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Chapter 7 Functions

Chapter 7 Functions. Chapter 7 Topics. Review of functions VERY IMPORTANT NEW TOPIC: The difference between passing parameters by value, and passing parameters by reference. Functions. every C++ program must have a function called main program execution always begins with function main

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Chapter 7 Functions

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  1. Chapter 7Functions

  2. Chapter 7 Topics • Review of functions • VERY IMPORTANT NEW TOPIC: • The difference between passing parameters by value, and passing parameters by reference

  3. Functions • every C++ program must have a function called main • program execution always begins with function main • any other functions are subprograms and must be called

  4. Function Calls One function calls another by using the name of the called function with parentheses ( ) enclosing an argument list. A function call temporarily transfers control from the calling function to the called function.

  5. Function Call Syntax FunctionName ( Argument List ) The argument list is a way for functions to communicate with each other by passing information. The argument list can contain 0, 1, or more arguments, separated by commas, depending on the function.

  6. Two Parts of Function Definition int Cube ( int n )heading { body return n * n * n ; }

  7. What is in a heading? type of value returned parameter list name of function int Cube ( int n )

  8. What is in a prototype? A prototype looks like a heading but must end with a semicolon; and its parameter list just needs to contain the type of each parameter. int Cube( int );// prototype OR: int Cube (int x); You can put in any variable name (optional)

  9. Prototypes • Prototypes of system functions (such as cin, cout, sqrt, open) are contained in header files (like iostream, cmath, fstream) • You include the proper header file for system functions • You must provide prototypes for your functions

  10. To Compile Successfully, • before a function is called in your program, the compiler must previously process either the function’s prototype, or the function’s definition (heading and body)

  11. When a function is called, temporary memory is set up ( for its value parameters and any local variables, and also for the function’s name if the return type is not void). Then the flow of control passes to the first statement in the function’s body. The called function’s body statements are executed until one of these occurs: return statement (with or without a return value), or, closing brace of function body. Then control goes back to where the function was called.

  12. #include <iostream> int Cube ( int ) ; // prototype using namespace std; void main ( ) { int yourNumber ; arguments int myNumber ; yourNumber = 14 ; myNumber = 9 ; cout << “My Number = “ << myNumber ; cout << “its cube is “ << Cube (myNumber)<< endl ; cout << “Your Number = “ << yourNumber ; cout << “its cube is “ << Cube (yourNumber)<< endl ; }

  13. A C++ function can return • at most 1 value of the type which was specified (called the return type) in its heading and prototype • but, a void-function cannot return any value

  14. Write a void function called DisplayMessage ( ) which you can call from main ( ) to describe the pollution index value it receives as a parameter. Your city describes a pollution Index less than 35 as “Pleasant”, 35 through 60 as “Unpleasant”, and above 60 as “Health Hazard.”

  15. parameter void DisplayMessage( int index ) { if ( index < 35 ) cout << “Pleasant”; else if ( index <= 60 ) cout << “Unpleasant”; else cout << “Health Hazard”; // return statement optional with void function }

  16. The Rest of the Program #include <iostream> void DisplayMessage (int); // prototype //void DisplayMessage (int anyName); // legal using namespace std; int main ( ) argument { int pollutionIndex; cout << “Enter air pollution index”; cin >> pollutionIndex; DisplayMessage(pollutionIndex);// call return 0; } 16

  17. return; • Causes control to leave the function and immediately return to the calling block leaving any subsequent statements in the function body unexecuted • There can be multiple return statements in a function • A void function does not need a return statement

  18. Example of using multiple returns void DisplayMessage( int index ) { if ( index < 35 ) { cout << “Pleasant”; return; } if ( index <= 60 ) { cout << “Unpleasant”; return; } cout << “Health Hazard”; return; } 18

  19. Program with Several Functions function prototypes main function Square function Cube function

  20. Value-returning Functions #include <iostream> int Square ( int ) ; // prototypes int Cube ( int ) ; using namespace std; int main ( ) { cout << “The square of 27 is “ << Square (27)<< endl; // function call cout << “The cube of 27 is “ << Cube (27)<< endl; // function call return 0; } 20

  21. Rest of Program int Square ( int n )// header and body { return n * n; } int Cube ( int n )// header and body { return n * n * n; }

  22. A void function call stands alone #include <iostream> void DisplayMessage ( int ) ; // prototype using namespace std; int main ( ) argument { DisplayMessage(15);// function call cout << “Good Bye“ << endl; return 0; } 22

  23. A void function does NOT return a value parameter void DisplayMessage ( int n ) { cout << “I have liked math for “ << n << “ years” << endl ; return ; // optional for void function }

  24. Pass by value or reference • Pass by value: The called function gets a copy of the variable. The variable in the caller cannot be changed. • Pass by reference: The called function gets the address (a reference) to the variable. If the called function changes the variable, it is changed in the caller.

  25. By default, parameters(of simple types like int, char, float, double) are always value parameters, unless you do something to change that. To get a reference parameter you need to place & after the type in the function heading and prototype.

  26. When to Use Reference Parameters • reference parameters should be used when you want your called function to change the value of a variable in the calling function

  27. Using a Reference Parameter • is like giving someone the key to your home • the key can be used by the other person to change the contents of your home!

  28. 4000 25 age MAIN PROGRAM MEMORY If you pass only a copy of 25 to a function, it is called “pass-by-value”and the function will not be able to change the contents of age. It is still 25 when you return.

  29. 4000 25 age MAIN PROGRAM MEMORY BUT, if you pass 4000, the address of age to a function, it is called “pass-by-reference”and the function will be able to change the contents of age. It could be 23 or 90 when you return.

  30. Pass-by-reference is also called . . . • pass-by-address, or • pass-by-location

  31. Example of Pass-by-Reference We want to find 2 real roots for a quadratic equation with coefficients a,b,c. Write a prototype for a void function named GetRoots( ) with 5 parameters. The first 3 parameters are type float (a,b, c variables of the quadratic equation). The last 2 are reference parameters of type float (the roots).

  32. // prototype void GetRoots ( float , float , float , float& , float& ); Now write the function definition using this information. This function uses 3 incoming values a, b, c from the calling block. It calculates 2 outgoing values root1 and root2 for the calling block. They are the 2 real roots of the quadratic equation with coefficients a, b, c.

  33. Function Definition void GetRoots( float a, float b, float c, float& root1, float& root2) { float temp; // local variable temp = b * b - 4.0 * a * c; if (temp >= 0) { // if real root root1=(-b + sqrt(temp) ) / ( 2.0 * a ); root2=(-b - sqrt(temp) ) / ( 2.0 * a ); } else { root1 = 999; root2 = -999; } return;} 33

  34. #include <iostream> #include <cmath> void GetRoots(float, float, float, float&, float&); using namespace std; int main ( ){ float a, b, c, first, second; int count = 0; cout << "Enter a,b,c " <<endl; cin >> a >> b >> c; while ( a != 0) { GetRoots(a, b, c, first, second); //call cout << "For variables: "; cout << a << " " << b << " " << c << endl; cout << "Roots: " ; cout << first << " " << second << endl; cout << "Enter a,b,c " <<endl; cin >> a >> b >> c; } return 0; } 34

  35. Calling a function to swap variables // Will Swap swap Number1,Number 2? int main() { int Number1, Number2; cin >> Number1 >> Number2; if (Number1 > Number2) { Swap(Number1, Number2);// Swap? } cout << "The numbers swapped:" << Number1 << ", " << Number2 << endl; return 0;}

  36. Will Swap swap? • IF parameters passed by value: NO • If parameters passes by reference: YES

  37. Using Pass by Value void Swap(int a, int b) { int Temp = a; a = b; b = Temp; return; }

  38. Doesn’t change the caller’s variables!

  39. Same Call to Swap But… void Swap(int &a, int &b); int main() { int Number1, Number2; cin >> Number1 >> Number2; if (Number1 > Number2) { Swap(Number1, Number2); } cout << "The numbers in sorted order: " << Number1 << ", " << Number2 << endl; return 0;}

  40. Swap defined as Pass by Reference void Swap(int &a, int &b) { int Temp = a; a = b; b = Temp; return; } Passed by reference -- in aninvocation the address is given ratherthan a copy Return statement notnecessary for void functions

  41. Pass-by-value “incoming” value of argument CALLING FUNCTION FUNCTION CALLED

  42. “incoming” original value of Passed argument Pass-by-reference CALLING FUNCTION FUNCTION CALLED “outgoing” changed value of passed argument

  43. Returning a value from function The returned value replaces the function name: int Square1(int n); // prototype ….. // Use function name like any int temp = Square1(b) - 4 * a* c;

  44. Pass by reference Pass by reference is sharing the variable void Square2(int & n); // prototype …. Square2(b); // Square modifies b in caller temp = b - 4*a*c;

  45. Square return vs. pass by reference int Square1(int n) // Square returns a value { return n*n; } // Square modifies n in caller void Square2(int &n) { n = n*n; }

  46. Questions • Why is a function used? • To cut down on the amount of detail in your main program (encapsulation). • So that it can be called multiple times • Can one function call another function? Yes • Can a function even call itself? Yes, that is called recursion. It is very useful (covered in CS 215).

  47. More Questions • Does it make any difference what names you use for passed parameters? NO. Just use them in function body. • Do the calling function and the called function have to use the same names for passed parameters? NO. Each function should use names meaningful to itself.

  48. Functions are written to specifications • the specifications state the return type, the parameter types, whether any parameters are “outgoing,” and what task the function is to perform with its parameters • the advantage is that teamwork can occur without knowing what the argument identifiers (names) will be

  49. Write prototype and function definition for • a void function called GetRating( ) with one reference parameter of type char • the function repeatedly prompts the user to enter a character at the keyboard until one of these has been entered: E, G, A, P to represent Excellent, Good, Average, Poor

  50. void GetRating( char& ); // prototype void GetRating (char& letter) { cout << “Enter employee rating.” << endl; cout << “Use E, G, A, or P : ” ; cin >> letter; while ( (letter != ‘E’) && (letter != ‘G’) && (letter != ‘A’) && (letter != ‘P’) ) { cout << “Rating invalid. Enter again: ”; cin >> letter; } } 50

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