CSCI 201 Study Aid: Definitions

• VARIABLE DECLARATIONS: Are used to define the variables that we need. A declaration includes the type of data we intend to work with and the name we'll use for the memory location to store that data.

• CONSTANT DECLARATIONS
  • Sometimes there are fixed values we want to use over and over again in a program, eg. for a program handling information about a class of students we might have a fixed limit on the class size
  • Such values are called constants, and we can assign a name and value to each constant at the start of a program, eg.

        const   int   ClassSize  = 30;
    

    This specifies a constant integer value, 30, which we can refer to within the program by the name of ``Size''.
  • The advantage of this are:
    • The name `ClassSize' is more meaningful when reading a program than a number.
    • To change the class size we only have to alter the program in one spot, rather than going through all the code changing every appearance of `30' to something else.

• FUNCTIONS: Functions are used to accomplish different tasks - each function has the same layout: a header including the return data type, the function name, and the function parameters. The header is followed by the function body which consists of declarations of variables and instructions for the function to perform.

• FUNCTION PROTOTYPES
  • The function prototype, or formal declaration, essentially describes an interface to a function.
  • It contains essentially the same information as the function header.
  • First we specify the type of data the function returns, eg. int, float, or char. If it does not return any values then we call the type `void'
  • Then we specify the name of the function itself
  • Then we give a list of values or parameters that the function expects to receive, and the names it is going to use internally for those values
  • All function prototypes end with a semi-colon.

• THE FUNCTION MAIN: Every program must have a function called main. It is where the execution of a program begins. It is the function that co-ordinates all the others, and this is where to look when deciding what order things happen in.

• PRE-PROCESSING DIRECTIVES Instructions from the programmer to the compiler, describing things it must do before starting the translation of C++ code into machine language code.

  There are three main kinds of directives we use:

  1. INCLUDES: tell the compiler we want to use certain `libraries' of pre-coded functions, the code below includes the library stored in a file named iostream.h

        #include < iostream.h >
     

  2. DEFINITIONS: tell the compiler to replace every appearance of one string with something else. The code below causes the compiler to replace "FALSE" with "0" when creating the executable or machine language version of the code.

          #define  FALSE   0
     

  3. CONDITIONAL DEFINITIONS: to make sure that a library file or a class definition is not included in a program twice

        #ifndef RATIONAL_H
        #define RATIONAL_H
     
        class Rational {
           ...
        }
        #endif