• Global variables

  - Declared outside of any function and are known (visible) to any function in the program

• Local variables

  - Declared inside a function and are known (visible) to this function only

  - The formal parameters of a function are local variables for the function

  - Variables declared inside a block are also local (they are legal inside the block only !!)

• Scope of a variable

  - The section of the program where it is legal to use the variable

  

• What if a variable is declared twice, once as global and once as local ?

  - The local declaration takes precedence over the global declaration (i.e., the local variable hides the global variable)

  
  #include <iostream.h>
  void SomeFunc (float);
  
  const int a=17;
  int b, c;
  
  void main()
  {
   b=4;
   c=6;
   SomeFunc(42.8);
  }
  
  void SomeFunc(float c)
  {
   float b;
  
   b=2.3;
   cout << "a= " << a;
   cout << "b= " << b;
   cout << "c= " << c;
  }
  
  

• Misuse of global variables

  - Rather than passing variables to a function, it is possible to make all variables global ones !!! Do not do this !!!

  

• Disadvantages of using global variables in a function

  - Functions which use global values are not self-contained !!

  - A function is self-contained if it does not rely on anything that is declared outside of it

  *Self contained functions can easily be integrated into different programs

  *Incorporating a function which uses global variables into a program requires us to incorporate the declarations of the global variables into the program as well

  - More difficult to track down errors

  - Naming conflict: some local variables might have the same name with some global variables !!

  

• Advantages of using global variables in a function

  - Useful when many functions need to share the same data

  - Less memory space (no memory is allocated for formal parameters)

  - Faster (no actual parameters are copied to the formal parameters)

• Design guidelines

  - Avoid using global variables for the functions that are intended to be reused in other programs later

  - If you use global variables, avoid using common names for them, to reduce the chance of naming conflicts

  Lifetime of Variables

• What is the lifetime of a variable ?

  - The period of time (during program execution) when a variable has memory allocated to it

• What is the lifetime of a global variable ?

  - It is the lifetime of the entire program

• What is the lifetime of a local variable ?

  - It is the lifetime of the function or the block within which the local variable has been declared

• Storage categories

  - Automatic variables

  - Static variables

• Initialization in declarations

  - Combine declaration and initialization in one statement
  

  #include <iostream.h>
  
  void main()
  {
   int x=5;
   int y, product;
  
   cout << "Enter a number";
   cin >> y;
   product = x * y;
   cout << "The result is" << product << endl;
  }
  

  Value returning functions (revisited)

• When is it appropriate to use them ?

  - When there is only one result to be returned by the function

  - When the result is going to be used within an expression

• Examples

  - Write a function to compute the factorial of a number (n!=1 x 2 x 3 ... x n)

  - Write a function named distance that accepts the rectangular coordinates of two points (x1,y1) and (x2,y2) and calculates and returns the distance between the two points

  - Write a function to compute the roots of the quadratic equation

     
Recommended exercises

- Quick Check excersices: 1-6

- Exam preparation excersices: 1, 3, 4, 8-10