Syntax to define variables

 

  • Syntax to define a variable in C (same as in Java): 

       data-type  variableName  [= initialValue] ;

    Examples: 

       int   x, y;
   float z = 4.0;

     

     

Where can you place variable definitions ?   (1) inside a function

  • A variable can be defined inside a function: 

      int  main(int argc, char *argv[] )               
  {
     int x;            // Local variables are defined      
     float y = 4.0;    // inside a function

     x = 0;   // OK to access inside the same function
  }

  • Local variables: (a concept !)

    • Variables defined inside a function are local variables

  • Property of Local variables:

      • Local variables can only be accessed (= used) by statements inside the same function (a.k.a.: local scope)

Where can you place variable definitions ?   (2) outside a function

  • A variable can be defined outside a function: 

      int x;            // Global variables are defined      
  float y = 4.0;    // outside a function

  int  main(int argc, char *argv[] )               
  {
     x = 0;  // OK
  }

  void func( )
  {
     y = 1;  // OK
  }

  • Variables defined outside a function are global variables

    • There is only 1 copy of a global variable with the same name in each C program

  • Global variables can be accessed (= used) by statements in any function

DEMO: demo/C/set1/variables1.c

A global variable is similar to a class (static) variable in Java

  • A global variable in a C program has the same properties as a class/static variable in a Java program

  • The variable x in these programs behave exactly the same way:

      C program Java program 
       int x = 1;      // Global variable 
    
 int main(int argc, char *argv[] )
 {
     x = 777;    // Update
                 // global variable
 }





       public class myClass
 {
     static int x = 1;

     static void main(String argv[] )
     {
         myClass.x = 777; 
                   // Update the class 
                   // (~= global) variable
     }
 }

  • Note:

    • There is only 1 copy of a class variable with the same name in a Java class

DEMO: demo/C/set1/variables1.java

Concept:   when are the local and global variables created

  • When are local variables created:

      • Each invocation of a function will create a new copy of all local variables defined in the function/method

      • I.e.:

        • Multiple invocations of a function/method (such as recursion) will create multiple copies of the local variables

  • When are global variables created:

      • Global variables are created at the start of execution of a computer program

      • There is one copy of each global variable throughout the entire execution of a computer program