Functions Overview

1. Functions Overview • Functions are • sequence of statements with its own localvariables • supports modularity, reduces code duplication • Data transfer between function to be called and caller function • by means of parameters • currently one-way • from caller function into function to be called • We will see how to return data back to the caller function

2. Function Prototype (from 2.6) • Functions definition has two parts • function heading • name, parameters, return type • function body (local variables and statements within curly brackets) void display (string name) { cout << “Hello ” << name << endl; } • Like variables, a function must be declared before its first call • Problem of function declaration order • You cannot call a function before you declare it • SOLUTION: You may define function prototypes (a copy of the heading) at the beginning without function declarations

3. Function Prototype – Example Problem • What is the problem below (program order.cpp) ? void Hi (string name){ cout << "Hi " << name << endl; Greetings();}void Greetings(){ cout << "Things are happening inside this computer" << endl;}int main(){ Hi("Fred"); return 0;} • Greetings() is called in Hi() but it is declared afterwards

4. Function Prototype – Solution • Add function prototypes to the beginning (order2.cpp) #include <iostream>#include <string>using namespace std;void Hi(string);void Greetings();void Hi (string name){cout << "Hi " << name << endl; Greetings();}void Greetings(){cout << "Things are happening inside this computer" << endl;}int main(){ Hi("Fred"); return 0;} Prototypes Function Declarations

5. Function Prototypes • *** Do not forget semicolon after the prototype definition *** • no semicolon after the parameters in normal definition • Sometimes prototypes are not necessary • if the order of function calls allows • but it is good programming practice to have them • Parameter names are not needed in prototypes • but it is OK if you have the parameter names • In #included files • we have the functions’ prototypes only • implementations of function bodies are in libraries or in other cpp files • they are linked together

6. Functions that return values • Functions we’ve written so far are void functions • They do a job and return back to caller, but without a value • Parameters are used for one-way data transfer • into the function to be called • How about transfer a computed value out of a function? • to the main program or to other function (the caller) • Non-void functions can return values of any type • function call becomes an expression • when the function finishes, the function call is replaced by the returned value • this way, values computed in functions can be transferred into the caller functions • void function call is not used in an expression, i.e. no value is associated with a void function Head(); DoThat(); Verse("cow", "moo");

7. Functions that return values • Example (see area_func_return.cpp): • suppose circlearea function takes the radius as parameter and returns the area. • In the program we call circlearea as an expression (you have to use the returned value somewhere) area = circlearea(r); cout << circlearea(12) << endl; if (circlearea(r/2) >= 100) { cout << “large circle” << endl; } circlearea(r); //syntax ok, but meaningless because //function call returned value is not used.

8. Math library functions • Mathematical functions like square root, logarithm, sin, cos, etc. • Prototypes are in header file cmath #include <cmath> • Full list is in page 758 (Table F.1) – partial list is in table 4.5. • correction in Table F.1: int abs (int x) • Keep thesemathlibraryfunctions on your cheat-sheet for the exam • Example use of function sqrt • see usemath.cpp • how did we use sqrt function? • in cout as an expression • could we use sqrt in assignment? How? • yes, let’s do it. • what happens if value is negative? • try and see! • we can add some if statements to display an error message in case of negative value

9. Function Syntax return-typefunc-name(parameters) { local variables statements } Example: Function to calculate volume of a sphere doubleSphereVol(double radius) { return4.0*radius*radius*radius*acos(-1)/3; } function body

10. Function Syntax doubleSphereVol(double radius) { return4.0*radius*radius*radius*acos(-1)/3; } • Function heading/prototype shows return type. • return type can be any type (including string) • theoretically return type may be Robot too, but in practice Robot class is not designed to be used as the return type • you do not see any syntax error, but execution may be problematic. So do not return a Robot from a function • Function body may have several statements in it • return statement is used to determine the value returned fromfunction, so the expression after it must be of the return type • Function body must include at least onereturn statement • The return statement causes the function to exit immediately and to return the value after return • A function can have more than one return statements, but only one is executed when the function is called (see next example) • Only one return is a good programming style to have control of bigger functions

11. Functions can return strings string WeekDay(int day) // precondition: 0 <= day <= 6 // postcondition: return "Sunday" for 0, // "Monday" for 1, // … "Saturday" for 6 { if (0 == day) return "Sunday"; else if (1 == day) return "Monday"; else if (2 == day) return "Tuesday"; else if (3 == day) return "Wednesday"; else if (4 == day) return "Thursday"; else if (5 == day) return "Friday"; else if (6 == day) return "Saturday"; } • A program piece that uses that function string dayName; int dayNum; cout << " enter day (0-6): "; cin >> dayNum; dayName = WeekDay(dayNum); • Which is/are correct use of WeekDay function? Why? cout << WeekDay(5) << endl; int j = WeekDay(0); cout << WeekDay(2.1) << endl; string s = WeekDay(22); WeekDay(3);

12. Function documentation • Functions usually have a precondition • What conditions (e.g. value of parameters) must be true for the function to work as intended? • If there are no parameters, then no precondition • Some functions work for every parameter value • no precondition • Functions always have a postcondition • If precondition is satisfied what does the function do? What does the function return?

13. Example – Compare cost of pizza sizes • Problem: Calculate and compare price per square inch of large and small size pizzas • Solution: • A function, say Cost, that takes the pizza radius and price as parameters and returns price per square inch • In main() • input radiuses and prices of large and small pizzas • calculate the per square inch costs by calling the cost function • display the results on screen • compare the unit costs to find out which one is best value • See pizza2.cpp

14. Example - When is a year a leap year? • Every year divisible by four is a leap year • Except years divisible by 100 are not • Except years divisible by 400 are • Alternatively: • Every year divisible by 400 is a leap year • Otherwise, years divisible by 100 are not leap years • Otherwise, years divisible by 4 are leap years • Otherwise, not a leap year • Boolean function bool IsLeapYear(int year); // pre: year > 0 // post: return true if year is a leap year

15. Implementation and use of leap year function bool IsLeapYear(int year)// precondition: year > 0// postcondition: returns true if year is a leap year, else returns false { if (year % 400 == 0) // divisible by 400 { return true; } else if (year % 100 == 0) // divisible by 100 { return false; } else if (year % 4 == 0) // divisible by 4 { return true; } return false;} int main(){ int year; cout << "enter a year "; cin >> year; if (IsLeapYear(year)) { cout << year << " has 366 days, it is a leap year" << endl; } else { cout << year << " has 365 days, it is NOT a leap year" << endl; } return 0;} Seeisleap.cpp

16. There’s more than one way • No if/else necessary in the function body bool IsLeapYear(int year) // precondition: year > 0 // post: return true if year is a leap year { return ( year % 400 == 0 ) || ( year % 4 == 0 && year % 100 != 0); } • How does this work? • Is this version more efficient? • Are these two versions different from user perspective?