As you’ll see in the next few paragraphs, you can specify both integers (whole numbers, like 255 or 2001) and floating-point numbers (real numbers with decimal points, like 3.14159, or 1.35 x 1025). But internally, Perl computes with double-precision floating-point values.^[3] This means that there are no integer values internal to Perl—an integer constant in the program is treated as the equivalent floating-point value.^[4] You probably won’t notice the conversion (or care much), but you should stop looking for distinct integer operations (as opposed to floating-point operations), because there aren’t any.^[5]

A literal is the way a value is represented in the source code of the Perl program. A literal is not the result of a calculation or an I/O operation; it’s data written directly into the source code.

Perl’s floating-point literals should look familiar to you. Numbers with and without decimal points are allowed (including an optional plus or minus prefix), as well as tacking on a power-of-10 indicator (exponential notation) with E notation. For example:

1.25
255.000
255.0
7.25e45  # 7.25 times 10 to the 45th power (a big number)
-6.5e24  # negative 6.5 times 10 to the 24th
         # (a big negative number)
-12e-24  # negative 12 times 10 to the -24th
         # (a very small negative number)
-1.2E-23 # another way to say that - the E may be uppercase

Integer literals are also straightforward, as in:

0
2001
-40
255
61298040283768

That last one is a little hard to read. Perl allows underscores for clarity within integer literals, so we can also write that number like this:

61_298_040_283_768

It’s the same value; it merely looks different to us human beings. You might have thought that commas should be used for this purpose, but commas are already used for a more-important purpose in Perl (as we’ll see in the next chapter).

Like many other programming languages, Perl allows you to specify numbers in other than base 10 (decimal). Octal (base 8) literals start with a leading 0, hexadecimal (base 16) literals start with a leading 0x, and binary (base 2) literals start with a leading 0b.^[6] The hex digits A through F (or a through f) represent the conventional digit values of ten through fifteen. For example:

0377       # 377 octal, same as 255 decimal
0xff       # FF hex, also 255 decimal
0b11111111 # also 255 decimal (available in version 5.6 and later)

Although these values look different to us humans, they’re all three the same number to Perl. It makes no difference to Perl whether you write 0xFF or 255.000, so choose the representation that makes the most sense to you and your maintenance programmer (by which we mean the poor chap who gets stuck trying to figure out what you meant when you wrote your code. Most often, this poor chap is you, and you can’t recall why you did what you did three months ago).

When a non-decimal literal is more than about four characters long, it may be hard to read. For this reason, starting in version 5.6, Perl allows underscores for clarity within these literals:

0x1377_0b77
0x50_65_72_7C

Perl provides the typical ordinary addition, subtraction, multiplication, and division operators, and so on. For example:

2 + 3      # 2 plus 3, or 5
5.1 - 2.4  # 5.1 minus 2.4, or 2.7
3 * 12     # 3 times 12 = 36
14 / 2     # 14 divided by 2, or 7
10.2 / 0.3 # 10.2 divided by 0.3, or 34
10 / 3     # always floating-point divide, so 3.3333333...

Perl also supports a modulus operator (%). The value of the expression 10 % 3 is the remainder when ten is divided by three, which is one. Both values are first reduced to their integer values, so 10.5 % 3.2 is computed as 10 % 3.^[7]

Additionally, Perl provides the FORTRAN-like exponentiation operator, which many have yearned for in Pascal and C. The operator is represented by the double asterisk, such as 2**3, which is two to the third power, or eight.^[8]

In addition, there are other numeric operators, which we’ll introduce as we need them.

A single-quoted string literal is a sequence of characters enclosed in single quotes. The single quotes are not part of the string itself—they’re just there to let Perl identify the beginning and the ending of the string. Any character other than a single quote or a backslash between the quote marks (including newline characters, if the string continues onto successive lines) stands for itself inside a string. To get a backslash, put two backslashes in a row, and to get a single quote, put a backslash followed by a single quote. In other words:

'fred'    # those four characters: f, r, e, and d
'barney'  # those six characters
''        # the null string (no characters)
'Don't let an apostrophe end this string prematurely!'
'the last character of this string is a backslash: '
'hello
' # hello followed by backslash followed by n
'hello
there'    # hello, newline, there (11 characters total)
'''    # single quote followed by backslash

Note that the within a single-quoted string is not interpreted as a newline, but as the two characters backslash and n. Only when the backslash is followed by another backslash or a single quote does it have special meaning.

A double-quoted string literal is similar to the strings you may have seen in other languages. Once again, it’s a sequence of characters, although this time enclosed in double quotes. But now the backslash takes on its full power to specify certain control characters, or even any character at all through octal and hex representations. Here are some double-quoted strings:

"barney"        # just the same as 'barney'
"hello world
" # hello world, and a newline
"The last character of this string is a quote mark: ""
"coke	sprite"  # coke, a tab, and sprite

Note that the double-quoted literal string "barney" means the same six-character string to Perl as does the single-quoted literal string 'barney'. It’s like what we saw with numeric literals, where we saw that 0377 was another way to write 255.0. Perl lets you write the literal in the way that makes more sense to you. Of course, if you wish to use a backslash escape (like to mean a newline character), you’ll need to use the double quotes.

The backslash can precede many different characters to mean different things (generally called a backslash escape ). The nearly complete^[10] list of double-quoted string escapes is given in Table 2-1.

f



a

e

07

x7f

cC

\

"

l

L

u

U

Q

E

Another feature of double-quoted strings is that they are variable interpolated, meaning that some variable names within the string are replaced with their current values when the strings are used. We haven’t formally been introduced to what a variable looks like yet, so we’ll get back to this later in this chapter.

String values can be concatenated with the . operator. (Yes, that’s a single period.) This does not alter either string, any more than 2+3 alters either 2 or 3. The resulting (longer) string is then available for further computation or to be stored into a variable. For example:

"hello" . "world"       # same as "helloworld"
"hello" . ' ' . "world" # same as 'hello world'
'hello world' . "
"    # same as "hello world
"

Note that the concatenation must be explicitly requested with the . operator, unlike in some other languages where you merely have to stick the two values next to each other.

A special string operator is the string repetition operator, consisting of the single lowercase letter x. This operator takes its left operand (a string) and makes as many concatenated copies of that string as indicated by its right operand (a number). For example:

"fred" x 3       # is "fredfredfred"
"barney" x (4+1) # is "barney" x 5, or "barneybarneybarneybarneybarney"
5 x 4            # is really "5" x 4, which is "5555"

That last example is worth spelling out slowly. The string repetition operator wants a string for a left operand, so the number 5 is converted to the string "5" (using rules described in detail later), giving a one-character string. This new string is then copied four times, yielding the four-character string 5555. Note that if we had reversed the order of the operands, as 4 x 5, we would have made five copies of the string 4, yielding 44444. This shows that string repetition is not commutative.

The copy count (the right operand) is first truncated to an integer value (4.8 becomes 4) before being used. A copy count of less than one results in an empty (zero-length) string.

For the most part, Perl automatically converts between numbers and strings as needed. How does it know whether a number or a string is needed? It all depends upon the operator being used on the scalar value. If an operator expects a number (like + does), Perl will see the value as a number. If an operator expects a string (like . does), Perl will see the value as a string. So you don’t need to worry about the difference between numbers and strings; just use the proper operators, and Perl will make it all work.

When a string value is used where an operator needs a number (say, for multiplication), Perl automatically converts the string to its equivalent numeric value, as if it had been entered as a decimal floating-point value.^[11] So "12" * "3" gives the value 36. Trailing nonnumber stuff and leading whitespace are discarded, so "12fred34" * " 3" will also give 36 without any complaints.^[12] At the extreme end of this, something that isn’t a number at all converts to zero. This would happen if you used the string "fred" as a number.

Likewise, if a numeric value is given when a string value is needed (say, for string concatenation), the numeric value is expanded into whatever string would have been printed for that number. For example, if you want to concatenate the string Z followed by the result of 5 multiplied by 7,^[13] you can say this simply as:

"Z" . 5 * 7 # same as "Z" . 35, or "Z35"

In other words, you don’t really have to worry about whether you have a number or a string (most of the time). Perl performs all the conversions for you.^[14] And if you’re worried about efficiency, don’t be. Perl generally remembers the result of a conversion so that it’s done only once.

You should generally select variable names that mean something regarding the purpose of the variable. For example, $r is probably not very descriptive but $line_length is. A variable used for only two or three lines close together may be called something simple, like $n, but a variable used throughout a program should probably have a more descriptive name.

Similarly, properly placed underscores can make a name easier to read and understand, especially if your maintenance programmer has a different spoken language background than you have. For example, $super_bowl is a better name than $superbowl, since that last one might look like $superb_owl. Does $stopid mean $sto_pid (storing a process-ID of some kind?) or $s_to_pid (converting something to a process-ID?) or $stop_id (the ID for some kind of “stop” object?) or is it just a stopid mispelling?

Most variable names in our Perl programs are all lowercase, like most of the ones we’ll see in this book. In a few special cases, capitalization is used. Using all-caps (like $ARGV) generally indicates that there’s something special about that variable. (But you can get into an all-out brawl if you choose sides on the $underscores_are_cool versus the $giveMeInitialCaps argument. So be careful.)

Of course, choosing good or poor names makes no difference to Perl. You could name your program’s three most-important variables $OOO000OOO, $OO00OO00, and $O0O0O0O0O and Perl wouldn’t be bothered—but in that case, please, don’t ask us to maintain your code.

The most common operation on a scalar variable is assignment , which is the way to give a value to a variable. The Perl assignment operator is the equals sign (much like other languages), which takes a variable name on the left side, and gives it the value of the expression on the right. For example:

$fred = 17;            # give $fred the value of 17
$barney = 'hello';     # give $barney the five-character string 'hello'
$barney = $fred + 3;   # give $barney the current value of $fred plus 3 (20)
$barney = $barney * 2; # $barney is now $barney multiplied by 2 (40)

Notice that last line uses the $barney variable twice: once to get its value (on the right side of the equals sign), and once to define where to put the computed expression (on the left side of the equals sign). This is legal, safe, and in fact, rather common. In fact, it’s so common that we can write it using a convenient shorthand, as we’ll see in the next section.

Expressions like $fred = $fred + 5 (where the same variable appears on both sides of an assignment) occur frequently enough that Perl (like C and Java) has a shorthand for the operation of altering a variable—the binary assignment operator . Nearly all binary operators that compute a value have a corresponding binary assignment form with an appended equals sign. For example, the following two lines are equivalent:

$fred = $fred + 5; # without the binary assignment operator
$fred += 5;        # with the binary assignment operator

These are also equivalent:

$barney = $barney * 3;
$barney *= 3;

In each case, the operator causes the existing value of the variable to be altered in some way, rather than simply overwriting the value with the result of some new expression.

Another common assignment operator is the string concatenate operator ( . ); this gives us an append operator ( .= ):

$str = $str . " "; # append a space to $str
$str .= " ";       # same thing with assignment operator

Nearly all binary operators are valid this way. For example, a raise to the power of operator is written as **=. So, $fred **= 3 means “raise the number in $fred to the third power, placing the result back in $fred“.

When a string literal is double-quoted, it is subject to variable interpolation^[16] (besides being checked for backslash escapes). This means that any scalar variable^[17] name in the string is replaced with its current value. For example:

$meal = "brontosaurus steak";
$barney = "fred ate a $meal";    # $barney is now "fred ate a brontosaurus steak"
$barney = 'fred ate a ' . $meal; # another way to write that

As you see on the last line above, you can get the same results without the double quotes. But the double-quoted string is often the more convenient way to write it.

If the scalar variable has never been given a value,^[18] the empty string is used instead:

$barney = "fred ate a $meat"; # $barney is now "fred ate a "

Don’t bother with interpolating if you have just the one lone variable:

print "$fred"; # unneeded quote marks
print $fred;   # better style

There’s nothing really wrong with putting quote marks around a lone variable, but the other programmers will laugh at you behind your back.^[19]

Variable interpolation is also known as double-quote interpolation , because it happens when double-quote marks (but not single quotes) are used. It happens for some other strings in Perl, which we’ll mention as we get to them.

To put a real dollar sign into a double-quoted string, precede the dollar sign with a backslash, which turns off the dollar sign’s special significance:

$fred = 'hello';
print "The name is $fred.
";    # prints a dollar sign
print 'The name is $fred' . "
"; # so does this

The variable name will be the longest possible variable name that makes sense at that part of the string. This can be a problem if you want to follow the replaced value immediately with some constant text that begins with a letter, digit, or underscore.^[20] As Perl scans for variable names, it would consider those characters to be additional name characters, which is not what you want. Perl provides a delimiter for the variable name in a manner similar to the shell. Simply enclose the name of the variable in a pair of curly braces. Or, you can end that part of the string and start another part of the string with a concatenation operator:

$what = "brontosaurus steak";
$n = 3;
print "fred ate $n $whats.
";          # not the steaks, but the value of $whats
print "fred ate $n ${what}s.
";        # now uses $what
print "fred ate $n $what" . "s.
";     # another way to do it
print 'fred ate ' . $n . ' ' . $what . "s.
"; # an especially difficult way

Operator precedence determines which operations in a complex group of operations happen first. For example, in the expression 2+3*4, do we perform the addition first or the multiplication first? If we did the addition first, we’d get 5*4, or 20. But if we did the multiplication first (as we were taught in math class), we’d get 2+12, or 14. Fortunately, Perl chooses the common mathematical definition, performing the multiplication first. Because of this, we say multiplication has a higher precedence than addition.

You can override the default precedence order by using parentheses. Anything in parentheses is completely computed before the operator outside of the parentheses is applied (just like you learned in math class). So if I really want the addition before the multiplication, I can say (2+3)*4, yielding 20. Also, if I wanted to demonstrate that multiplication is performed before addition, I could add a decorative but unnecessary set of parentheses, as in 2+(3*4).

While precedence is simple for addition and multiplication, we start running into problems when faced with, say, string concatenation compared with exponentiation. The proper way to resolve this is to consult the official, accept-no-substitutes Perl operator precedence chart, shown in Table 2-1.^[21] (Note that some of the operators have not yet been described, and in fact, may not even appear anywhere in this book, but don’t let that scare you from reading about them in the perlop manpage.)

In the chart, any given operator has higher precedence than all of the operators listed below it, and lower precedence than all of the operators listed above it. Operators at the same precedence level resolve according to rules of associativity instead.

Just like precedence, associativity resolves the order of operations when two operators of the same precedence compete for three operands:

4 ** 3 ** 2 # 4 ** (3 ** 2), or 4 ** 9 (right associative)
72 / 12 / 3 # (72 / 12) / 3, or 6/3, or 2 (left associative)
36 / 6 * 3  # (36/6)*3, or 18

In the first case, the ** operator has right associativity, so the parentheses are implied on the right. Comparatively, the * and / operators have left associativity, yielding a set of implied parentheses on the left.

So should you just memorize the precedence chart? No! Nobody actually does that. Instead, just use parentheses when you don’t remember the order of operations, or when you’re too busy to look in the chart. After all, if you can’t remember it without the parentheses, your maintenance programmer is going to have the same trouble. So be nice to your maintenance programmer.

For comparing numbers, Perl has the logical comparison operators that remind you of algebra: < <= == >= > !=. Each of these returns a true or false value. We’ll find out more about those return values in the next section. Some of these may be different than you’d use in other languages. For example, == is used for equality, not a single = sign, because that’s used for another purpose in Perl. And != is used for inequality testing, because <> is used for another purpose in Perl. And you’ll need >= and not => for “greater than or equal to”, because the latter is used for another purpose in Perl. In fact, nearly every sequence of punctuation is used for something in Perl. So, if you get writers’ block, just let the cat walk across the keyboard, and debug what results.

For comparing strings, Perl has an equivalent set of string comparison operators which look like funny little words: lt le eq ge gt ne. These compare two strings character by character to see whether they’re the same, or whether one comes first in standard string sorting order. (In ASCII, the capital letters come before the lowercase letters, so beware.)

The comparison operators (for both numbers and strings) are given in Table 2-3.

Here are some example expressions using these comparison operators:

35 != 30 + 5         # false
35 == 35.0           # true
'35' eq '35.0'       # false (comparing as strings)
'fred' lt 'barney'   # false
'fred' lt 'free'     # true
'fred' eq "fred"     # true
'fred' eq 'Fred'     # false
' ' gt ''            # true

You may actually use any scalar value as the conditional of the if control structure. That’s handy if you want to store a true or false value into a variable, like this:

$is_bigger = $name gt 'fred';
if ($is_bigger) { ... }

But how does Perl decide whether a given value is true or false? Perl doesn’t have a separate Boolean data type, like some languages have. Instead, it uses a few simple rules:

So, if your scalar value is undef, 0, '', or '0', it’s false. All other scalars are true—including all of the types of scalars that we haven’t told you about yet.

If you need to get the opposite of any Boolean value, use the unary not operator, !. If what follows it is a true value, it returns false; if what follows is false, it returns true:

if (! $is_bigger) {
  # Do something when $is_bigger is not true
}