1 Engineering Problem Solving With C++ An Object Based Approach Chapter 5 Functions

2 A program can be thought of as a collection of sub parts or sub tasks: input data analyze data output results In C++ these sub tasks are often organized into functions.

3 Functions Complex problems can be broken down into sub tasks, each of which is easy to implement in C++. What are some other advantages to using functions, as opposed to writing the entire solution in main? –Multiple programmers –Testing/Debugging/Maintaining –Reduce duplication of code

4 Functions Pre-defined –standard libraries User defined

5 Pre-defined Functions - Example #include using namespace std; int main() { double angle; cout << “input angle in radians: “; cin >> angle; cout << “\nthe sine of the angle is “ << sin(angle) << endl; return 0; }//end main

6 Programmer Defined Functions Terminology Function Prototype –describes how a function is called Function Call Function Arguments –used in the function call Function Definition –function header –function body Formal Parameters –used in function definition Formal parameters must agree with arguments in order, number and data type, but the identifiers can be different.

7 Programmer Defined Functions Can be defined to –return a single value to the calling function –perform a task –change the value of multiple variables

8 Value Returning Functions A function returns a single value to the calling program The function header declares the type of value to be returned A return statement is required in the body of the function

9 n! example n! = n*(n-1)*(n-2)*…*1 n is a positive integer 0! is 1 by definition

10 //function definition: n! = n*(n-1)*(n-2)*…*1, // 0! is 1 by definition //Function fact returns n! //Function fact assumes n is non-negative integer int fact(int n) //function header, NO SEMICOLON {int nfact = 1; while(n>1) {nfact = nfact*n; n--; }//end while block return(nfact); }//end fact Example - factorial function

11 Calling a function - a function prototype is required int fact(int n);//function prototype, semicolon required // parameter identifier is optional #include using namespace std; int main() { int n; cin >> n; if(n>=0) cout<<n<<“! is ” <<fact(n)<<endl; //n is the argument else cout <<“factorial not defined for negative numbers” <<endl; return 0; }//end main

12 Calling a function- second example int fact(int);//function prototype, //parameter identifier is optional #include using namespace std; int main() { int n, factorial; cin >> n; if(n>=0) {factorial = fact(n);//function call cout << n <<“! is ” << factorial << endl; } else cout << “factorial not defined for negative numbers” << endl; return 0; }//end main

13 2 Points of Style When Writing Value Returning Functions Formal parameters are used to pass information to the function. cin statements are usually not required. A return statement returns a value to the calling program. cout statements are usually not required. Use library functions as model (sin, log, etc)

14 void Functions A void function may be called to perform a particular task (clear the screen) modify data perform input and output A void function does not return a value to the calling program return statement is optional if a return statement is used, it has the following form –return;

15 Example of void function //output formatted date //function definition void print_date(int mo, int day, int year) //function header {string month; switch(mo) {case 1: month = “January”; break; case 2: month = “February”; break; … case 12: month = “December”; }//end switch cout << month << ‘ ’ << day << “, << year << endl; return;//return is optional } //end print date

16 Parameter Passing - pass by value Pass by value –the default in C++ (except when passing arrays as arguments to functions) –formal parameter receives the value of the argument –changes to the formal parameter do not affect the argument

17 #include using namespace std; int fact(int);//function prototype int main() {int n, factorial; cin >> n; if(n>=0) {factorial = fact(n);//function call cout << n <<“! is “ << factorial << endl; }//end if return 0; }//end main int fact(int n)//function header, NO SEMICOLON { int nfact = 1; while(n>1) {nfact = nfact*n; n--; }//end while block return(nfact); } //end fact

18 Parameter Passing - pass by reference Pass by reference –append an & to the parameter data type in both the function prototype and function header void get_date(int& day, int& mo, int& year) –formal parameter receives the address of the argument –any changes to the formal parameter directly change the value of the argument

19 Example - pass by reference #include using namespace std; void swap(double&, double&); //function prototype int main() { double x=5, y=10; swap(x,y);//function call; x y are arguments cout >> “x = “ << x << ‘,’ << “ y= “ << y << endl; return 0; }//end main Output is: x = 10, y = 5

20 Example - pass by reference //Function swap interchanges the values of two variables //function definition void swap(double& x, double& y)//function header { double temp;//local variable temp temp = x; x=y; y=temp; return;//optional return statement }//end swap

21 Practice! - What is the output? #include using namespace std; void fun(int&, int&, int); int main() {int c1=1, c2=2, c3=3; cout << c1 << ‘,’ << c2 << ‘,’ << c3 << endl; fun(c1,c2,c3); cout << c1 << ‘,’ << c2 << ‘,’ << c3 << endl; fun(c3, c2, c1); cout << c1 << ‘,’ << c2 << ‘,’ << c3 << endl; return 0; } void fun(int& a1, int& a2, int a3) {a1++; a2++; a3--; } 1,2,3 2,3,3 2,4,4

22 Storage Class and Scope Scope refers to the portion of the program in which it is valid to reference a function or a variable Storage class refers to the lifetime of a variable

23 Scope Local scope - a local variable is defined within a function or a block and can be accessed only within the function or block that defines it Global scope - a global variable is defined outside the main function and can be accessed by any function within the program file. Strongly discouraged!!