EECE 230 Chapter 6- Functions, Spring 2012

Chapter 6: Functions C++ Programming: Program Design Including Data Structures, Second Edition 2 Objectives In this chapter you will: • Learn about standard (predefined) functions • Learn about user-defined functions • Examine value-returning functions, including actual and formal parameters • Discover the difference between value and reference parameters • Learn about the scope of an identifier C++ Programming: Program Design Including Data Structures, Second Edition 3 Objectives • Examine the difference between local and global identifiers • Discover static variables • Learn function overloading • Explore functions with default parameters C++ Programming: Program Design Including Data Structures, Second Edition 4 Functions • Functions are like building blocks, Called modules • They allow complicated programs to be divided into manageable pieces C++ Programming: Program Design Including Data Structures, Second Edition 5 Predefined Functions • Some of the predefined mathematical functions are: − sqrt(x) − pow(x,y) • Predefined functions are organized into separate libraries • I/O functions are in iostream header • Math functions are in cmath header C++ Programming: Program Design Including Data Structures, Second Edition 6 The Power and sqrt Functions • pow(x,y) calculates xy, pow(2,3) = 8.0 • pow returns a value of the type double • x and y are called the parameters (or arguments) of the function pow • The square root function sqrt(x) − Calculates the non-negative square root of x, for x >= 0.0 − sqrt(2.25) is 1.5 − Type double and has only one parameter C++ Programming: Program Design Including Data Structures, Second Edition 8 Value-Returning Functions • The syntax is: functionType functionName(formal parameter list) { statements } • functionType: type of the value returned by the function, Also called the data type • Void functions: do not have a data type C++ Programming: Program Design Including Data Structures, Second Edition 9 Value-Returning Functions (continued) • Heading: first line • Formal Parameter: variable declared in the heading • Actual Parameter: variable or expression listed in a call to a function A function call in a program results in the execution of the body of the called function C++ Programming: Program Design Including Data Structures, Second Edition 10 Syntax • The syntax of the formal parameter list is: dataType identifier, dataType identifier, ... • The syntax for a function call is: functionName(actual parameter list) • The syntax for the actual parameter list is: expression or variable,expression or variable, ... C++ Programming: Program Design Including Data Structures, Second Edition 11 Functions • The formal parameter list can be empty • functionType functionName() • functionType functionName(void) • The call is: functionName() C++ Programming: Program Design Including Data Structures, Second Edition 12 The return Statement • Once the function computes the value, the function returns the value via the return statement • The syntax of the return statement is: return expression or variable; • When a return statement executes − Function immediately terminates − Control goes back to the caller • When a return statement executes in the function main, the program terminates C++ Programming: Program Design Including Data Structures, Second Edition 13 Function Prototype • Function Prototype: function heading without the body of the function • The syntax is: functionType functionName(parameter list); • Function prototypes appear before any function definition • It is not necessary to specify the variable name in the parameter list • The data type of each parameter must be specified C++ Programming: Program Design Including Data Structures, Second Edition 14 Function Overloading • If several functions have the same name, every function must have: − A different set of parameters − If the number of formal parameters is the same, then the data type of the formal parameters, in the order you list, must differ in at least one position • Overloading a function refers to the creation of several functions with the same name C++ Programming: Program Design Including Data Structures, Second Edition 15 Flow of Execution • A function call statement results in − Transfer of control to the first statement in the body of the called function • After the last statement of the called function is executed − Control is passed back to the point immediately following the function call C++ Programming: Program Design Including Data Structures, Second Edition 16 Flow of Execution • A value-returning function returns a value • After executing the function − The value that the function returns replaces the function call statement C++ Programming: Program Design Including Data Structures, Second Edition 17 Recursion • Recursive functions − Functions that call themselves • Example: factorial n! = n * ( n – 1 ) * ( n – 2 ) * … * 1 − Recursive relationship ( n! = n * ( n – 1 )! ) 5! = 5 * 4! 4! = 4 * 3!… − Base case (1! = 0! = 1) C++ Programming: Program Design Including Data Structures, Second Edition 18 Recursion (continued) • Fibonacci series: 0, 1, 1, 2, 3, 5, 8... − Each number sum of two previous ones − Example of a recursive formula: • fib(n) = fib(n-1) + fib(n-2) C++ Programming: Program Design Including Data Structures, Second Edition 19 Recursion: Towers of Hanoi − N disks of different diameters are placed on peg A so that a larger disk is always below a smaller disk. − Only the top disk on any peg may be moved to any other peg, and a larger disk may never rest on a smaller one. − Problem: move the n disks to peg c using peg b as an auxiliary: • it can be solved for n=1 • we suppose it can be solved for n-1 then show it can be solved for n Solution: − n=1: move the single disk from A to C. − n: move the top n-1 disks from A to B using C as the auxiliary − move the remaining disk from A to C − move the n-1 disks from B to C using A as the auxiliary C++ Programming: Program Design Including Data Structures, Second Edition 20 Inline Functions • Inline functions − Keyword inline before function − Asks the compiler to copy code into program instead of making function call • Reduce function-call overhead • Compiler can ignore inline − Good for small, often-used functions C++ Programming: Program Design Including Data Structures, Second Edition 21 Value Parameters • If a formal parameter is a value parameter − The value of the corresponding actual parameter is copied into it • The value parameter has its own copy of the data • During program execution − The value parameter manipulates the data stored in its own memory space C++ Programming: Program Design Including Data Structures, Second Edition 22 Reference Variables as Parameters • If a formal parameter is a reference parameter − It receives the address of the corresponding actual parameter • A reference parameter stores the address of the corresponding actual parameter C++ Programming: Program Design Including Data Structures, Second Edition 23 Reference Variables as Parameters (continued) • During program execution to manipulate the data − The address stored in the reference parameter directs it to the memory space of the corresponding actual parameter C++ Programming: Program Design Including Data Structures, Second Edition 24 Reference Variables as Parameters (continued)  A reference parameter receives the address of the actual parameter  Reference parameters can:  Pass one or more values from a function  Change the value of the actual parameter C++ Programming: Program Design Including Data Structures, Second Edition 25 Reference Variables as Parameters (continued)  Reference parameters are useful in three situations:  Returning more than one value  Changing the actual parameter  When passing the address would save memory space and time C++ Programming: Program Design Including Data Structures, Second Edition 26 Parameters & Memory Allocation • When a function is called − Memory for its formal parameters and variables declared in the body of the function (called local variables) is allocated in the function data area • In the case of a value parameter − The value of the actual parameter is copied into the memory cell of its corresponding formal parameter C++ Programming: Program Design Including Data Structures, Second Edition 27 Parameters & Memory Allocation (continued) • In the case of a reference parameter − The address of the actual parameter passes to the formal parameter • Content of the formal parameter is an address C++ Programming: Program Design Including Data Structures, Second Edition 28 Parameters & Memory Allocation (continued) • During execution, changes made by the formal parameter permanently change the value of the actual parameter • Stream variables (for example, ifstream and ofstream) should be passed by reference to a function C++ Programming: Program Design Including Data Structures, Second Edition 29 Scope of an Identifier • The scope of an identifier refers to where in the program an identifier is accessible • Local identifier - identifiers declared within a function (or block), accessible only within the block until the end of the block • Global identifier – identifiers declared outside of every function definition accessible everywhere in the program from the point it is declared C++ Programming: Program Design Including Data Structures, Second Edition 30 Scope of an Identifier • Nested blocks: With nested blocks, if a variable in an inner block has the same name as the outer block, the outer is hidden until the inner block terminates C++ Programming: Program Design Including Data Structures, Second Edition 31 Global Variables • The operator :: is called the scope resolution operator • By using the scope resolution operator − A global variable declared before the definition of a function (block) can be accessed by the function (or block) even if the function (or block) has an identifier with the same name as the variable C++ Programming: Program Design Including Data Structures, Second Edition 32 Side Effects of Global Variables • Using global variables has side effects • Any function that uses global variables − Is not independent − Usually it cannot be used in more than one program C++ Programming: Program Design Including Data Structures, Second Edition 33 Side Effects of Global Variables (continued) • If more than one function uses the same global variable and something goes wrong − It is difficult to find what went wrong and where • Problems caused by global variables in one area of a program might be misunderstood as problems caused in another area C++ Programming: Program Design Including Data Structures, Second Edition 34 Static and Automatic Variables • Automatic variable - memory is allocated at block entry and deallocated at block exit • Static variable - memory remains allocated as long as the program executes • Variables declared outside of any block are static variables • By default variables declared within a block are automatic variables • Declare a static variable within a block by using the reserved word static C++ Programming: Program Design Including Data Structures, Second Edition 35 Static and Automatic Variables (continued) • The syntax for declaring a static variable is: static dataType identifier; • The statement static int x; declares x to be a static variable of the type int • Static variables declared within a block are local to the block • Their scope is the same as any other local identifier of that block C++ Programming: Program Design Including Data Structures, Second Edition 36 Functions with Default Parameters • When a function is called − The number of actual and formal parameters must be the same • C++ relaxes this condition for functions with default parameters • You specify the value of a default parameter when the function name appears for the first time, such as in the prototype C++ Programming: Program Design Including Data Structures, Second Edition 37 Functions with Default Parameters (continued) • If you do not specify the value of a default parameter − The default value is used • All of the default parameters must be the rightmost parameters of the function • In a function call where the function has more than one default parameter and a value to a default parameter is not specified − You must omit all of the arguments to its right C++ Programming: Program Design Including Data Structures, Second Edition 38 Functions with Default Parameters (continued) • Default values can be constants, global variables, or function calls • The caller has the option of specifying a value other than the default for any default parameter • You cannot assign a constant value as a default value to a reference parameter