Chapter 11. Using Functions for Procedural Abstraction (Part 1)

Transcription

1 Christian Jacob Chapter 11 Using Functions for Procedural Abstraction (Part 1) 11.1 Function Prototypes 11.2 Scope Rules of Functions Local Variables Formal Parameters and Local Variables Global Variables 11.3 The return Statement Returning from a Function Functions Returning void Chapter Overview

2 Christian Jacob Returning Values 11.4 References Chapter Overview

3 Page 3 Chapter 11: Functions Christian Jacob The functional structure of Mine Sweeper: InitializeField needs DistributeMines needs RandomInteger main() CountNeighbours needs Play GetCoordinates MarkMine UncoverSquare First Back TOC Prev Next Last

4 Page 4 Chapter 11: Functions Christian Jacob The functional (C++) structure of Mine Sweeper: void InitializeField() void void DistributeMines (int howmany) int int RandomInteger (int low, int high) int main() void CountNeighbours() void Play() void void GetCoordinates (int &i, int &j) void MarkMine (int i, int j) void UncoverSquare (int i, int j) First Back TOC Prev Next Last

5 Page 5 Chapter 11: Functions Christian Jacob 11.1 Function Prototypes In C++, all functions must be declared before they are used. Typically, this is accomplished by use of a function prototype. Prototypes specify the data interface of a function by its return type, the type of its parameters, and the number of its parameters. Example: int square(int i); The use of parameter names in a prototype is optional: int square(int); First Back TOC Prev Next Last

6 Page 6 Chapter 11: Functions Christian Jacob General Form of a Function Prototype Definition Prototype definitions have to appear before a function is used in a program. A function prototype definition is the same as a function definition, except that no body is present: type function_name( type parameter_name 1, type parameter_name 2,... type parameter_name N ); First Back TOC Prev Next Last

7 Page 7 Chapter 11: Functions Christian Jacob Prototypes provide information for the compiler Prototypes allow the compiler to perform three important operations, namely: Detect differences between the number of arguments used to call a function and the number of parameters in the function. Find and report any illegal type conversions between the type of arguments used to call a function and the type of definition of its parameters. What type of code to generate when a function is called. Different return types must be handled differently by the compiler. First Back TOC Prev Next Last

8 Page 8 Chapter 11: Functions Christian Jacob The following program uses a function prototype to enforce type checking: float square(int i); // prototype void main() int i, x = 10; float s[x]; for (i=0; i < x; i++) s[i] = square(i); float square(int i) return(float(i * i)); First Back TOC Prev Next Last

9 Page 9 Chapter 11: Functions Christian Jacob 11.2 Scope Rules of Functions The scope rules of a programming language are the rules that govern how an object a variable, a data structure, a function may be accessed by various parts of a program. The scope rules determine what code has access to a variable and the lifetime of a variable. There are three types of variables: local variables, global variables, and formal parameters. First Back TOC Scope Rules of Functions Prev Next Last

10 Page 10 Chapter 11: Functions Christian Jacob Local Variables Variables that are declared inside a function are called local variables. int f() int a, b;... int f ( ) parameters: local: int a int b Memory In this example, a and b are local variables of the function f(). That is, the function body is the the scope of variables a and b. First Back TOC Scope Rules of Functions Prev Next Last

11 Page 11 Chapter 11: Functions Christian Jacob Variables can also be localized to a block 1 (example 1): int f() int a=1, b=2, c; cout << c = a * b; int a=11, b=12, c=13; int f ( ) parameters: local: int a = 1 int b = 2 int c cout << a * b << ":"; cout << c; local: int a = 11 int b = 12 int c = 13 cout << a * b << ":"; cout << c; 1. A block begins with an opening curly brace and ends with a closing curly brace. First Back TOC Scope Rules of Functions Prev Next Last

12 Page 12 Chapter 11: Functions Christian Jacob Scopes of variables (example 2): int f() int a=1, b=2, c=4; cout << c = a * b; int a=11, b=12; int f ( ) parameters: local: int a = 1 int b = 2 int c cout << a * b << ":"; cout << c; cout << a * b << ":"; cout << c; local: int a = 11 int b = 12 a: 11 b: 12 c: 4 First Back TOC Scope Rules of Functions Prev Next Last

13 Page 13 Chapter 11: Functions Christian Jacob Examples of Blocks: A block is any code section delimeted by opening and closing curly braces: if ( condition ) switch ( condition ) while ( condition ) do while ( condition ) for ( init; condition; inc_dec ) type function ( arguments ) First Back TOC Scope Rules of Functions Prev Next Last

14 Page 14 Chapter 11: Functions Christian Jacob Local variables are created upon entry into a block are local to this block are destroyed upon exit from this block Hence, local variables exist only while the block of code in which they are declared is executing. and functions Each function defines its own scope. All variables needed within a function should be declared at the beginning of that function s code block. The variables declared within one function have no effect on those declared in another even if the variables share the same name. First Back TOC Scope Rules of Functions Prev Next Last

15 Page 15 Chapter 11: Functions Christian Jacob Formal Parameters and Local Variables If a function uses arguments, it must declare variables that will accept the values of those arguments. These variables are called the formal parameters of the function. A formal parameter behaves like any other local variable inside the function. The scope of a formal parameter is local to its function. Example: int f(int a, float b, double c)... This function has three formal parameters a, b, and c of type integer, float, and double. First Back TOC Scope Rules of Functions Prev Next Last

16 Page 16 Chapter 11: Functions Christian Jacob float f1(int a, float b) float c = a + b; return c; float f1 (... ) parameters: int a, float b local: float c float f2(int a, float b) c = a + b; return 2*c; void main() float a=1, b=2, c=5; cout << f1(a,b) + f2(a,c) + c; float f2 (... ) parameters: int a, float b local: - void main ( ) parameters: - local: float a = 1 float b = 2 float c = 4 First Back TOC Scope Rules of Functions Prev Next Last

17 Page 17 Chapter 11: Functions Christian Jacob During execution of the main() function we get the following variable scopes and values: float f1(int a, float b) float c = a + b; return c; float f2(int a, float b) c = a + b; return 2*c; void main() float a=1, b=2, c=5; cout << f1(a,b) + f2(a,c) + c; void main ( ) a : 1 b : 2 c : 5 float f1( a, b ) a : 1 b : 2 c : 3 local: int a = 11 Returns: 3 float f2( a, b ) a : 1 b : 2 c : 3 Returns: 6 c : 3 First Back TOC Scope Rules of Functions Prev Next Last

18 Page 18 Chapter 11: Functions Christian Jacob 11.3 Global Variables Global variables are known throughout the entire program, can be used by any piece of code, maintain their values during the entire execution of the program. The scope of global variables extends to the entire program. Global variables are created by declaring them outside of any function. When a global and a local variable share the same name, the local variable has precedence. First Back TOC Scope Rules of Functions Prev Next Last

19 Page 19 Chapter 11: Functions Christian Jacob float c = 10; float f1(int a, float b) float c = a + b; return c; float f2(int a, float b) c = a + b; return 2*c; void main() float a=1, b=2; cout << f1(a,b) + f2(a,c) + c; float c = 10 float f1 (... ) parameters: int a, float b local: float c float f2 (... ) parameters: int a, float b local: - void main ( ) parameters: - local: float a = 1 float b = 2 First Back TOC Scope Rules of Functions Prev Next Last

20 Page 20 Chapter 11: Functions Christian Jacob During execution of the main() function we get the following variable scopes and values: float c = 10; float f1(int a, float b) float c = a + b; return c; float f2(int a, float b) c = a + b; return 2*c; void main() float a=1, b=2; cout << f1(a,b) + f2(a,c) + c; c : 10 void main ( ) a : 1 b : 2 float f1( a, b ) a : 1 b : 2 c : 3 local: int a = 11 Returns: 3 float f2( a, b ) a : 1 b : 2 c : 3 Returns: 6 c : 3 First Back TOC Scope Rules of Functions Prev Next Last

21 Page 21 Chapter 11: Functions Christian Jacob 11.4 The return Statement Returning from a Function A function returns to its calling routine either when the function s closing curly brace is encountered or when a return statement is executed. The return statement can be used with or without an associated value. However, functions that are declared as returning a value (with nonvoid return type) must return a value. Only functions declared as void can use return without any value. First Back TOC The return Statement Prev Next Last

22 Page 22 Chapter 11: Functions Christian Jacob Functions Returning void For void functions, the return statement is mostly used as a programcontrol structure, e.g., to prevent part of a function from executing, as the following example demonstrates. void power(int base, int exp) int i = 1; if(exp < 0) return; // No negative exponents // Bypass rest of function for( ; exp; exp--) i = base * i; cout << base << ^ << exp << is << i; First Back TOC The return Statement Prev Next Last

23 Page 23 Chapter 11: Functions Christian Jacob Returning Values Every function of a non-void type has to return a value by the return statement. Any of these value-returning functions can be used as an operand in an expression. Examples: z = power(x, y); if(max(x, y) > 100) cout <<... ; switch(abs(x))... First Back TOC The return Statement Prev Next Last

24 Page 24 Chapter 11: Functions Christian Jacob The function power as a value-returning function: int power(int base, int exp) int i = 1; if(exp < 0) return(0); // No negative exponents for( ; exp; exp--) i = base * i; return(i); void main() int z; z = power(2,3);... First Back TOC The return Statement Prev Next Last

25 Page 25 Chapter 11: Functions Christian Jacob 11.5 References G. Blank and R. Barnes, The Universal Machine, Boston, MA: WCB/ McGraw-Hill, Chapter 9. H. Schildt, C++ from the Ground Up, McGraw-Hill, Berkeley, CA, Chapter 7. First Back TOC References Prev Next Last