Notes

Outline

COM1100 Fundamentals of Computer Science –Winter 2000 Lecture 6 01/20/99

Reading material this week Textbook Chapter 4 : Decision 4.1 The if statement 4.2 Relational Operator Chapter 5 : Functions (5.1 ~ 5.4) Chapter 3 : (optional) is not exactly like our graphics package Package 1 – Patterns NameUseFunctions Package 2 – Recitation 2 Package 3 – Lab 2

Tuesday’s lecture Review of previous lectures C++ programming style Flow of control if statement

Today’s lecture Functions Constant C++ source file Layout More on graphic functions

Functions The typical way of getting something done in C++ is to call (invoke) a function to do it. In addition to required main() function, C++ program can also contain any number of additional function. Purpose Make code reusable Make program understandable A function cannot be called unless it has been previously declared and implemented.

Function Declaration Function declaration is also referred as function signature (or prototype). The function signature specifies the interface between the function writer and the function user. Syntax: return_type function_name(para1, para2, … , paraN);

Function Signature --1 return_type function_name(para1, para2, … , paraN); The function signature consists of three parts: Function name: Typically something meaningful. By convention it starts with a capital (PaintCircle, GetMouse) built-in functions start with lowercase (sin, cos) Function name follows the same rule as identifier.

Function Signature --2 return_type function_name(para1, para2, … , paraN); Function return value type : Specifies what type of value function returns when it completes its task. Function return value can be used in all situations where a value of the return type can be used (assignment of the value, output, expressions).

Function Signature --3 return_type function_name(para1, para2, … , paraN); Function arguments : Specifies all values (and their types) that the caller needs to supply (value arguments), as well as variables (and their types) that will be used to return results back to the caller (reference arguments).

Function Signature -- Examples Examples: int Increment(int n); int Max(int x, int y); void DrawLine(short x1, short y1, short x2, short y2); void means nothing will be returned by the function.

Function implementation return_type function_name(para1, para2, …, paraN) { statements; } double abs(double x); // function declaration double abs(double x) { // function implementation if ( x >=0 )   return x; else return –x; }

Function implementation – return statement Syntax: return expression; Purpose: Exit a function returning the value of the expression as the function result. When the return statement is processed, the function exists immediately.

Function comments Comments are for human reader, not compilers, and there is no universal standard for the layout of a function comment. Basically, you may indicate: Purpose Receives(arguments) Returns of a function.

Function comments -- Example int Max(int x, int y); // function declaration int Max(int x, int y) /* Purpose: computer the maximum of two integers      Receives: x, y – two integers      Returns: the larger of the two inputs */ { if (x > y)  return x; else     return y; }

Using a function Once a function has been defined, it can be called by the user, provided the right number and kind of arguments is supplied. The user (caller) of a function maybe the main() function or other function (including itself -- recursive). Functions with defined return value can be used anywhere where a data value of the specified type can be used. Functions that return void are called in a standalone function call.

Using a function -- Example int Max(int x, int y); // function declaration int Max(int x, int y) { // function implementation if (x > y)    return x; else        return y; } Int main() { int m = 5, n = 9; int z = Max(m, n); // function used in an assignment cout << “The larger one is : ” << z << endl; m = RequestInt(“Enter number 1 : “); n = RequestInt(“Enter number 2 : “); // function used in an output statement cout << “The larger number is : ” << Max(m, n) << endl; return 0; }

Magic number -- 1 Circum = 2 * 3.1416 * radius; Age = 2000 – YearOfBirth; Quite frequently, literal data used within a program have a more general meaning that is recognized outside the context of the program. Numbers are normally referred by programmers as magic numbers.

Magic number -- 2 To avoid the problem of having a magic number spread throughout a program in many places and to permit clear identification of more universal constants, such as p, C++ allows the programmer to give these constant these own symbolic name. Instead of using the number throughout the program, symbolic name is used instead.

const declaration and use const float PI = 3.1416; const int currentYear = 2000; Once declared, a const variable can be used in any C++ statement in place of the number it represents. Circum = 2 * PI * radius; Age = currentYear – YearOfBirth;

C++ source file layout Header comment #include statements Constants Global variables Function declarations Function implementations main() function

Header comments /******************************* Project: COM1100 Lab 1 File: Shell.cpp Purpose: Warmup VC++ IDE Compiler: Visual C++ 6.0 Programmer: Yuhong Yin & Alice Brown Sequence: 8 (or 9) Date : 01/17/2000 ******************************/

#include statements This section lists all included header files #include <iostream.h> used for pre-defined headers <>  - Standard directory, #include "IOTools.h" #include "Graphics.h“ used for user-defined headers “ ”  - current directory & standard directory,

Constants  This section contains constants that are needed throughout the program file.  Constant names are uppercase. const int MAX_GRADE = 100; const double CLOCK_RADIUS = 5.0; const double PI = 3.14156;

Global Variables This section contains the definitions of global variables double annual_raise; //  this year’s raise for all     //  employee Every global variable must have a comment explaining its purpose

Function declarations This section lists all functions’ declarations of the program. double distance(double x1, double y1, double x2, double y2); int max(int x, int y);

Function implementations double distance(double x1, double y1, double x2, double y2) { implementation;        } int max(int x, int y) { implementation; } int main() { …. return 0; }

Picture Lab First defines a function that paints a simple square shaped picture at a given location and of a specified size. The main program then calls this function several times, testing that the function is well defined and illustrating simple user interfaces.

Picture Lab Define the function Picture to display a square picture that will fit inside the selected square. You must use at least one rectangle, one oval, one circle, and two lines and at least three different colors.

More graphic functions ClearDrawing(); ClearGraphicsWindow(); DrawLine(x1, y1, x2, y2); FrameRect(x1, y1, x2, y2); FrameOval(x1, y1, x2, y2); FrameCircle(x, y, r); ShowText(“const string”); MoveTo(x, y);

Today’s lecture Functions Constant C++ Source File Layout More on Graphic functions

Lecture next week More on functions Arguments pass by value Arguments pass by reference Flow of control Repetition – for statement Format output