C++ Functions.ppt
- 2. Agenda 1. What is a function? 2. Types of C++ functions: 1. Standard functions 2. User-defined functions 3. C++ function structure 1. Function signature 2. Function body 4. Declaring and Implementing C++ functions 5. Sharing data among functions through function parameters – Value parameters – Reference parameters 2
- 3. Functions and subprograms • The Top-down design appeoach is based on dividing the main problem into smaller tasks which may be divided into simpler tasks, then implementing each simple task by a subprogram or a function • A C++ function or a subprogram is simply a chunk of C++ code that has – A descriptive function name, e.g. • computeTaxes to compute the taxes for an employee • isPrime to check whether or not a number is a prime number – A returning value • The computeTaxes function may return with a double number representing the amount of taxes • The isPrime function may return with a Boolean value (true or false) 3
- 4. C++ Standard Functions • C++ language is shipped with a lot of functions which are known as standard functions • These standard functions are groups in different libraries which can be included in the C++ program, e.g. – Math functions are declared in <math.h> library – Character-manipulation functions are declared in <ctype.h> library – C++ is shipped with more than 100 standard libraries, some of them are very popular such as <iostream.h> and <stdlib.h>, others are very specific to certain hardware platform, e.g. <limits.h> and <largeInt.h> 4
- 5. Example of Using Standard C++ Math Functions #include <iostream.h> #include <math.h> void main() { // Getting a double value double x; cout << "Please enter a real number: "; cin >> x; // Compute the ceiling and the floor of the real number cout << "The ceil(" << x << ") = " << ceil(x) << endl; cout << "The floor(" << x << ") = " << floor(x) << endl; } 5
- 6. Example of Using Standard C++ Character Functions #include <iostream.h> // input/output handling #include <ctype.h> // character type functions void main() { char ch; cout << "Enter a character: "; cin >> ch; cout << "The toupper(" << ch << ") = " << (char) toupper(ch) << endl; cout << "The tolower(" << ch << ") = " << (char) tolower(ch) << endl; if (isdigit(ch)) cout << "'" << ch <<"' is a digit!n"; else cout << "'" << ch <<"' is NOT a digit!n"; } 6 Explicit casting
- 7. How to define a C++ Function? • Generally speaking, we define a C++ function in two steps (preferably but not mandatory) – Step #1 – declare the function signature in either a header file (.h file) or before the main function of the program – Step #2 – Implement the function in either an implementation file (.cpp) or after the main function 7
- 8. 8 • Functions – Modularize a program – Software reusability • Call function multiple times • Local variables – Known only in the function in which they are defined – All variables declared in function definitions are local variables • Parameters – Local variables passed to function when called – Provide outside information
- 9. 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
- 10. 10 Function Definitions • Function prototype – Tells compiler argument type and return type of function – int square( int ); • Function takes an int and returns an int – Explained in more detail later • Calling/invoking a function – square(x); – Parentheses an operator used to call function • Pass argument x • Function gets its own copy of arguments – After finished, passes back result
- 11. 11 Function Definitions • Format for function definition return-value-type function-name( parameter-list ) { declarations and statements } – Parameter list • Comma separated list of arguments – Data type needed for each argument • If no arguments, use void or leave blank – Return-value-type • Data type of result returned (use void if nothing returned)
- 12. A C++ function can return • in its identifier 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 in its identifier
- 13. Classified by Location Always appear in a function call within the calling block. Always appear in the function heading, or function prototype. Arguments Parameters
- 14. Example of User-defined C++ Function double computeTax(double income) { if (income < 5000.0) return 0.0; double taxes = 0.07 * (income-5000.0); return taxes; } 14
- 15. Example of User-defined C++ Function double computeTax(double income) { if (income < 5000.0) return 0.0; double taxes = 0.07 * (income-5000.0); return taxes; } 15 Function header Function body
- 16. Function Signature • The function signature is actually similar to the function header except in two aspects: – The parameters’ names may not be specified in the function signature – The function signature must be ended by a semicolon • Example double computeTaxes(double) ; 16 Unnamed Parameter Semicolon ;
- 17. Example #include <iostream> #include <string> using namespace std; // Function Signature double getIncome(string); double computeTaxes(double); void printTaxes(double); void main() { // Get the income; double income = getIncome("Please enter the employee income: "); // Compute Taxes double taxes = computeTaxes(income); // Print employee taxes printTaxes(taxes); } double computeTaxes(double income) { if (income<5000) return 0.0; return 0.07*(income-5000.0); } double getIncome(string prompt) { cout << prompt; double income; cin >> income; return income; } void printTaxes(double taxes) { cout << "The taxes is $" << taxes << endl; } 17
- 18. Sharing Data Among User-Defined Functions • There are two ways to share data among different functions –Using global variables (very bad practice!) –Passing data through function parameters • Value parameters • Reference parameters 18
- 19. II. Using Parameters • Function Parameters come in two flavors: – Value parameters – which copy the values of the function arguments – Reference parameters – which refer to the function arguments by other local names and have the ability to change the values of the referenced arguments 19
- 20. Example of Defining and Using Global and Local Variables #include <iostream.h> int x; // Global variable Void fun(); // function signature void main() { x = 4; fun(); cout << x << endl; } void fun() { int x = 10; // Local variable cout << x << endl; } 20
- 21. Local Variables • Local variables are declared inside the function body and exist as long as the function is running and destroyed when the function exit • You have to initialize the local variable before using it • If a function defines a local variable and there was a global variable with the same name, the function uses its local variable instead of using the global variable 21
- 22. Example of Defining and Using Global and Local Variables #include <iostream.h> int x; // Global variable Void fun(); // function signature void main() { x = 4; fun(); cout << x << endl; } void fun() { int x = 10; // Local variable cout << x << endl; } 22 x 0 Global variables are automatically initialized to 0
- 23. Example of Defining and Using Global and Local Variables #include <iostream.h> int x; // Global variable Void fun(); // function signature void main() { x = 4; fun(); cout << x << endl; } void fun() { int x = 10; // Local variable cout << x << endl; } 23 x 0 void main() { x = 4; fun(); cout << x << endl; } 1
- 24. Reference Parameters • In value parameters any changes in the value parameters don’t affect the original function arguments • Sometimes, we want to change the values of the original function arguments or return with more than one value from the function, in this case we use reference parameters – A reference parameter is just another name to the original argument variable – We define a reference parameter by adding the & in front of the parameter name, e.g. double update (double & x); 24
- 25. Example of Reference Parameters #include <iostream.h> void fun(int &y) { cout << y << endl; y=y+5; } void main() { int x = 4; // Local variable fun(x); cout << x << endl; } 25 void main() { int x = 4; fun(x); cout << x << endl; } 1 x ? x 4