Local and Global variabel in c++

Local variables:

Local variables must always be defined at the top of a block.
When a local variable is defined - it is not initalised by the system, you must initalise it yourself.
A local variable is defined inside a block and is only visable fromwithin the block. 

main()
   {
       int i=4;
       i++;
   }

When execution of the block starts the variable is available, and when the block ends the variable 'dies'.
A local variable is visible within nested blocks unless a variable with the same name is defined within the nested block.

main()
   {
       int i=4;
       int j=10;
       
       i++;
      
       if (j > 0)
       {
          printf("i is %d\n",i);      /* i defined in 'main' can be seen      */
       }
     
       if (j > 0)
       {
          int i=100;                  /* 'i' is defined and so local to 
                                       * this block */
          printf("i is %d\n",i);      
       }                              /* 'i' (value 100) dies here            */
     
       printf("i is %d\n",i);         /* 'i' (value 5) is now visable.        */
   }

Global variables:

Global variables ARE initalised by the system when you define them!

Data Type	Initialser
int	0
char	'\0'
float	0
pointer	NULL

n the next example i is a global variable, it can be seen and modified by  main and any other functions that may reference it.

int i=4; main() { i++; }

Now, this example has global and Internal variables.

   int i=4;          /* Global definition   */
   
   main()
   {
       i++;          /* global variable     */
       func
   }

   func()
   {
       int i=10;     /* Internal declaration */
       i++;          /* Internal variable    */
   }