CSIS 113A

1. CSIS 113A Lecture 5 Functions

2. Introduction to Functions • Building Blocks of Programs • Other terminology in other languages: • Procedures, subprograms, methods • In C++: functions • I-P-O • Input – Process – Output • Basic subparts to any program • Use functions for these ‘pieces’

3. Predefined Functions • Libraries full of functions for our use! • Two types: • Those that return a value • Those that do not (void) • Must ‘#include’ appropriate library • e.g.: • <cmath>, <cstdlib> (Original ‘C’ libraries) • <iostream> (for cout, cin)

4. Using Predefined Functions • Math functions very plentiful • Found in library <cmath.h> • Most return a value (the ‘answer’) • Example: theRoot = sqrt(9.0); • Components:sqrt = name of library functiontheRoot = variable used to assign ‘answer’ to9.0 = argument or ‘starting input’ for function • In I-P-O: • I = 9.0 • P = ‘compute the square root’ • O = 3, which is returned & assigned to theRoot

5. The Function Call • Back to this assignment:theRoot = sqrt(9.0); • The expression ‘sqrt(9.0)’ is known as afunction call, or function invocation • The argument in a function call (9.0) can be aliteral, a variable, or an expression • The call itself can be part of an expression: • bonus = sqrt(sales)/10; • A function call is allowed wherever it’s legal to usean expression of the function’s return type

6. An Example #include <iostream>#include <cmath> using namespace std;int main(){int numLoop;   cout << "Enter the number of iterations ";   cin >> numLoop;    for(int i = 0; i < numLoop; i++)    {      cout << "The square of " << i << "is " << pow(i, 2) << endl;      cout << "The cube of " << i << " is " << pow(i, 3) << endl;      cout << "The sqrt of " << i << " is " << sqrt(i) << endl;    }    return 0; }

7. More Predefined Functions • #include <cstdlib> • Library contains functions like: • abs() // Returns absolute value of an int • labs() // Returns absolute value of a long int • *fabs() // Returns absolute value of a float • *fabs() is actually in library <cmath>! • Can be confusing • Remember: libraries were added after C++ was‘born’, in incremental phases • Refer to appendices/manuals for details

8. More Math Functions • pow(x, y) • Returns x to the power ydouble result, x = 3.0, y = 2.0;result = pow(x, y);cout << result; • Here 9.0 is displayed since 3.02.0 = 9.0 • Notice this function receives twoarguments • A function can have any number ofarguments, of varying data types

9. Even More Math Functions

10. Predefined Void Functions • No returned value • Performs an action, but sends no ‘answer’ • When called, it’s a statement itself • exit(1); // No return value, so not assigned • This call terminates program • void functions can still have arguments • All aspects same as functions that ‘returna value’ • They just don’t return a value!

11. Ok, So What Is This void Thing? • Some functions don't return anything • An example from the previous table is the exit function. • Notice that the return type is void. void is a type in C++ and it is used to indicate nothing. • So, why do you need void it if it means nothing? • Well that is a tough question. It all comes down to the fact that the compiler needs a definition of the function before you can use it. • To make all functions similar (meaning they all have a return type) void is used to indicate the function returns nothing.

12. Programmer Defined Functions • Like small sub programs that allow you to organize your code in a logical fashion. • What are you really doing when you write a piece of software? • The answer is that you are solving some sort of problem. • The easiest way to tackle the problem is to break the large problem into several smaller problems and then tackle them one at a time. • In computer science, this is called stepwise refinement

13. Function Syntax

14. User Defined Functions II • Return Type – • All functions declarations must have a return type. If the function doesn't return anything then you need to use the keyword void. You can return any valid C++ type from a function. Later on you will learn that you can return you own custom made types. • Function Name – • You can name a function anything that you like but you must also follow the same rules for naming variables. • Formal Parameters – • The formal parameters (formal arguments) must be enclosed inside the parentheses following the function name. If you write a function that doesn't need any parameters then you just leave the parentheses empty. If you need multiple parameters sent to your function they must be separated by commas. • Curyly Braces – • The body of the function is always surrounded by open and closed curly braces. If you omit one or all of the braces then you will get a big syntax error. • Function Body – • All of the code you want executed when the function is called (invoked) goes between the open and closed curly braces.

15. Simple Example #include <iostream> using namespace std; void helloWorld(); int main(){    helloWorld();   return 0; } void helloWorld(){   cout << "Hello World" << endl;}

16. The return Statement • Two ways that program can branch back to where it came from. • 1. when the closing brace of the function is reached. • In this case the flow of the program will branch back to where it was called from. • 2. The return statement. You can place the return statement anywhere you want within a function and the program will branch back to where it came from.

17. Another Example #include <iostream> using namespace std; void helloWorld(); int main(){    helloWorld();   return 0; } void helloWorld(){   cout << "Hello World" << endl;   return; // Force the flow of the program to brach back to where it came from }

18. Returning A Value • Some functions produce values. • Data gets returned to calling function • Place a value after the return statement • return10; • Value must match the return type • Value returned gets assigned through equal sign • int x = foo(); // Assigned just like value to a variable

19. Another Example #include <iostream> using namespace std; int getValue(); int main(){int someValue;    someValue = getValue();   cout << "The value entered was " << someValue << endl; return 0; } int getValue(){int val;   cout << "Please Enter an Integer " << endl;   cin >> val;   return val;}

20. Modified Example #include <iostream> using namespace std; int getValue(); int main(){   cout << "The value entered was " << getValue() << endl; return 0; } int getValue(){int val;   cout << "Please Enter an Integer " << endl;   cin >> val;   return val;}