Local Variables
All the variables we have used thus far have been local variables. A local variable is a variable which is either a variable declared within the function or is an argument passed to a function. As you may have encountered in your programming, if we declare variables in a function then we can only use them within that function. This is a direct result of placing our declaration statements inside functions.Consider a program that calls a swap() function.
#include <iostream.h>
void swap(int x, int y) {
int temp = x;
x = y;
y = temp;
}
int main()
{
int A, B;
cin >> A >> B;
cout << "A: " << A << "." << endl;
cout << "B: " << B << "." <<
endl;
swap(A, B);
cout << "A: " << A << "." << endl;
cout << "B: " << B << "." << endl;
return 0;
}
Consider
variables which are in scope (DEF: portion of a program in which a
variable is legally accessible) in the main() and swap() functions:
| Function | Accessible variables |
| main | A, B |
| swap | temp, x, y |
If you attempted to use A was in swap(), a compilation error would result ("undeclared identifier") since it is a local variable in only main().
Global Variables
A global variable (DEF) is a variable which is accessible in multiple scopes. It is important to note that global variables are only accessible after they have been declared. That is, you can use a variable is is declared below its usage in the text. For instance, if we consider this simple program, we have one global variable, w.#include <iostream.h>
double w;
void Z() {
w = 0;
return;
}
void Q() {
int index;
w = 2;
return;
}
int main() {
int i;
w = 4;
return 0;
}
w is a global variable which is in scope in all three functions in this program: Z(), Q(), and main(). This program also has local variables: index is local to Q(), and i is local to main().
Scoping can viewed easily in the following table.
| Function | Accessible Variables |
| main | w, i |
| Z | w |
| Q | w, index |
Another example.
#include <iostream.h>int sum;
void Z(int alpha) {
sum += 1;
alpha *= 3;
return;
}
double total;
void Q() {
int index;
total = index;
return;
}
int main() {
int i;
int w = 10;
sum = 0;
total = 0;
return 0;
}
We construct the scoping table.
| Function | Accessible Variables |
| main | sum total, i, w |
| Q | sum, total, index |
| Z | sum, alpha |
Note that since the variable total is declared below Z() in the text, total is not in scope in Z().