Functions ROBERT REAVES

Functions  Interface – the formal description of what a subprogram does and how we communicate with it  Encapsulation – Hiding a module implementation in a separate block with a formally specified interface.  Module parameter list – a set of variables within a module that hold values that are either incoming to the module or outgoing to the caller, or both.  Data flow – the direction of flow of information between the caller and a module through each parameter.

Communicating with a module  Incoming values are values received from the caller.  Outgoing values are values produced and returned to the caller.  Incoming/outgoing values are values that the caller has that the module changes. (receives and returns.)

Writing functions  void x();  This would come before main and is called the function prototype.  Must include the data types of the parameters for this function.  x();  This would be invoked where ever you needed to call your function, called function call.  void x() {  // some code.  }  This would come after main and the “some code” would be what your function did. Called the function definition.

Void Functions  Do NOT return a value to caller.  void x() { // after the name is the parameter list, this one is empty.  // Some code here…  }  “void” signals the compiler that this is not a value-returning function.  Can use inside function:  return; // returns to the caller of the function immediately.

Function Parameters  The code between parentheses that looks like a variable declaration, called parameter declaration.  void x(int y, int t) { // int y and int t would be the parameter declaration.  // some code.  }  Argument is a variable or expression listed in a call to a function.  x(height, width); // height and width would be arguments.  Parameter is a variable declared in a function definition.

Local Variables  Local variable is a variable declared within a block and not accessible outside of that block.  void x();  int main() {  int x = 5; // local variable to main.  x();  return 0;  }  void x() {  cout << x << endl; // will say x is undefined, x is local to main.  }

Parameters  Value parameter – a parameter that receives a copy of the value of the corresponding argument.  void x(int y); // y is a value parameter.  Reference parameter – a parameter that receives the location (memory address) of the caller’s argument.  void x(int& y); // y is a reference parameter.

Documentation  Use preconditions and post-conditions as comments to document function interfaces.  Pre is an assertion describing everything the function requires to be true at the moment the caller invokes the function.  Post describes the state of the program the moment the function finishes executing.  // Pre: sum has been assigned and count is greater than 0.  // Post: The average has been output on one line.  void PrintAverage(float sum, int count) {  cout << “Average is “ << sum / float(count) << endl;  }