CPS120: Introduction to Computer Science

1. CPS120: Introduction to Computer Science Functions

2. Subprogram Statements • We can give a section of code a name and use that name as a statement in another part of the program • When the name is encountered, the processing in the other part of the program halts while the named code is executed • -- Invocation

3. Subprogram Statements • There are times when the calling unit needs to give information to the function to use in its processing • A parameter list is a list of the identifiers with which the subprogram is to work, along with the types of each identifier placed in parentheses beside the function name

4. Subprogram / Function Statements Subprogram flow of control

5. Subprogram Statements Subprogram flow of control

6. Subprogram Statements • Parameters: The identifiers listed in parentheses beside the function name; sometimes they are called formal parameters • Arguments: The identifiers listed in parentheses on the function call; sometimes they are called actual parameters

7. Subprogram Statements • Value parameter: a parameter that expects a copy of its argument to be passed by the calling unit (put on the message board) • Reference parameter: a parameter that expects the address of its argument to be passed by the calling unit (put on the message board)

8. Subprogram Statements

9. Functions • Every C++ program must have a main function • Each of the smaller tasks (let's say, subtasks) can be coded as C++ functions that go together with the to make up a structured program.

10. Functions & Structured Programs • Using functions makes it easier to code programs. • Using functions also makes it easier to debug large programs. • Using functions makes it easier to maintain and upgrade a program after the first version has been finished

11. Guidelines for Building Programs With Multiple Functions • Organization: - programs with functions are easier to read and modify • Autonomy:- functions should not depend on data or code outside of the function any more than necessary • Encapsulation: - functions should keep all of their "details" to themselves • Reusability: functions may be reused in other programs or, even, by other programmers

12. Function Syntax • Functions are given valid identifier names, just like variables and constants. • A function name must be followed by parentheses • Coded functions to the bottom of a C++ program, after the main function. • If the functions are listed after the main function, then function prototypes must be included at the top of the program, above the main function • An error will definitely result if you call a function from within the main function that has not been prototyped.

13. Structure of Functions in C++ function-name(argument list)argument declaration;{ local variable declarations; executable statement1; executable statement2; ---------- ----------return (expression);}             

14. An Example of A Function #include <iostream.h> void printMyMessage(int numOfTimes); // PROTOTYPE and NAME int main( ) { int userInput = 0; cout << "Enter a number between 1 and 10 (0 to Exit): " ; cin >> userInput; if (userInput != 0) { printMyMessage (userInput); // CALL STATEMENT WITH ACTUAL PARAMETER } else cout << "Thanks, Bye!"; return 0; } // end of main void printMyMessage(int numOfTimes) // FUNCTION HEADER WITH RETURN TYPE AND // ACTUAL PARAMETER { int i=0; // LOCAL VARIABLE WITHIN THE FUNCTION for (i=0; i<= numOfTimes; i++) // BODY {cout << "Let's Go State!!" << endl;} // OF THE } //end of printMyMessage // FUNCTION

15. Describing a Function • The first line of a function is called the function header • Before the name of the function you must specify a "return type." • The return type is the data type of the value that is returned by the function to the calling function • If the function does not return any value, you must type the word void as the return type • After the name of the function in the function header, you must include a parameter list. • Immediately preceding each parameter, you must identify the data type of that parameter.

16. Returning Values • If the function is not a void function, there must be a return statement at the end of the function

17. Scope of Variables • The scope of a variable is the area in which it can be legally referenced • Variables are either global or local in nature • Global variables are ones that are declared outside and above the main function • They can be used in any function throughout the program. • It is not wise to use global variables any more than you have to. • Local variables are ones that are declared inside of a function, including main. They cannot be used or referred to in other functions

18. Using Functions • Avoid using cin or cout statements within functions • Unless the whole purpose of a function is to obtain (and or validate) a user's input, you should not use a cin statement within a function (besides main). • Unless the whole purpose of a function is to display something such as a menu, you should avoid using a cout statement within a function besides main.

19. Passing Data • Data is passed to functions as arguments • When a function is "called" by the main function one or more arguments are passed to the function • On the receiving end, the function accepts these arguments • The variable names of the arguments from the "calling" function do not have to be the same as the names in the "called" function. • The data types of the arguments and the parameters should match exactly

20. More About Passing Arguments • There are technically three ways to pass data as arguments to functions • passing by value is the preferred method. You simply use a variable name, an actual numeric literal, or an expression in the parentheses of the call statement • passing by reference is to be used when you want the function to actually and permanently change the values of one or more variables • You must use an ampersand (&) before the formal parameter names in the function header (the first line of the function definition) to denote passing by reference • passing by address is technically what happens when you pass an array to a function

21. Another Look at Return Values • Often, though, you want your function to return a computed value to the calling function • It is not possible in C++ to execute tworeturn statements within a function • It is not possible to return two values in the same return statement

22. Library Functions • There are many functions available to C++ programmers which were written by other programmers • Use the #include compiler directive at the top of your program to take advantage of these functions. • To use the you do not even have to know exactly how they work • You do have to know how many arguments to send to the functions and what datatypes to use for those functions.

23. Recursion • Recursion: the ability of a subprogram to call itself • Each recursive solution has at least two cases • base case: the one to which we have an answer • general case: expresses the solution in terms of a call to itself with a smaller version of the problem • For example, the factorial of a number is defined as the number times the product of all the numbers between itself and 0: N! = N * (N  1)!