University:
California State University, Northridge
Course:
COMP 380/L | Introduction to Software Engineering and Lab
Academic year:

2006
Views:

214

Pages:

5
Author:

Amani Galvan

Outline defi ned Introduction to U Usser er--defin Funct Functiions • • • • • • Why we use functions Writing and calling a function Header and body Function prototype Passing information to a function Returning values in the function name 2 Introduction to Functions Why do we write functions? • Library functions like pow, atan and sqrt used previously • Statement to set x = yz3: • First use of functions was for code like mathematical function calculations – x = y * pow( z, 3 ); – Note order of arguments important; the call pow( 3, z ) gives 3z – Use #include for this function • You can write your own functions – Why do we write functions? – How do we write code to use functions? – Specialized calculation done repeatedly – Want to write code only one time – Want to be able to pass values of parameters to code and get value back • As programs got more complex, breaking code into functions provided a way to organize complex code 3 How do we write functions? 4 Using Functions • C++ code is a collection of functions • Each function, including main, has the same level of importance in writing code – Complete code for each function before starting a new function int main() { // body of main } int myFunction( …… ) { // body of myFunction } 5 • User-defined functions are used in the same way as library functions – Functions are used (“called”, “invoked”) by placing the name of the function at the correct location in the code – The function name is usually followed by a list of variables or constants in parentheses whose values are passed to the function – The result of the function is returned in the function name 6 1 Operation of Functions Operation of Functions • Any function (the caller) can call another function by using the name of the function being called in an expression • The statement calling the function sends information from the caller • Execution control is transferred to the function being called • The function being called returns control and (usually) results to the caller • The function being called uses the information it receives to do a set of calculations or procedures • In the usual case, the function being called returns a result to the caller in the location of the function name • Example: d = pow(b,2) – 4 * a * c; 7 8 Writing and Using Functions – calls the pow function with values of b and 2 – and the result b2 is returned in function name, pow, for further use in an expression Writing and Using Functions II • Organize the program into individual functions that are called by main • Library functions, such as pow(x, y) to compute xy, transfer information based on the order of the variables • This is true for user-defined functions as well – Simple example: main calls three functions: (1) input function, (2) calculation function and (3) output function • Write code for each function (and main) – Write function header to specify information received from calling function – Write function body to calculate results and return them to “calling” function • Write function calls that pass data and get results – Information transferred from a list of variables in the calling function to a list of variables in the function called – Correspondence based on order of variables in function header and statement calling the function 9 Writing Functions 10 Function Basics • Each function has a header and a body • The code in a function is works in the same way that the code you have prepared for a single main function • All the conventional structures can be used • What is new? – Header specifies • Name of function • Type of value returned by the function name • List of variables in the function whose values are determined by the calling program – Body gives code executed by the function – The first line of the function, the function header, contains a declaration of the variables whose values are passed into the function when it is called 11 • Function prototypes at start of code provide information to compiler – Same as header except a semicolon is added at the end – Can omit variable names 12 2 Function Basics II Function Header • Header has following syntax: • Example of header and body – ( ) – specifies the type of value returned by the function – is the name you choose for your function; this name is used to call the function from another function – specifies type and names of variables in function whose values come from the calling program – There is no semicolon at the end of the function header double myPow ( double number, double power ) // header { // Body,in braces contains // actual code for function } • Header declares data type for function value, function name, and variables whose values will be received from calling function 13 Function Header II 14 Argument List • Example of function, myPow, a userdefined function to replace pow • Example on previous chart had two parameters in argument list – Pass the number and the power to the function as type double – Return the result as type double – ( ) – double myPow ( double number, double power ) – General header syntax from previous chart ( ) – double myPow ( double number, double power ) • Function will use number and power as type double variables • Values for these variables set by other function that calls (uses) myPow 15 16 Passing arguments The return Statement • Based on order of arguments in function header and in calling statement • Recall the library pow function was called as pow(number, power) • We have used this statement in main as return EXIT_SUCCESS; • The general syntax of this statement is return ; • may be a constant, a variable or an expression • This is value returned to calling program in function name • return always transfers control to calling function – pow(3,4) = 34 but pow(4,3) = 43 – What is result of following code double number = 3, power = 4; cout << pow( power, number) Result is 43; only the order counts! 17 18 3 Organization of Function Code Alternative Function Code double myPow( double number, double power ) { double result = exp( power * log( number ) ); return result; } double myPow( double number, double power ) { return exp( power * log( number ) ); } • Place following prototype at top of code double myPow( double number, double power ); • Can use following prototype without variable names at top of code double myPow( double, double ); 19 20 Exercise Another Function Example • Write a statement that uses the function myPow(double number, double power) to compute z = x + yz3 z = x + y * myPow( z, 3 ); • Write the header for a type bool function named equal that whose first parameter is x, an int variable and whose second parameter, a, is type double bool equal( int x, double a ) Type Name Argument List bool leap( int year ) Header { if ( year % 4 != 0 ) return false; else if ( year % 400 == 0 ) return true; else if ( year % 100 == 0 ) return false; Multiple else return true; returns } Body 21 Yet Another Function Example Type Name Argument List Header bool leap( int year ) { return year % 4 == 0 && ( year % 100 != 0 || year % 400 == 0 ); } Body • This function will have same behavior as one on previous chart • User of function does not have to know its internal code, only its input and output 23 22 Use of bool leap( int year ) bool leap ( int year );// prototype int main() // examples of use { cout << “Enter a year: “; int y; cin >> y; bool cond = leap( y ); if ( leap( y ) ) {…} if ( leap( y ) && month == 2 ) {…} return EXIT_SUCCESS } // leap and other functions go here 24 4 Exercise Exercise • Write a function that takes two type int arguments and returns their difference int diff( int a, int b) { return a – b; } • Use this function to compute and print the difference 3 – 5 cout << diff( 3, 5 ); • Write the prototype int diff( int a, int b); 25 • Write a function that takes two type double arguments and returns their quotient • Use this function to compute 5/3 • Write the prototype double div( double a, double b) { return a / b; } //use: cout << div( 5, 3 ); // prototype: double div(double a, double b); 26 5

Introduction to User-Defined Functions

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