Identifiers and Variables and Constants, Oh My!

• The book defines an Identifier as a name associated with a function or data object and used to refer to that function or data object.
  • I like a name associated with a user defined object in a program.
• We have seen identifiers associated with
  • Constants : COMPUTER_ITEM_1
  • And standard identifiers : cin, endl, main, std
• Syntax rules vs conventions.
  • Syntax rules MUST be followed or the program will not compile.
  • Conventions should be followed, they make programs easier to read or understand.
• Syntax rules for forming an identifier
  • They must only contain letters, numbers and the underscore.
  • They must start with a letter or the underscore
    • Convention: Don't use the underscore to start identifiers
  • They must not be a reserved word.
• Conventions for identifiers
  • They should indicate how the identifier is used
  • They should follow the naming conventions
  • See This page for more information.
• Constants hold "permanent" information in a program.
  • The value of a constant may not change as the program is executed.
• Constants must be declared before they are used.
• For now, declare constants at the "top" of the code
  • after using namespace std.
• To declare a constant
  • const datatype identifier = value
  • const datatype identifier {value} // this is preferred
  • There are others, but use one of these.
• The conventions for constants are
  • Use all upper case letters
  • Use a _ to delimit words.
  • Use an identifier that describes the constant.
• Constant identifiers are tricky
  • If a number has a meaning, it should be defined as a constant.
  • const float TAX_RATE {0.06};
  • But don't do this
  • const int FIVE{5};
• When to use a constant is tricky as well
  • I would use a constant for the computer items in the trunk
  • But I would not use a constant for a question to print.
• Watch what I do, read the book and you will catch on.
• Variables hold "temporary" information in a program.
  • The value of a variable may change as the program is executed.
• Variables must be declared before they are used.
• To declare a variable
  • datatype identifier;
    • string item1;
  • datatype identifier = value;
    • string item1 = "rope";
  • datatype identifier {value}; // this is preferred
    • string item1 {"rope"};
  • datatype identifier, identifier ...;
    • string item1, item2, item3;
  • datatype identifier = value, identifier = value ...;
    • string item1 = "rope", item2 = "rock", item3 = "string";
  • datatype identifier {value}, identifier {value} ...;
    • string item1 {"rope"}, item2 {"rock"}, item3 {"string"};
• I would wrap the last four over multiple lines.

  •  
         string item1 {"rope"}, 
                item2 {"rock"}, 
                item3 {"string"};

• By convention variables should
  • start with a lower case. (item1, animal)
  • Each additional word should start with upper case (playerItem1)
  • Be descriptive of what they will hold.
• You can think of a variable as a box, with a label over it, that holds data.
  • The label is the identifier
  • The box is a location in memory
• Only declare variables inside of code blocks.
  • Do not declare them at the top of the code like constants.
  • Such variables are called global variables and are bad.
• In general only use a variable to perform a single task.
• You can reuse variables if they are performing the same task in different places in the code.