University:
California State University, Northridge
Course:
COMP 110/L | Introduction to Algorithms and Programming and Lab
Academic year:

2005
Views:

259

Pages:

12
Author:

radobetzce2

Functions Outline User User--defined Functions • Writing and calling a function – Header and body – Function prototype – Passing information to a function – Returning values in the function name • void functions with no return value • Use pass-by-reference to change variables in the calling program • Variable scope and global variables 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 for 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 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 • C++ code is a collection of functions • Each function, including main, has the same level of importance – Close code for each function before starting a new function int main() { // body of main } int myFunction( …… ) { // body of myFunction } 5 6 1 Operation of Functions Writing and Using Functions • 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; – 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 • Organize the program into individual functions that are called by main – Simple example: main calls three functions: (1) input function, (2) calculation function and (3) output funciton • 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 to exchange data 7 8 Writing and Using Functions II Function Basics • 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 • Each function has a header and a body – 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 – 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 • 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 9 Function Basics II 10 Function Header • Example of header and body double myPow ( double number, double power ) // header { // Body,in braces contains // actual code for function } • Header defines information that will be received from calling function 11 • Header has following syntax: – ( ) – 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 in the function header 12 2 Function Header II 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 13 14 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 – 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! 15 16 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 ); 17 • Can use following prototype without variable names at top of code double myPow( double, double ); 18 3 Functions Another function Example 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 } Use of bool leap( int year ) Body 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 19 20 Exercise Exercise • Write a function that takes two type int arguments and returns their difference • Use this function to compute 3 – 5 • Write the prototype int diff( int a, int b) { return a – b } //use: cout << diff( 3, 5 ); // prototype: int diff( int a, int b); • 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); 21 void functions with no return • The type void used for functions that do not return a value • Example: error message function void printError( int code ) { if ( code == 1) cout << “Type one error\n”; else if ( code == 2 ) cout << “Error two is … else if // additional code } // no return needed here 22 The return Statement • The return statement returns control and a value to the calling program – Functions, other than void functions use the syntax return to return a value to the calling function in the function name – Void functions may have a simple return statement without a value to return control to the calling program at some point before the end of the function • Functions may have more than one return statement – return transfers control immediately 23 24 4 Functions Empty Argument List Kinetic Energy Function • If a function does not need any values from the calling program an empty set of parentheses is required • Example is function with several output statements to describe purpose of code void describeCode() { cout << “This code ….. cout << “Still more output // No return needed for type void } 25 Kinetic Energy Function II • Write a function that takes two type double parameters, mass and velocity and computes kinetic energy = mV2/2 double KE( double m, double V ) { return m * V * V / 2; } • Possible calls to this function totalE = KE( 4, 3 ) + PE; • What is result of this call result = 50 + KE( 5, 2 ); 50 + 5 * 22 / 2 = 60 26 Data Validation Function • The function getValidInt( int xMin, int xMax,string name ) does the following tasks double KE( double m, double V ) { return m * V * V / 2; } • What is output from these calls? double mass = 5, velocity = 2; 10 cout << KE( velocity, mass ); double e = PE + KE( 2*pow( velocity, 2), velocity); double total = KE( mass * velocity, mass ); e = KE(2*22,2) = KE(8,2)=8*22/2 = 16 – Prompts the user for an input variable (named in the string passed in the third parameter) within a range defined by the first and second parameters – Gets the input from the user – Tells the user if there is an error and gets new input from the user in this case – Returns valid input to the calling function total = KE(5*2,5) = KE(10,5)=10*52/2 = 12527 Data Validation Function II • The function getValidInt described on the previous chart is used in exercise seven and project one • Examples of its use int month = getValidInt( 1, 12, “month”); int mayDay = getValidInt( 1, 31, “day of the month” ); int year = getValidInt( 1901, 2000, “year in the 20th century” ); 28 int getValidInt( int xMin, int xMax, string name ) { // Function used to input integer // data within a stated range // Example function call to input a // value for a variable named // hour with range between 0 and 23: // int hour = // getValidInt( 0, 23, “hour” ); int x; // Input data value bool badData; // Bad data flag // continued on next chart 29 5 Functions do // Loop until user data in range { cout << "Enter a value for " << name << " between " << xMin << " and " << xMax << ": "; cin >> x; badData = x < xMin || x > xMax; if ( badData ) if ( badData ) // print error message { cout << << << << << << // error message code on next chart } while ( badData ); } "\n\nIncorrect data; you " "entered " << name " = “ << x << "\n" name << "must be between " xMin << " and " << xMax " Reenter the data now.\n"; } // end of function getValidInt Screen Results • Call to getValidInt int mayDay = getValidInt( 1, 31, “day of the month” ); • Screen prompt showing parameters and user input Enter a value for day of the month between 1 and 31: 0 Incorrect data; you entered day of the month = 0. day of the month must be between 1 and 31. Reenter the data now. Enter a value for day of the month between 1 and 31: 1 33 int getValidInt( int, int, string); // prototype above int main() { int month = getValidInt( 1, 12, “month” ); ……………… // other code return EXIT_SUCCESS } int getValidInt( int xMin, int xMax, string name ) { int x; ……………………………… // other code cin >> x; ……………………………… // other code return x; } Program Structure Example • Next chart: example for getValidInt • Function prototype before program that uses function • In this example main calls function • Complete code for main is written start of code for function • A call to the function transfers control to function with values in the function header variables from the caller • Function returns value to main 34 Passing Information to Functions • Parameters in function header: formal parameters or dummy parameters (also called formal or dummy arguments) • Values sent to function by calling program: actual parameters or actual arguments • Pass by value is default process: when a function is called a copy of the value of the argument is passed to the function 36 6 Functions More on Information to Functions • In pass-by-value, the values of the actual arguments in the calling program are not changed • The alternative to pass by value is pass by reference – The memory address of the actual parameter is passed to the function – Changes to the dummy parameter in the function change the actual parameter in the calling program Pass-by-Value Example //calling program double x = 10, y = 2; cout << “fake = “ << fake( x, y cout << “, x = “ << “, y = “ << // what is printed? //function double fake( double x, double y { x +=10; y *= x; return 3 * } ); y; ) y; 37 Pass-by-value Operation 38 Pass-by-reference • The code on the previous chart does not change the x and y values in the calling program • Only values of x and y from the calling program are passed to the function • Functions cannot changed values of variables that are passed by value • How do we use pass by reference to change the values of parameters passed into a function? • To use pass by reference place an ampersand (&) between the type and the parameter name in the function header: int f1( int& x, int& y ) – Not a preferred programming style – Used only when we have to change more than one parameter (e.g., input routine, vector components, etc.) – Exercise seven uses an input function which must have pass by reference 39 Pass-by-reference II 40 Pass-by-Value Example • Default is pass-by-value where changes to parameters do not affect variables in the calling program double fake1 ( int x, double y ) { x++; y += x; return x * y; } • Ampersand (&) gives pass by reference that changes program variables double fake2 ( int& x, double& y ) { x++; y += x; return x * y; } 41 //calling program segment double u = 5, v = 2; cout << “fake = “ << fake( u, v cout << “\nu =“ << u << “, v =“ // what is printed? fake = 90 u = 5, v = 2 //function double fake( double x, double y { x +=10; y *= x; return 3 * } x=5+ 10 = 15 y=2* 15 = 30 ); << v; ) y; fake( u, v ) = 3 * 30 = 90 42 7 Functions Pass-by-Reference Example Pass-by-Reference Example II //calling program segment double u = 3, v = 4; cout << “fake = “ fake( u, v ); cout << “\nu =“ << u << “, v =“ << v; // what is printed? fake = 156 //function u = 13, v = 52 double u = 3, v = 4; cout << “fake = “ fake( u, v ); cout << “\nu =“ << u << “, v =“ << v; double fake( double& x, double& y ) { x +=10; y *= x; return 3 * y; } // at start fake has x = 3, y = 4 // fake code sets x = x + 10 = 13 // and y = y * x = 4 * 13 = 52 // fake returns 3 * 52 = 156 and // changes u to 13 and v to 52 double fake(double& x, double& y ) { x +=10; y *= x; return 3 * y; } x=3+ y = 13 * fake( u, v ) = 10 = 13 4 = 52 3 * 52 = 156 43 44 Example: Converting Vectors • Convert different representations of twodimensional vectors – Polar: magnitude, |F| and direction, θ Converting Vectors II • Write two functions to convert between the two different representations y Fy |F| θ – Rectangular: components, Fx, and Fy, along the x and y axes x Fx • Conversion equations – |F| = (Fx2 + Fy2)1/2, θ = tan-1(Fy / Fx ) – Fx = |F|cos θ, Fy = |F|sin θ, – Polar to rectangular function has magnitude and direction as inputs and returns x-component and y-component – Rectangular to polar function has xcomponent and y-component as inputs and returns magnitude and direction – Use atan2 function for θ = tan-1(Fy / Fx ) to get full 2π result (-π/2 < atan result < π/2) 45 46 Converting Vectors III Converting Vectors IV • Each function has two input values and computes two results • Use pass by reference to get results back to calling program • Inputs to function are pass by value • Function type can be void since function name need not return a value void polarToRectangular ( double magnitude, double angle, double& xComponent, double& yComponent ) { xComponent = magnitude * cos( angle ); yComponent = magnitude * sin( angle ); } – Functions using pass by reference to return values sometimes return an error code in the function name 47 48 8 Functions Converting Vectors V Use of Conversion Functions void rectangularToPolar ( double xComponent, double yComponent, double& magnitude, double& angle ) { magnitude = sqrt( pow( xComponent, 2 ) + pow( yComponent, 2 ) ); angle = atan2 ( yComponent, xComponent ); } const double PI = 4 * atan(1); double x = 3, y = 4; double A, theta; rectangularToPolar(x, y, A, theta ); cout << x << “ “ << y << “ “ << A << “ “ << theta << endl; double size = 10, direction = PI / 8; double xComp, yComp; polarToRectangular( size, direction, xComp, yComp ); cout << size << “ “ << direction << “ “ << xComp << “ “ << yComp; 49 50 Pass-by-Reference Exercise Pass-by-Reference Exercise • Write an input function that prompts the user to enter two type double variables, x and y, and returns these values to the calling program void input( double& x, double& y) { cout << “Enter x and y: “; cin >> x >> y; } void input( double& x, double& y) { cout << “Enter x and y: “; cin >> x >> y } • Write the prototype for this function and a call to the function to get x and y void input( double& x, double& y ); double x, y; Optional input ( x, y ); 51 Use of Pass by Reference 52 Scope of a Variable • Calling programs use same approach for pass by reference and pass by value • Variable or expression is placed as one of the arguments to the function • Do not use a constant in a function call unless it is passed by value • Data types (and ampersands for pass by reference) are not used in function call 53 • Scope of a variable is the part of program that can use the variable • We see that we can have the same variable name in different functions • These names, although the same, occupy two different memory locations in the computer and are not related • Even within a single function we can limit the part of a function in which a variable is in scope (exists) 54 9 Functions Background Basic Rule for Scope • All variables must be declared (given a type) before they are used • Variables can be declared given a value when declared or later in the code • Usually assign a value before first use • Scope refers only to declaring a variable, not to assigning it a value • A variable defined in a set of braces only exists within those braces • It can be used anywhere in the program below its initial declaration – This is just a reminder that we have to initialize variables as well as declare them – This includes sets of braces that are opened below the initial declaration • After close of brace where variable is declared, the variables “goes out of scope” it cannot be used 55 56 Example of Scope Another example of Scope double x; if ( c == 4 ) { x = 12; double y = 2; // limited scope } cout << x << “ “ << y; // statement above will give syntax // error; y is not defined here double y = 0, c = 4; if ( c == 4 ) { double y = 2; // different var// iable with limited scope } cout << “y = “ << y; // statement above will print y = 0 // from initial declaration of y 57 58 Where to Declare Variables Another Example • Current programming practice declares variables as close to first time of use as possible • May have to be declared earlier in the code to give appropriate scope – First use of variable may be inside a loop – We must declare it prior to the loop if we want to use if following the loop • Code below will not work because yesNo goes out of scope after closing brace do { // program code cout << “Another run(Y/N)? “; char yesNo; // bad location cin >> yesNo; } while( yesNo == ‘Y’ || yesNo == ‘y’ ); 59 60 10 Functions Another Example Corrected Global Variables • Global variables have scope of more than one function • Code below works because yesNo is declared before brace opening the loop char yesNo; // correct location do { // program code cout << “Another run(Y/N)? “; cin >> yesNo; } while( yesNo == ‘Y’ || yesNo == ‘y’ ); – Declared outside function boundaries – Have scope of all functions from declaration to end of file – Usually declared at top of program to be present in all functions – Considered bad programming practice – Use only when variable must be accessed by several functions or there are problems in passing the variable 61 Trace Global Variables 62 Project Two Global Variables • What is program output? int status = 0; // global int main() { cout << status << “ “; f1(); f2(); cout << “ “ << status // more } void f1() { status = 1; } void f2() {cout << status << endl;} • In project two the main function calls a function which calls a third function • We want to get data from main to the third function • We do not want to rewrite the second function, but it does not allow us to pass the necessary information • Use global variables to get the information from main to third function • Program output is 0 1 1 63 Summary 64 Summary Continued • Use functions to organize code • Elements of a function • Pass information to function through argument list in function header – Header with type, name, and argument list – Body with code that function executes – Statement to return information through function name in calling program must be included in function body – Prototype at start of program which is header with a semicolon • Function name calls function and returns value 65 – Correspondence by position of arguments in header and position of arguments in calling function – Default of pass by value will not change arguments in calling function – Pass by reference (requires ampersand(&) in function header and prototype) changes arguments in calling function 66 11 Functions Summary Concluded Review Function Introduction • Scope of variables is part of program where a variable can be used • Variables can only be used within braces where there are declared and only following the declaration • Global variables, declared outside any function, can be used by any function following the declaration • A C++ program is a collection of functions – Each function is written as a unit – Complete code for one function before starting to write a new one – Execution starts in main function • Upon calls to a function, information and control is transferred to the function • Value returned in function name 67 68 Information Transfer • Function header has argument list • Variables in that list (called dummy parameters or dummy arguments) are determined by call to function • Call to function has actual arguments or actual parameters in same order that dummy arguments appear – Order is all that matters in transferring information to a function 69 12

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