For those of you who are not familiar with
non-decimal numbering systems, here is a short explanation.
In decimal notation - or base ten - when you see the value 15 it
signifies (1 * 10) + 5 or 15<FONT size=1><SUB>10</SUB></FONT>. The
subscript indicates which base is being used.
In octal notation - or base eight - when you see the value 15 it
signifies (1 * 8) + 5 or 13<FONT size=1><SUB>10</SUB></FONT>.
In hexadecimal notation - or base 16 - when you see the value 15 it
signifies (1 * 16) + 5 or 21<FONT size=1><SUB>10</SUB></FONT>. Base 16
needs an 6 characters in addition to 0 to 9 so that each position can have
a total of 16 values. The letters A-F are used to represent 10-15. So the
value BD<FONT size=1><SUB>16</SUB></FONT> is equal to (B<FONT
size=1><SUB>16</SUB></FONT> * 16) + D<FONT size=1><SUB>16</SUB></FONT> or
(11<FONT size=1><SUB>10</SUB></FONT> * 16) + 13<FONT
size=1><SUB>10</SUB></FONT> which is 189<FONT
size=1><SUB>10</SUB></FONT>.</TD></TR></TBODY></TABLE>
If you will be using very large or very small numbers, you might also
find scientific notation to be of use.
| Note |
| If you're like me, you probably forgot most of the
math you learned in high school. However, scientific notation has always
stuck with me. Perhaps because I liked moving decimal points around.
Scientific notation looks like 10.23E+4, which is equivalent to 102,300.
You can also represent small numbers if you use a negative sign. For
example, 10.23E-4 is .001023. Simply move the decimal point to the right
if the exponent is positive and to the left if the exponent is negative.
|
| Errata
Note |
| The printed version of this book indicated that
10.23E+4 was equivalent to 1,023,000 which was incorrect. The correct
number is 102,300. This problem was spotted by Andy
Poulsen. |
2.1.1. Example: Numbers
Let’s take a look at some different types of numbers that you can use in your program code.
First, here are some integers.
| Pseudocode |
An integer. Integers are numbers with no
decimal components.
An integer in octal format. This number is 35, or (4 * 8) + 3, in base
10.
An integer in hexadecimal format. This number is also 35, or (2 * 16) +
3 in base 10. </TT></TD></TR></TBODY></TABLE><PRE><B>123
043
0x23Now, some numbers and fractions - also called floating point
values. You will frequently see these values referred to as a float
value for simplicity’s sake.
| Pseudocode |
A float with a value in the tenths place.
You can also say 100 and 5/10.
A float with a fraction value out to the thousandths place. You can
also say 54 and 534/1000. </TT></TD></TR></TBODY></TABLE><PRE><B>100.5
54.534Here’s a very small number.
| Pseudocode |
| A very small float value. You can represent
this value in scientific notation as 3.4E-5. | .000034
2.2. String Literals
String Literals are groups of characters surrounded by quotes so that they can be used as a single
datum. They are frequently used in programs to identify filenames, display
messages, and prompt for input. In Perl you can use single quotes (‘), double
quotes(“), and back quotes (`).
2.2.1. Example: Single-quoted Strings
The following examples show you how to use string literals.
String literals are widely used to identify file names or when messages are
displayed to users. First, we’ll look at single-quoted strings, then
double-quoted strings.
| Note |
| The real value of single-quoted strings won't become
apparent until you read about variable interpolation in the section "Example:
Variable Interpolation" in [](./variables.md). |
| Pseudocode |
A literal that describes one of my favorite
role-playing characters.
A literal that describes the blessed cleric that frequently helps
WasWaldo stay alive. </TT></TD></TR></TBODY></TABLE><PRE><B>'WasWaldo the Illusionist'
‘Morganna the Fair’Strings are pretty simple, huh? But what if you
wanted to use a single quote inside the literal? If you did this, Perl would
think you wanted to end the string early and a compiler error would result. Perl
uses the backslash () character to indicate that the normal function of the
single quote - ending a literal - should be ignored for a moment.
| Tip |
| The backslash character is also called an escape
character - perhaps because it lets the next character escape from its
normal interpretation. |
| Pseudocode |
A literal that comments on WasWaldo's
fighting ability. Notice how the single quote is used.
Another comment from the peanut gallery. Notice that double quotes can
be used directly inside single-quoted strings.
| ’WasWaldo can’t hit the broad side of a barn.’
‘Morganna said, “WasWaldo can’t hit anything.”’ The single-quotes are
used here specifically so that the double-quotes can be used to surround the
spoken words. Later in the section on double-quoted literals, you’ll see that
the single-quotes can be replaced by double-quotes if you’d like.
You must know only one more thing about single-quoted strings. You can add a
line break to a single-quoted string simply by adding line breaks to your source
code - as demonstrated by Listing 2.1.
| Pseudocode |
Tell Perl to begin printing.
More Lines for Perl to display
The single quote ends the string literal. </TT></TD></TR></TBODY></TABLE>
| Listing 2.1 - 02LST01.PL -
Using Embedded Line Breaks to Skip to a New Line |
print 'Bill of Goods
Bread: $34.45
Fruit: $45.00
======
$79.45';
|
Figure 2.1 shows a bill of goods displayed on one long, single-quoted
literal.
Fig. 02.1 - A bill of goods displayed one long single-quoted literal.
You can see that with single-quoted literals, even the line breaks in your
source code are part of the string.
2.2.2. Example: Double-Quoted Strings
Double-quoted strings start out simple, then become a bit more involved than single-quoted strings. With double-quoted strings, you can use the backslash to add some special characters to your string. Variables will talk about how double-quoted strings and variables interact.
| Note |
| Variables - which are described in [](./variables.md) - are simply
locations in the computer's memory where Perl holds the various data
types. They're called variables because the content of the memory can
change as needed. |
The basic double-quoted string is a series of characters surrounded by double
quotes. If you need to use the double quote inside the string, you can use the
backslash character.
| Pseudocode |
This literal is similar to one you've
already seen. Just the quotes are different.
Another literal that uses double quotes inside a double-quoted string.
</TT></TD></TR></TBODY></TABLE><PRE><B>"WasWaldo the Illusionist"
“Morganna said, “WasWaldo can’t hit anything.””Notice how the
backslash in the second line is used to escape the double quote characters. And
the single quote can be used without a backslash.
One major difference between double- and single-quoted strings is that
double-quoted strings have some special escape sequences that can be
used. Escape sequences represent characters that are not easily entered using
the keyboard or that are difficult to see inside an editor window. Table 2.1
shows all of the escape sequences that Perl understands. The examples following
the table will illustrate some of them.
| Table 2.1 - Escape
Sequences |
| Escape Sequences |
Description or Character |
| \a |
Alarm\bell |
| \b |
Backspace |
| \e |
Escape |
| \f |
Form Feed |
| \n |
Newline |
| \r |
Carriage Return |
| \t |
Tab |
| \v |
Vertical Tab |
| \$ |
Dollar Sign |
| \@ |
At sign (@) |
| \% |
Percent Sign
| Errata
Note |
| The \% is not a valid escape sequence
for Perl. It was included erroneously. Please ignore this
entry. If you need to use the % character in double-quoted
strings, go ahead and use it. Later in the book, you'll read
about the printf() function. If you want to actually use the %
character in the printf() format string, use the %% sequence.
- Randal Schwartz was kind enough to identify this
error. | |
| \0nnn |
Any Octal byte |
| \xnn |
Any Hexadecimal byte |
| \cn |
Any Control character |
| \l |
Change the next character to lower case |
| \u |
Change the next character to upper case |
| \L |
Change the following characters to lowercase until a
\E sequence is encountered. Note that you need to use an uppercase E
here, lowercase will not work. |
| \Q |
Quote meta-characters as literals. See [](./regular-expressions.md) for more information on meta-characters. |
| \U |
Change the following characters to uppercase until a
\E sequence is encountered. Note that you need to use an uppercase E
here, lowercase will not work. |
| \E |
Terminate the \L, \Q, or \U sequence. Note that you
need to use an uppercase E here, lowercase will not work. |
| \\ |
Backslash | |
| Tip |
| In [](./variables.md) you'll see why you might need to use a backslash when using the $ and @ characters.
|
| Pseudocode |
This literal represents the following:
WasWaldo is 34 years old. The \u is used twice in the first word to
capitalize the w characters. And the hexadecimal notation is used to
represent the age using the ASCII codes for 3 and 4.
This literal represents the following: The kettle was HOT!. The \U
capitalizes all characters until a \E sequence is seen.
| ”\uwas\uwaldo is \x33\x34 years old.”
“The kettle was \Uhot\E!” Actually, this example isn’t too difficult,
but it does involve looking at more than one literal at once and it’s been a few
pages since our last advanced example. Let’s look at the \t and \n escape
sequences. Listing 2.2 - a program displaying a bill with several items - will
produce the output shown in Figure 2.2.
| Pseudocode |
Display a literal as the first line, second
and third of the output.
Display literals that show what was purchased
Display a separator line.
Display the total. </TT></TD></TR></TBODY></TABLE>
| Listing 2.2 - 02LST02.PL -
Using Tabs and Newline Characters to Print |
print "Bill of Goods
Bread:\t\$34.45\n";
print "Fruit:\t";
print "\$45.00\n";
print "\t======\n";
print "\t\$79.45\n";
|
| Errata
Note |
| The printed version of this book was missing a
semi-colon in the third line of the above listing. |
Fig. 02.2 - A bill of goods displayed using newline and tab
characters.
| Tip |
| Notice that Figures 2.1 and 2.2 look identical. This
illustrates a cardinal rule of Perl - there's always more than one
way to do something. |
This program uses two methods to cause a line break.
- The first is simply to include the line break in the source code.
- The second is to use the \n or newline character.
I recommend using the \n character so that when looking at your code in the
future, you can be assured that you meant to cause a line break and did not
simply press the ENTER key by mistake.
| Caution |
| If you are a C/C++ programmer, this material is not new
to you. However, Perl strings are not identical to C/C++ strings
because they have no ending NULL character. If you are thinking of
converting C/C++ programs to Perl, take care to modify any code that
relies on the NULL character to end a string. |
2.2.3. Example: Back-Quoted Strings
It might be argued that back-quoted strings are not really a
data type. That’s because Perl uses back-quoted strings to execute system
commands. When Perl sees a back-quoted string, it passes the contents to
Windows, UNIX, or whatever operating system you are using.
Let’s see how to use the back-quoted string to display a directory listing of
all text files in the perl5 directory. Figure 2.3 shows what the output of such
a program might look like.
| Pseudocode |
| Print the directory listing.
| print `dir *.txt`; Fig. 02.3 - Using a back-quoted string to
display a directory.
All of the escape sequences used with double-quoted strings can be used with
back-quoted strings.
2.3. Array Literals
Perl uses arrays - or lists - to store a series of items. You could use an array to hold all of the
lines in a file, to help sort a list of addresses, or to store a variety of
items. We’ll look at some simple arrays in this section. In the next chapter,
“Variables” , you’ll see
more examples of how useful arrays can be.
2.3.1. Example: Printing an Array
In this section, we’ll look at printing an array and see how arrays are represented
in Perl source code.
This example shows an empty array, an array of numbers and an array of
strings. Figure 2.4 shows the output of Listing 2.3.
| Pseudocode |
Print the contents of an empty array.
Print the contents of an array of numbers.
Print the contents of an array of strings.
Print the contents of a array with different data types.
|
| Listing 2.3 - 02LST03.PL -
Printing Some Array Literals |
print "Here is an empty array:" . () . "<-- Nothing there!\n";
print (12, 014, 0x0c, 34.34, 23.3E-3);
print "\n";
print ("This", "is", 'an', "array", 'of', "strings");
print "\n";
print ("This", 30, "is", 'a', "mixed array", 'of', 0x08, "items");
|
Fig. 02.4 - The output from Listing 2.3, showing different array
literals.
The fourth line of this listing shows that you can mix single- and
double-quoted strings in the same array. You can also mix numbers and strings
interchangeably, as shown in the last line.
| Note |
Listing 2.3 uses the period, or
concatenation, operator to join a string representation of the
empty array with the string "Here is an empty array:" and the
string "<-- Nothing there!\n". You can read more about
operators in [](./operators.md).
In this and other examples in this chapters, the elements of an array
will be printed with no spaces between them. You will see how to print
with spaces in the section "Strings Revisited" in [](./variables.md).
|
2.3.2. Example: Nesting Arrays
Many times a simple list is not enough. If you’re a painter, you might have one array that
holds the names of orange hues and one that holds the names of yellow hues. To
print them, you can use Perl’s ability to specify a sub-array inside your main
array definition.
While this example is not very “real-world,” it gives you the idea behind
specifying an array by using sub-arrays.
| Pseudocode |
Print an array that consists of two
sub-arrays.
Print an array that consists of an array, a string, and another array.
</TT></TD></TR></TBODY></TABLE><PRE><B>print (("Bright Orange", "Burnt"), ("Canary Yellow", "Sunbeam"));
print ((“Bright Orange”, “Burnt”), ” Middle “, (“Canary Yellow”, “Sunbeam”));So
far, we haven’t talked about the internal representations of data types. That’s
because you almost never have to worry about such things with Perl. However, it
is important to know that, internally, the sub-arrays are merged into the main
array. In other words, the array:
((“Bright Orange”, “Burnt”), (“Canary Yellow”, “Sunbeam”))
is exactly equivalent to
(“Bright Orange”, “Burnt”, “Canary Yellow”, “Sunbeam”)
2.3.3. Example: Using a Range of Values
At times you might need an array that consists of sequential
numbers or letters. Instead of making you list out the entire array, Perl has a
shorthand notation that you can use.
Perl uses two periods (..) to replace a consecutive series of values. Not
only is this method quicker to type - and less prone to error - it is easier to
understand. Only the end points of the series are specified; you don’t need to
manually verify that every value is represented. If the .. is used, then
automatically you know that a range of values will be used.
| Pseudocode |
Print an array consisting of the numbers
from 1 to 15.
Print an array consisting of the numbers from 1 to 15 using the
shorthand method. </TT></TD></TR></TBODY></TABLE><PRE><B>print (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15);
print “\n”;
print (1..15);The two arrays used in the previous example are
identical, but they were specified differently.
| Note |
| The double periods in the array specification are
called the range operator. The range operator is also discussed in
Chapter 4, "Operators," |
You can also use the shorthand method to specify values in the middle of an
array.
| Pseudocode |
Print an array consisting of the numbers 1,
2, 7, 8, 9, 10, 14, and 15.
Print an array consisting of the letters A, B, F, G, H, Y, Z
</TT></TD></TR></TBODY></TABLE><PRE><B>print (1, 2, 7..10, 14, 15);
print “\n”;
print (“A”, “B”, “F”..”H”, “Y”, “Z”);The range operator works by
taking the lefthand value, adding one to it, then appending that new value to
the array. Perl continues to do this until the new value reaches the righthand
value. You can use letters with the range operator because the ASCII table uses
consecutive values to represent consecutive letters.
2.4. Summary
This chapter introduced you to both numeric
and string literals. You learned that literals are values that are placed
directly into your source code and never changed by the program. They are
sometimes referred to as hard-coded values.
You read about numbers and the three different bases that can be used to
represent them - decimal, octal, and hexadecimal. Very large or small numbers
can also be described using scientific notation.
Strings were perhaps a bit more involved. Single-, double-, and back-quoted
strings are used to hold strings of characters. Back-quoted strings have an
additional purpose. They tell Perl to send the string to the operating system
for execution.
Escape sequences are used to represent characters that are difficult to enter
through the keyboard or that have more than one purpose. For example, using a
double quote inside a double-quoted string would end the string before you
really intended. The backslash character was introduced to escape the double
quote and change its meaning.
The next chapter, “Variables,” will show you how
Perl uses your computer memory to store data types and also will show you ways
that you can manipulate data.
2.5. Review Questions
- What are the four types of literals?
- What is a numeric literal?
- How many types of string literals are there?
- What is the major difference between single- and double-quoted strings?
- What are three escape sequences and what do they mean?
- What would the following one-line program display?
print `dir /*.log`;
- What is scientific notation?
- How can you represent the number 64 in hexadecimal inside a double-quoted string?
- What is the easiest way to represent an array that includes the numbers 56 to 87?
2.6. Review Exercises
- Write a program that prints the decimal number 32. However, in the print command, specify the value of 32 using hexadecimal notation.
- Create program that uses the tab character in three literals to align text.
- Write a program that prints using embedded new lines in a single-quoted literal.
- Convert the number 56,500,000 into scientific notation.
- Write a program that prints an array that uses the range operator. The Left value should be AA and the right value should be BB. What happens and why?
- Write a program that prints its own source code using a back-quoted
string.
| | | | | | | | | |
