Understanding Variable Scope in Programming: Types and Importance

1. CSCI 130 Scope of Variables Chapter 6

2. What is scope • Parts of program which can access a variable • accessibility • visibility • How long variable takes up system resources (memory)

3. Scope Program 1 (compiles) | Program 2 (does not compile) int x = 999; | void squareX() | void squareX(); | void main() { | int x = 999; void main() { | x = x * x; x = x * x; | } } | | squareX() { squareX () { | x = x * x; x = x * x; | } } |

4. Importance of scope • Modularization • each function should be independent • variables isolated from interference • Scope allows for control over degree of isolation

5. External (Global variables) • Declared outside any function (including main) • Visible to any functions within that file • extern keyword • note: Listing 6.3 in book is incorrect • int x = 999; should be before main() • Use globals for variables accessed by many functions (i.e. PI)

6. Example of Global variable int x = 999; void squareX(); void main() { extern int x; //This line is not necessary, but adds to structure x = x * x; } squareX () { extern int x; //This line is not necessary, but adds to structure x = x * x; }

7. Local Variables • Defined within a function • Visibility limited to that function • Usually local variables created each time function is called, destroyed when function ends • Can request variable not be destroyed • static keyword

8. Local variables - example void main() { outputNumbers(); outputNumbers(); outputNumbers(); } ____________________________ void outputNumbers() { | Program output: static int x = 0; | 0 0 int y = 0; | 1 0 printf(“\n%d %d”, x++, y++); | 2 0 } |

9. Parameters and Scope • A variable in the parameter list is local • void function1(int x) { • int y = 0; • printf(“%d %d”, x, y); • } • In the preceding, x and y act as local variables

10. Static Variables • Static external variables apply only to variables within the file • static float pi = 3.14; • void main() { • …. • Area = pi * radius * radius; • }

11. Register variables • Suggests that a local variable be stored in processor register (not regular memory) • void function1() { • register int x; • …. • } • Any processing with x will be done quicker • Can only be used with simple numeric variables • can’t use with arrays, structures, etc.

12. Program Blocks • Variables can be declared as local within program blocks (statements enclosed in {}) • void main() { • int count = 0; • printf(“%d”, count); • { • int count = 10; • printf(“%d”, count); • } • printf(“%d”, count); • }

13. Guidelines • Initialize all variables • Pass data as function parameters unless most functions use the data • Use globals for data that is used in most of the functions • Put definitions at beginning of scope • functions, files, etc.