push(@food, "eggs", "lard");
push(@food, ("eggs", "lard"));
push(@food, @morefood);

The push function returns the length of the new list. 

To remove the last item from a list and return it use the pop function. From our original list the pop
function returns eels and @food now has two elements: 

$grub = pop(@food);     # Now $grub = "eels"

It is also possible to assign an array to a scalar variable. As usual context is important. The line 

$f = @food;

assigns the length of @food, but 

$f = "@food";

turns the list into a string with a space between each element. This space can be replaced by any
other string by changing the value of the special $" variable. This variable is just one of Perl's many
special variables, most of which have odd names. 

Arrays can also be used to make multiple assignments to scalar variables: 

($a, $b) = ($c, $d);            # Same as $a=$c; $b=$d;
($a, $b) = @food;               # $a and $b are the first two
                                # items of @food.
($a, @somefood) = @food;        # $a is the first item of @food
                                # @somefood is a list of the
                                # others.
(@somefood, $a) = @food;        # @somefood is @food and
                                # $a is undefined.

The last assignment occurs because arrays are greedy, and @somefood will swallow up as much of
@food as it can. Therefore that form is best avoided. 

Finally, you may want to find the index of the last element of a list. To do this for the @food array
use the expression 

$#food



Displaying arrays

Since context is important, it shouldn't be too surprising that the following all produce different
results: 

print @food;    # By itself
print "@food";  # Embedded in double quotes
print @food.""; # In a scalar context



Exercise

Try out each of the above three print statements to see what they do. 

 File handling



Here is the basic perl program which does the same as the UNIX cat command on a certain file. 

#!/usr/local/bin/perl
#
# Program to open the password file, read it in,
# print it, and close it again.

$file = '/etc/passwd';          # Name the file
open(INFO, $file);              # Open the file
@lines = <INFO>;                # Read it into an array
close(INFO);                    # Close the file
print @lines;                   # Print the array

The open function opens a file for input (i.e. for reading). The first parameter is the filehandle
which allows Perl to refer to the file in future. The second parameter is an expression denoting the
filename. If the filename was given in quotes then it is taken literally without shell expansion. So
the expression '~/notes/todolist' will not be interpreted successfully. If you want to force shell
expansion then use angled brackets: that is, use <~/notes/todolist> instead. 

The close function tells Perl to finish with that file. 

There are a few useful points to add to this discussion on filehandling. First, the open statement can
also specify a file for output and for appending as well as for input. To do this, prefix the filename
with a > for output and a >> for appending: 

open(INFO, $file);      # Open for input
open(INFO, ">$file");   # Open for output
open(INFO, ">>$file");  # Open for appending
open(INFO, "<$file");   # Also open for input

Second, if you want to print something to a file you've already opened for output then you can use
the print statement with an extra parameter. To print a string to the file with the INFO filehandle
use 

print INFO "This line goes to the file.\n";

Third, you can use the following to open the standard input (usually the keyboard) and standard
output (usually the screen) respectively: 

open(INFO, '-');        # Open standard input
open(INFO, '>-');       # Open standard output

In the above program the information is read from a file. The file is the INFO file and to read from
it Perl uses angled brackets. So the statement 

@lines = <INFO>;

reads the file denoted by the filehandle into the array @lines. Note that the <INFO> expression
reads in the file entirely in one go. This because the reading takes place in the context of an array
variable. If @lines is replaced by the scalar $lines then only the next one line would be read in. In
either case each line is stored complete with its newline character at the end. 



Exercise

M