Engineering Problem Solving With C++ An Object Based Approach

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

2. Functions • 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 called 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 <iostream> #include <cmath> 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. Cast Functions • Return value as new data type, • Have no affect on the argument • Note the syntax-this is preferred usage #include <iostream> using namespace std; int main() { int x=9, y=2; cout << x/y << endl; cout << (double)x/(double)y << endl; cout << (double)x/y << endl; cout << (double)x/y << endl; return 0; }//end main Output? 4.5 4 4.5

7. Programmer Defined FunctionsTerminology • 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 datatype, but the identifies can be different.

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

9. 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

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

11. Example - factorial function //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

12. Calling a function - a function prototype is required int fact(int n);//function prototype, semicolon required // parameter identifier is optional #include <iostream> 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

13. Calling a function- second example int fact(int); //function prototype, //parameter identifier is optional #include <iostream> 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

14. 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)

15. 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;

16. 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

17. 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

18. #include <iostream> 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

19. 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

20. Example - pass by reference #include <iostream> 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

21. 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

22. Practice! - What is the output? #include <iostream> 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

23. 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

24. 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.

25. Storage Class - 4 types • automatic - key word auto - default for local variables • Memory set aside for local variables is not reserved when the block in which the local variable was defined is exited. • external - key word extern - used for global variables • Memory is reserved for a global variable throughout the execution life of the program. • static- key word static • Requests that memory for a local variable be reserved throughout the execution life of the program. The static storage class does not affect the scope of the variable. • register - key word register • Requests that a variable should be placed in a high speed memory register.

26. //Scope Example - What is the output #include <iostream> using namespace std; void a(); //Function prototypes void b(); int x=1; //Global (file scope) int main() { int x=5; //Local to main cout << "before call to a, x= " << x << endl; cout << "global x is " << ::x << endl; a(); cout << "after call to a, x= " << x << endl; cout << "global x is " << ::x << endl; b(); cout << "after call to b, x= " << x << endl; cout << "global x is " << ::x << endl; b(); cout << "after 2nd call to b, x= " << x << endl; cout << "global x is " << ::x << endl; return(0); } void a() { int x=25; //Local to a x++; } void b() { x++; }

27. Scope Example - Output before call to a, x= 5 global x is 1 after call to a, x= 5 global x is 1 after call to b, x= 5 global x is 2 after 2nd call to b, x= 5 global x is 3

28. //Storage Class Example - What is the output #include <iostream> using namespace std; void donothing(); int main() { donothing(); donothing(); donothing(); return(0); }//endmain void donothing() { int x=0; static int y=0; x++; y++; cout <<“x is ” x << “, y is ” << y << endl; return; }//end donothing

29. Storage Class Example - Output x is 1, y is 1 x is 1, y is 2 x is 1, y is 3