Functions Spring 2005
Outline User-defined Functions • Writing and calling a function – Header and body – Function prototype Larry Caretto – Passing information to a function Computer Science 106 – Returning values in the function name Computing in Engineering and Science • void functions with no return value • Use pass-by-reference to change Spring 2005 variables in the calling program • Variable scope and global variables
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Introduction to Functions Why do we write functions? • Library functions like pow, atan and • First use of functions was for code like sqrt used previously mathematical function calculations • Statement to set x = yz3: – Specialized calculation done repeatedly – x = y * pow( z, 3 ); – Want to write code only one time – Note order of arguments important; the call – Want to be able to pass values of pow( 3, z ) gives 3z parameters to code and get value back – Use #include
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How do we write functions? Operation of Functions • C++ code is a collection of functions • Any function (the caller) can call another • Each function, including main, has the function by using the name of the same level of importance function being called in an expression – Close code for each function before • The statement calling the function starting a new function sends information from the caller int main() { // body of main • Execution control is transferred to the } function being called int myFunction( …… ) { // body of myFunction • The function being called returns control } and (usually) results to the caller
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1 Functions Spring 2005
Operation of Functions Writing and Using Functions • The function being called uses the • Organize the program into individual information it receives to do a set of functions that are called by main calculations or procedures – Simple example: main calls three functions: (1) input function, (2) calculation • In the usual case, the function being function and (3) output funciton called returns a result to the caller in the • Write code for each function (and main) location of the function name – Write function header to specify • Example: d = pow(b,2) – 4 * a * c; information received from calling function – calls the pow function with values of b and 2 – Write function body to calculate results and return them to “calling” function – and the result b2 is returned in function name, pow, for further use in an expression • Write function calls to exchange data 7 8
Writing and Using Functions II Function Basics • Library functions, such as pow(x, y) to • Each function has a header and a body compute xy, transfer information based – Header specifies on the order of the variables • Name of function • Type of value returned by the function name • This is true for user-defined functions as • List of variables in the function whose values well are determined by the calling program – Information transferred from a list of – Body gives code executed by the function variables in the calling function to a list of • Function prototypes at start of code variables in the function called provide information to compiler – Correspondence based on order of – Same as header except a semicolon is variables in function header and statement added at the end calling the function – Can omit variable names 9 10
Function Basics II Function Header • Example of header and body • Header has following syntax: double myPow ( double number, –
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2 Functions Spring 2005
Function Header II Argument List • Example of function, myPow, a user- • Example on previous chart had two defined function to replace pow parameters in argument list – Pass the number and the power to the –
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Passing arguments The return Statement • Based on order of arguments in function • We have used this statement in main as header and in calling statement return EXIT_SUCCESS; • Recall the library pow function was • The general syntax of this statement is called as pow(number, power) return
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Organization of Function Code Alternative Function Code
double myPow( double number, double myPow( double number, double power ) double power ) { { double result = exp( power * return = exp( power * log( number ) ); log( number ) ); return result; } } • Can use following prototype without • Place following prototype at top of code variable names at top of code double myPow( double number, double myPow( double, double ); double power );
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3 Functions Spring 2005
Another function Example Use of bool leap( int year ) bool leap ( int year );// prototype bool leap( int year ) Header int main() // examples of use Type { { cout << “Enter a year: “; if ( year % 4 != 0 ) int y; cin >> y; Name return false; bool cond = leap( y ); else if ( year % 400 == 0 ) return true; if ( leap( y ) ) {…} Argu- else if ( year % 100 == 0 ) if ( leap( y ) && month == 2 ) {…} ment return false; return EXIT_SUCCESS List Body else Multiple } return true; returns // leap and other functions go here }
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Exercise Exercise • Write a function that takes two type int • Write a function that takes two type arguments and returns their difference double arguments and returns their quotient • Use this function to compute 3 – 5 • Use this function to compute 5/3 • Write the prototype • Write the prototype int diff( int a, int b) double div( double a, double b) { return a – b } { return a / b } //use: cout << diff( 3, 5 ); //use: cout << div( 5, 3 ); // prototype: int diff( int a, int b); // prototype: double div(double a, double b); 21 22
void functions with no return The return Statement • The type void used for functions that do • The return statement returns control not return a value and a value to the calling program – Functions, other than void functions use • Example: error message function the syntax return
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4 Functions Spring 2005
Empty Argument List Kinetic Energy Function • If a function does not need any values • Write a function that takes two type from the calling program an empty set double parameters, mass and velocity of parentheses is required and computes kinetic energy = mV2/2 • Example is function with several output double KE( double m, double V ) statements to describe purpose of code { void describeCode() return m * V * V / 2; { } cout << “This code ….. • Possible calls to this function cout << “Still more output totalE = KE( 4, 3 ) + PE; // No return needed for type void • What is result of this call } result = 50 + KE( 5, 2 ); 2 25 50 + 5 * 2 / 2 = 60 26
Kinetic Energy Function II Data Validation Function
double KE( double m, double V ) • The function getValidInt( int xMin, { int xMax,string name ) does the return m * V * V / 2; following tasks } – Prompts the user for an input variable • What is output from these calls? (named in the string passed in the third parameter) within a range defined by the double mass = 5, velocity = 2; 10 cout << KE( velocity, mass ); first and second parameters double e = PE + KE( 2*pow( velocity, – Gets the input from the user 2), velocity); – Tells the user if there is an error and gets double total = KE( mass * velocity, new input from the user in this case e = KE(2*22,2) = KE(8,2)=8*22/2 = 16 mass ); – Returns valid input to the calling function
total = KE(5*2,5) = KE(10,5)=10*52/2 = 12527 28
Data Validation Function II int getValidInt( int xMin, int xMax, string name ) • The function getValidInt described { on the previous chart is used in exercise // Function used to input integer seven and project one // data within a stated range • Examples of its use // Example function call to input a // value for a variable named int month = getValidInt( 1, 12, “month”); // hour with range between 0 and 23: // int hour = int mayDay = getValidInt( 1, 31, “day of the month” ); // getValidInt( 0, 23, “hour” ); int year = getValidInt( 1901, 2000, “year in the 20th century” ); int x; // Input data value bool badData; // Bad data flag // continued on next chart 29
5 Functions Spring 2005
do // Loop until user data in range if ( badData ) { // print error message cout << "Enter a value for " << name { << " between " << xMin << " and " cout << "\n\nIncorrect data; you " << xMax << ": "; << "entered " << name cin >> x; badData = x < xMin || x > xMax; << " = “ << x << "\n" if ( badData ) << name << "must be between " // error message code on next chart << xMin << " and " << xMax } << " Reenter the data now.\n"; while ( badData ); }
} // end of function
getValidInt Screen Results Program Structure Example • Call to getValidInt • Next chart: example for getValidInt int mayDay = getValidInt( 1, 31, “day of the month”); • Function prototype before program that • Screen prompt showing parameters and uses function user input • In this example main calls function Enter a value for day of the month • Complete code for main is written start between 1 and 31: 0 of code for function Incorrect data; you entered day of the month = 0. day of the month must be • A call to the function transfers control to between 1 and 31. Reenter the data now. function with values in the function Enter a value for day of the month header variables from the caller between 1 and 31: 1 • Function returns value to main
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int getValidInt( int, int, string); // prototype above Passing Information to Functions int main() { • Parameters in function header: formal int month = getValidInt( 1, 12, parameters or dummy parameters (also “month” ); called formal or dummy arguments) ……………… // other code return EXIT_SUCCESS • Values sent to function by calling } program: actual parameters or actual int getValidInt( int xMin, int xMax, arguments string name ) { int x; • Pass by value is default process: when ……………………………… // other code a function is called a copy of the value cin >> x; of the argument is passed to the ……………………………… // other code function return x;
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6 Functions Spring 2005
More on Information to Functions Pass-by-Value Example • In pass-by-value, the values of the //calling program actual arguments in the calling program double x = 10, y = 2; are not changed cout << “fake = “ << fake( x, y ); • The alternative to pass by value is pass cout << “, x = “ << “, y = “ << y; by reference // what is printed? – The memory address of the actual //function parameter is passed to the function double fake( double x, double y ) – Changes to the dummy parameter in the { function change the actual parameter in x +=10; y *= x; return 3 * y; the calling program }
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Pass-by-value Operation Pass-by-reference • The code on the previous chart does • To use pass by reference place an not change the x and y values in the ampersand (&) between the type and calling program the parameter name in the function • Only values of x and y from the calling header: int f1( int& x, int& y ) program are passed to the function – Not a preferred programming style • Functions cannot changed values of – Used only when we have to change more variables that are passed by value than one parameter (e.g., input routine, vector components, etc.) • How do we use pass by reference to – Exercise seven uses an input function change the values of parameters which must have pass by reference passed into a function?
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Pass-by-reference II Pass-by-Value Example • Default is pass-by-value where changes //calling program segment to parameters do not affect variables in double u = 5, v = 2; the calling program cout << “fake = “ << fake( u, v ); double fake1 ( int x, double y ) cout << “\nu =“ << u << “, v =“ << v; { x++; y += x; return x * y; } // what is printed? fake = 90 • Ampersand (&) gives pass by reference //function u = 5, v = 2 that changes program variables double fake( double x, double y ) double fake2 ( int& x, double& y ) { { x++; y += x; return x * y; } x +=10; y *= x; return 3 * y; } x = 5 + y = 2 * fake( u, v ) = 41 10 = 15 15 = 30 3 * 30 = 90 42
7 Functions Spring 2005
Pass-by-Reference Example Pass-by-Reference Example II
//calling program segment double u = 3, v = 4; double u = 3, v = 4; cout << “fake = “ fake( u, v ); cout << “fake = “ fake( u, v ); cout << “\nu =“ << u << “, v =“ << v; cout << “\nu =“ << u << “, v =“ << v; double fake( double& x, double& y ) // what is printed? fake = 156 { x +=10; y *= x; return 3 * y; } //function u = 13, v = 52 // at start fake has x = 3, y = 4 double fake(double& x, double& y ) // fake code sets x = x + 10 = 13 { // and y = y * x = 4 * 13 = 52 x +=10; y *= x; return 3 * y; // fake returns 3 * 52 = 156 and } x = 3 + y = 13 * fake( u, v ) = // changes u to 13 and v to 52 10 = 13 4 = 52 3 * 52 = 156 43 44
Example: Converting Vectors Converting Vectors II
• Convert different y • Write two functions to convert between representations of two- the two different representations dimensional vectors – Polar to rectangular function has Fy – Polar: magnitude, |F| and |F| magnitude and direction as inputs and direction, θ θ x returns x-component and y-component – Rectangular to polar function has x- F – Rectangular: components, Fx, x component and y-component as inputs and and Fy, along the x and y axes returns magnitude and direction -1 • Conversion equations – Use atan2 function for θ = tan (Fy / Fx ) to 2 2 1/2 -1 –|F| = (Fx + Fy ) , θ = tan (Fy / Fx ) get full 2π result (-π/2 < atan result < π/2) –Fx = |F|cos θ,Fy = |F|sin θ,
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Converting Vectors III Converting Vectors IV • Each function has two input values and void polarToRectangular ( computes two results double magnitude, double angle, • Use pass by reference to get results double& xComponent, back to calling program double& yComponent ) { • Inputs to function are pass by value xComponent = magnitude • Function type can be void since function * cos( angle ); name need not return a value yComponent = magnitude – Functions using pass by reference to * sin( angle ); return values sometimes return an error } code in the function name
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8 Functions Spring 2005
Converting Vectors V Use of Conversion Functions
void rectangularToPolar ( const double PI = 4 * atan(1); double xComponent, double x = 3, y = 4; double yComponent, double A, theta; double& magnitude, double& angle ) rectangularToPolar(x, y, A, theta ); { cout << x << “ “ << y << “ “ magnitude = sqrt( << A << “ “ << theta << endl; pow( xComponent, 2 ) + double size = 10, direction = PI / 8; pow( yComponent, 2 ) ); double xComp, yComp; angle = atan2 ( yComponent, polarToRectangular( size, direction, xComp, yComp ); xComponent ); cout << size << “ “ << direction << } “ “ << xComp << “ “ << yComp; 49 50
Pass-by-Reference Exercise Pass-by-Reference Exercise • Write an input function that prompts the void input( double& x, double& y) user to enter two type double variables, x { and y, and returns these values to the cout << “Enter x and y: “; calling program cin >> x >> y } void input( double& x, double& y) { • Write the prototype for this function and a call to the function to get x and y cout << “Enter x and y: “; void input( double& x, double& y ); cin >> x >> y; double x, y; } input ( x, y ); Optional 51 52
Use of Pass by Reference Scope of a Variable
• Calling programs use same approach for • Scope of a variable is the part of program that can use the variable pass by reference and pass by value • We see that we can have the same • Variable or expression is placed as one variable name in different functions of the arguments to the function • These names, although the same, • Do not use a constant in a function call occupy two different memory locations unless it is passed by value in the computer and are not related • Data types (and ampersands for pass by • Even within a single function we can reference) are not used in function call limit the part of a function in which a variable is in scope (exists)
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9 Functions Spring 2005
Background Basic Rule for Scope • All variables must be declared (given a • A variable defined in a set of braces type) before they are used only exists within those braces • Variables can be declared given a value • It can be used anywhere in the program when declared or later in the code below its initial declaration • Usually assign a value before first use – This includes sets of braces that are • Scope refers only to declaring a opened below the initial declaration variable, not to assigning it a value • After close of brace where variable is – This is just a reminder that we have to initialize variables as well as declare them declared, the variables “goes out of scope” it cannot be used
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Example of Scope Another example of Scope double x; double y = 0, c = 4; if ( c == 4 ) if ( c == 4 ) { { x = 12; double y = 2; // different var- double y = 2; // limited scope // iable with limited scope } } cout << x << “ “ << y; cout << “y = “ << y; // statement above will give syntax // statement above will print y = 0 // error; y is not defined here // from initial declaration of y
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Where to Declare Variables Another Example
• Current programming practice declares • Code below will not work because yesNo variables as close to first time of use as goes out of scope after closing brace possible do • May have to be declared earlier in the { // program code code to give appropriate scope cout << “Another run(Y/N)? “; – First use of variable may be inside a loop char yesNo; // bad location – We must declare it prior to the loop if we cin >> yesNo; want to use if following the loop } while( yesNo == ‘Y’ || yesNo == ‘y’ );
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10 Functions Spring 2005
Another Example Corrected Global Variables
• Code below works because yesNo is • Global variables have scope of more than one function declared before brace opening the loop – Declared outside function boundaries char yesNo; // correct location – Have scope of all functions from do declaration to end of file { // program code – Usually declared at top of program to be cout << “Another run(Y/N)? “; present in all functions cin >> yesNo; – Considered bad programming practice – Use only when variable must be accessed } by several functions or there are problems while( yesNo == ‘Y’ || yesNo == ‘y’ ); in passing the variable
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Trace Global Variables Project Two Global Variables • What is program output? • In project two the main function calls a int status = 0; // global function which calls a third function int main() { • We want to get data from main to the cout << status << “ “; third function f1(); f2(); • We do not want to rewrite the second cout << “ “ << status // more function, but it does not allow us to pass } the necessary information void f1() { status = 1; } • Use global variables to get the void f2() {cout << status << endl;} information from main to third function • Program output is 0 1 1
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Summary Summary Continued • Use functions to organize code • Pass information to function through • Elements of a function argument list in function header – Header with type, name, and argument list – Correspondence by position of arguments in header and position of arguments in – Body with code that function executes calling function – Statement to return information through – Default of pass by value will not change function name in calling program must be arguments in calling function included in function body – Pass by reference (requires ampersand(&) – Prototype at start of program which is in function header and prototype) changes header with a semicolon arguments in calling function • Function name calls function and returns value 65 66
11 Functions Spring 2005
Summary Concluded Review Function Introduction • Scope of variables is part of program • A C++ program is a collection of where a variable can be used functions • Variables can only be used within – Each function is written as a unit braces where there are declared and – Complete code for one function before only following the declaration starting to write a new one • Global variables, declared outside any – Execution starts in main function function, can be used by any function • Upon calls to a function, information and following the declaration control is transferred to the function • Value returned in function name
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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
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