1 Lecture 16:User-Definded function I Introduction to Computer Science Spring 2006

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3 Predefined Functions To use these functions you need to: Include the correct header file Know the name of the function Know the number of parameters, if any Know the data type of each parameter Know the data type of the value computed by the function, called the type of the function

4 Predefined Functions : Value-Returning Functions Because the value returned by a value- returning function is unique, we must: Save the value for further calculation Use the value in some calculation Print the value A value-returning function is used in an assignment or in an output statement

5 Use of standard function pow #include using namespace std; int main() { double u,v; double result; u = 4.2; v = 3.0; return 0; } #include result=pow(u, v); cout << u << " to the power of " << v << " = " << pow(u, v) << endl; cout << "u = " << u << endl; u = u + pow(3, 3);

6 User-Defined Functions Void functions: do not have a data type Value-returning functions: have a data type

7 #include using namespace std; int main() { int i, j; int k; cin>>i>>j; if (i > j) { k = i; } else { k = j; } cout << "The max is " << k << endl; return 0; } #include using namespace std; /* Function Declarations */ int FindMax(int n1, int n2); void PrintMax(int someNumber); int main() { int i, j; int k; cin>>i>>j; k = FindMax(i,j); PrintMax(k); // Prints Max Value return 0; } /* Function Definitions */ int FindMax(int n1, int n2) { if (n1 > n2) { return n1; } else { return n2; } /* Function Definitions */ void PrintMax(int someNumber) { cout << "The max is " << someNumber << endl; } Value-returning functions Void functions

8 Value-Returning Functions Because the value returned by a value- returning function is unique, we must: Save the value for further calculation Use the value in some calculation Print the value A value-returning function is used in an assignment or in an output statement

9 #include using namespace std; /* Function Declarations */ int FindMax(int n1, int n2); int main() { int i, j; int k; cin>>i>>j; k = FindMax(i,j); return 0; } /* Function Definitions */ int FindMax(int n1, int n2) { if (n1 > n2) { return n1; } else { return n2; } cout << "The max is "<< FindMax(i,j) << endl; k =FindMax(i, j) + 1;

10 Value-Returning Functions: function definition Properties that form the function definition: Heading: 1. Name of the function 2. Number of parameters 3. Data type of each parameter 4. Function Type (type of the value returned by the function) Body: 1. Code required to accomplish the task (the body of the function) The syntax of the function definition is: functionType functionName(formal parameter list) { statements } The syntax of the formal parameter list is: dataType identifier, dataType identifier,...

11 Value-Returning Functions: function definition int FindMax(int n1, int n2) { if (n1 > n2) { return n1; } else { return n2; } Function Heading int FindMax(int n1, int n2) Function Body Function Type Function Name Formal parameter list

12 Value-Returning Functions: call a value-returning function To call a value-returning function: Use its name, with the actual parameters (if any) in parentheses The syntax for a function call is: functionName(actual parameter list) The syntax for the actual parameter list is: expression or variable,expression or variable,...

13 #include using namespace std; /* Function Declarations */ int FindMax(int n1, int n2); int main() { int i, j; int k; cin>>i>>j; k = FindMax( i, j ) ; return 0; } /* Function Definitions */ int FindMax( int n1, int n2 ) { if (n1 > n2) { return n1; } else { return n2; } Actual parameter list Function Name Call a Value-returning Function

14 Function Prototype Function Prototype: function heading without the body of the function The syntax is: functionType functionName(parameter list); It is not necessary to specify the variable name in the parameter list The data type of each parameter must be specified Function prototypes appear before any function definition The compiler translates these first The compiler can then correctly translate a function call

15 #include using namespace std; /* Function Declarations */ int FindMax(int n1, int n2); int main() { int i, j; int k; cin>>i>>j; k = FindMax( i, j ) ; return 0; } /* Function Definitions */ int FindMax( int n1, int n2 ) { if (n1 > n2) { return n1; } else { return n2; } Function Prototype

16 Value-Returning Functions (continued) Formal Parameter: variable declared in the heading Actual Parameter: variable or expression listed in a call to a function There is a one-to-one correspondence between actual and formal parameters

17 #include using namespace std; /* Function Declarations */ int FindMax(int n1, int n2); int main() { int i, j; int k; cin>>i>>j; k = FindMax( i, j ) ; return 0; } /* Function Definitions */ int FindMax( int n1, int n2 ) { if (n1 > n2) { return n1; } else { return n2; } Actual parameter list Formal parameter list

18 Functions The formal parameter list can be empty If the formal parameter list is empty Parentheses are still needed Function heading of the value-returning function takes either of the following forms: functionType functionName() functionType functionName(void) In a function call the actual parameter is empty A call to a value-returning function with an empty formal parameter list is: functionName()

19 Value-Returning Functions : The return Statement Once the function computes the value, the function returns the value via the return statement The syntax of the return statement is: return expression or variable; When a return statement executes Function immediately terminates Control goes back to the caller When a return statement executes in the function main, the program terminates

20 Flow of Execution Execution always begins at The first statement in the function main no matter where main is placed in the program Other functions are executed only when they are called A function call statement results in Transfer of control to the first statement in the body of the called function After the last statement of the called function is executed Control is passed back to the point immediately following the function call A value-returning function returns a value After executing the function The value that the function returns replaces the function call statement

21 #include using namespace std; /* Function Declarations */ int FindMax(int n1, int n2); int main() { int i, j; int k; cin>>i>>j; k = FindMax( i, j ) ; return 0; } /* Function Definitions */ int FindMax( int n1, int n2 ) { if (n1 > n2) { return n1; } else { return n2; }

22 End of lecture 16 Thank you!