Perl CGI Happy Path: Creating Constants

Creating Constants (Numbers)

• Everybody has to start somewhere, and your Perl program is no exception. It's normal to want to set a few variables to particular values each time your program starts up.
• Also, any time you want to try and decide if a value you are using is equal to something, you need to be able to specify what the "something" is that you want to compare, so you may need to create a constant.
• These are called "literals" or "constants" when you want to impress the person interviewing you, or be taken seriously by geeks.
• In Perl, we call numbers, characters, or lines of text Scalars, so we call these "Scalar Constants" or "Scalar Literals". We will learn "Scalar Variables" soon, so don't worry about them for now.
• Valid Perl numbers include whole numbers, floating point numbers, and numbers in scientific notation.
• Internally to Perl, all scalar numbers are stored as double precision floating point values (as defined by your operating system). Technical types can no doubt argue the pro's and con's of this approach until the end of time, but all you have to know is that this means that you can store numbers that are incredibly large, and that your math will be incredibly accurate.
• Example Scalar literal numbers, and how to write them in your Perl program:
  

   
  1                 # one
  1.2               # one and two tenths
  6.02e-23          # Avagadros number (6.02 times 10 to the -23 power)
  1.0E6             # One million
  -5.3              # Negative 5 and 3 tenths 
  1234567.890       # Just what it appears
  

• Perl also allows octal representation of literals (only) by use of a leading zero. For example, 010 is octal 10 (the decimal value of 8). This is rarely useful unless you are doing low level system tasks.
• Perl also allows hexadecimal representation of literals (only) by use of a leading zero x. For example, -0xff would be negative 255 decimal. This is rarely useful unless you are doing low level system tasks.
• This should look remarkably familiar to you C programmers. You will find a great deal in common with C and Perl.

Creating Constants (Strings)

• The traditional programmer name for a sequence of characters (letters, numbers, and symbols) is a "string".
• Valid Perl strings are any sequences of zero or more characters.
• The longest possible Perl string is one that fills all available system memory. Perl is weird this way (and I mean that endearingly).
• Any of the 256 ASCII characters are legal to put into Perl strings. Basically, this means every symbol on your keyboard can be put in a Perl string (and there are a few that aren't on your keyboard that can be used as well).
• You use Single quoted string literals when you don't want Perl to go messing with the things inside the strings. Perl will not try and replace any variable names with the variable contents, and will not try and discover and any magic characters (like carriage returns or the "bell" character).
• To get the actual single quote character to appear inside a single quoted string, preceed it with the backslash ("\") character. If you don't preceed it with a backslash, Perl thinks you are ending your string and gets confused by the following characters. To get a backslash into a single quoted string, just preceed it with a backslash as well ("\\").
• Example single quoted strings (shown as string literals):
  

   
  'hello'           # hello
  'hello world'     # hello, a space, and world
  'Hit the \\ key to continue'   # Hit the \ key to continue
  'Perl\'s Diner'   # Perl's Diner
  'Hello \nWorld'   # Literally Hello \nWorld (no internal newline)
  'Welcome
  To
  Perl'  # Welcome, newline, To, newline, Perl.
  

Double Quoted String Literals

• Double quoted string literals consist of every character between the starting and ending quote, but Perl will try and "interpret" the supplied characters. This "interpretation" consists of replacing any supplied variable names with the contents of that variable, and looking for special "metacharacters" that should be changed to special (normally non-printable) ASCII characters.
• Example double quoted strings (shown as string literals):

   
  "hello world\n"     # hello, a space, and world, then a newline
  # The word Ding, an exclamation point,
  # and the ASCII character with octal value 007
  "Ding! \007"
  "one\ttwo\tthree"   # One, a tab, two, a tab, and three
  

• Escape sequences and their Actions:
  

  Table of escape sequences and their actions

  Escape Sequence	Effect
  \n	Newline (correct for your system)
  \r	Carriage return
  \t	Horizontal tab
  \f	Form feed
  \b	Backspace
  \a	Bell
  \e	Escape
  \007	Character corresponding to any octal value (octal 7 = bell)
  \x4a	Character corresponding to any hexadecimal value (hex 4a = "J")
  \cN	Any control character (here ctrl-N)
  \\	Backslash ("\")
  \"	Double Quote (""")
  \l	Lowercase next letter
  \L	Lowercase all following characters until \E
  \u	Uppercase next letter
  \U	Uppercase all following characters until \E
  \Q	Backslash-quote all non-alphanumerics characters until \E
  \E	Stop \L, \U, \Q