Introduction to Functions

• main is a function
• place code in units based on purpose of code
• Readable code
• functions can be reused
• User defined
• Built-in

Defining a Function

• Place at same "level" as main
• Return answer type
  int, bool, double, ...
  void
• name
  Use same rules for naming variable identifiers
• Optional parameters
  list of variable identifiers separated by commas
• Body of Code
  Any C++ code (except another function definition)
• Template
  returnType functionName (params)
  {
      FunctionBody
  }

// This function rounds a double number and
// returns an integer
int round (double number)  // number passed by value
{
   int answer;

   answer = (int) (number + 0.5);

   return answer;
}

Calling a Function

• functionName (paramValues); // ONLY for void functions
• var = functionName (paramValues);
• Sample 1

  double input;
  int grade;
  cout << "enter a number to be rounded =>";
  cin >> input;
  grade = round (input);
  cout << "the rounded number is: " << grade;
  

• Sample 2

  double input;
  cout << "enter a number to be rounded =>";
  cin >> input;
  cout << "the rounded number is: " << round (input);
  

• Complete Program Rounding Program Code   Output

Function Semantics

1. The function call
1. Control is transferred from the caller to callee
2. Parameters are passed in in-order
3. Memory is allocated for local variables (including parameters passed by value)
2. The body of the function is executed
3. Function Return
1. If it is not a void function, the return value is calculated and returned to caller
2. Memory for local variables is deallocated
3. Control is transferred back to the caller

• A copy of the value of the actual argument is placed in the memory location for the formal parameter
• Default type of passing parameters
• The only technique that has been discussed so far
• Value arguments are like local variables, but function gets them automatically
• Common Mistake:Declaring parameter again inside function
  double fee(int hoursWorked, int minutesWorked)
  {
     int quarterHours;    // local variable
     int minutesWorked;   // NO!
  
  
• Choosing Formal Parameter Names
  Use a meaningful name within the function
  It does not need to be the same name as the calling function

• Instead of putting function code before main, put it afterwards
• Cleaner program code - start in main, it should be first
• Need to "declare" the header of the functions that will appear after main
• Same as function header, EXCEPT a semicolon is needed
• Do not need to provide parameter names, just types
• Improved Rounding Program Code

• Overloading:Giving two or more functions the same name
  Some type must be different (return type, parameter type)
• Automatic Type Conversion (bool-to-int, char-to-int, int-to-double)
• Rules for Resolving Overloading
  1. Exact match
  2. Match using type conversion
• Default Arguments
  Values for actual arguments may be given in the function prototype
  These default values are used, it the call doesn't include all of the parameters (remember: pass in order)
• Sample calls and prototypes
  myFunction(3,4,5);         // a is used
  myFunction(3.0, 4.0);    // c is used
  myFunction(11.1, 9, 2);    // b is used
  myFunction(23.56);         // d is used
  1. double myFunction(int, int, int).
  2. int myFunction(double, int, int)
  3. int myFunction (double, double)
  4. void myFunction(double)

• include <cmath>
• abs(x)
• floor(x) and ceil(x)
• pow(x,y)
• sqrt(x)
• rand()
• srand(seed)
  frequently used with time(0) found in ctime

  max(a, b)

  min(a,b)

• include <cctype>
• isupper(c)
• islower(c)
• isdigit(c)
• isspace(c)
• isalpha(c)
• ispunct(c)
• toupper(c)
• tolower(c)
• isupper(c)