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CS156 Scope

The Scope of Variables

• All variables in a C program have a scope that determines where they are accessible.
• For example, variables declared outside the main function have global scope.
• We have also seen variables with local scope declared in the main function.

An Example

    int glow;              // a global scope variable
    int main() {
       int loco = 1;       // a local scope var with
                           // valid scope in main
       glow = 2;
       {                   // braces introduce a new scope
          int engineer;    // a variable in this scope
          engineer = loco; // accessing a local
          glow = 3;        // accessing a global
       }
       engineer = 2;       // ERROR: variable not in scope 
       return 0;
    }

Variable Names and Scope

• Variables can have the same names if they are defined in different scopes.
• The variable with the most local scope is active in any given block.

int main() {
   int n = 111;
   printf("n = %d\n", n);
   {
      int n = 222;
      printf("n = %d\n", n);
   }
   printf("n = %d\n", n);
   return 0;
}

n = 111
n = 222
n = 111

• These are two different variables, each called n.
• We can’t access the first n within the inner block.

Some Tips on Scope

• Not everything you can do in C, you should do.
• Declaring variables with the same name usually leads to code that is hard to understand.
• Global variables become hard to manage when you begin to work with multiple files.
• Declare variables in the smallest, most restrictive, scope that you can manage.