CS 102/ECE 206 Lecture Notes: Chapter 2, Part 1
C++ Programming

American National Standards Institute

International Organization for Standardization

Preprocessor Directives

The preprocessor is a specialized text editor that is not (usually) part of the compiler. It uses a different syntax than C++.

Instructions to the preprocessor begin with # ;
they are terminated at the end of the line, but may be continued to the next line with a slash (/);
they come first, before C++ code;
they cause modification of code before the program is compiled.

Examples:

Each library has a standard header file. The #include gives a program access to a library and causes the preprocessor to insert definitions from its standard header file into a program before compilation. The directive #include<iostream> gives a program access to the I/O library, its constants, and input/output functions and objects.

The directive #define YEAR 1776 is a constant macro; YEAR is replaced everywhere it appears in the code by the value 1776 before the program is sent to the compiler. By convention, constant macros are all capital letters.

Complete C++ program:

/*********************************************** Read in 3 integers, display average input: 3 integers output: average of input integers ***********************************************/ #include <iostream> #define NUMBER 3.0 using namespace std; int main() { int x, y, z; // integers to read and add double avg; // average of three integers cout << "Please enter three integers: "; cin >> x >> y >> z; avg = (x + y + z) / NUMBER; cout << "The average is " << avg << ".\n"; return 0; // successful termination; exit function }

Input: -23 107 50 Output: The average is 44.6667.

A comment is used to document a program and to help the reader understand the code. Comments are ignored by the compiler and are not part of the object code.
```
    // This is one kind of comment, usually used for one-line comments. 
    /* This kind is usually used for multi-line comments */
    /****     So is this.     ****/
    /* We cannot /* embed */ comments */
  
```
In the program above, the first 7 lines are header block comments. The header block contains:
- WHAT the program does (NOT HOW)
- input required
- output expected
- For homework labs, include your name, your TA's name, the lab number, and the due date.
using namespace std; allows us to use cin and cout rather than std::cin and std::cout. The std namespace is where the entire Standard C++ Library is declared. The :: is the scope resolution operator. More later.
int main() is the function header for function main(). It indicates the type of value the function will return (int), the function name (main), and arguments to the function (here, there are none). Execution begins here.
{} Braces begin and end the body of the function. The body has variable declarations, which are compiler directives and executable statements. Declarations allocate storage, name it, and type it. Executable statements do the work. Note that comments and spacing matter to us but often not to g++. Punctuation and case do matter to the compiler.

Variables and Constants

A variable is a named memory location associated with a particular memory address where a value of the specified type may be stored. Thus, a variable is a symbolic name associated with a physical address. Initially, variables are undefined; that is, the value is unpredictable.
A symbolic constant is associated with a particular value; that value cannot be changed. Thus, it is a symbolic name associated with a constant value.
By convention, variable names are mostly lower case; constants are upper case. Variables and constants are typed (int, float, double, char, etc.). Each holds a single value of its type. Named entities (variables, constants, function names, classes, objects, etc.) are identifiers, and the names must follow certain rules. Use mnemonic names!

Naming Conventions

Start with a letter or underscore
Contain letters, digits, __
No blanks, commas, special characters
Cannot be more than 31 characters
Must conform to case sensitivity

Cannot be a C++ keyword

Valid:	`_OK`	`num6`	`one_time`	`oneTime`	`a724xyz_97`
Invalid:	`*bucks`	`4Aces`	`A-Z`
Note: Linux and C++ are case sensitive!

Data Types
- float, double Data Types
- int Data Type (4 bytes)
  Range of Values
- char Data Type (1 byte)
Declaring Program Variables
```
        int age;             // value of age is undefined 
        char letter; 
        int Sum, sum, SUM;   // valid but BAD FORM 
        int num = 100;       // value of num is 100, assigned at compile time 
        char flag = '4';     // character '4' does NOT equal integer 4!! 
        double rate = 0.25; 
        double cost, net_pay, average; 
        bool sorted;         // can be false (0) or true (1)
                
```
Symbolic Constants
Any value that will not change during the life of a function should be a symbolic constant or a constant macro, not a variable. The use of constants improves readability and helps to prevent errors. To use a different constant value, make the change in the declaration and have that change perpetuate throughout the program.
- At compile time, a programmer may define a symbolic name for a constant value by using the compiler directive const.
- Syntax: const <type> <symbolic name> = <value>;
- Symbolic constants cannot be changed by the program.
- By convention, constant names are uppercase.
Constant Declaration Examples:
const int CARDS = 52; // cards in the deck
const float PI = 3.14159; // math constant to five decimal places
const char CODE = 'd'; // abbreviation for Thursday
const int FALSE = 0; // expressions evaluated to zero are false

What is the meaning of each?
What happens if you omit the quotes?
Notice in #define there are no types, get-symbols, or semi-colons. This is because of the way the preprocessor replaces the constant macro name with the constant value before the program is compiled.
It is legal to use arithmetic expressions and other defined constants in #define directives.
#define PI 3.14159
#define RADIUS 100
#define AREA PI * RADIUS * RADIUS

Assignment Statements
cout Examples: cout << "I have " << number << " baseball cards.\n"; cout << "Average: " << average << " Grade: " << letter << endl; cout << "It costs $" << cost << ".\n"; cout << "Hello!"; cout << "Hello!"; cout << "\n"; // go to a new line cout << "\n\n"; // skip a line cout << "\\ \' \"\n"; // display characters \ ' " Assume these values: number: 500 letter: B average: 89.965 cost: 6.4 Output: I have 500 baseball cards. Average: 89.965 Grade: B It costs $6.4. Hello!Hello! -- blank line -- -- blank line -- \ ' " [output cursor] ______________________________________________ cin Examples: cin >> x; cin >> x >> y; cin >> code >> cost >> debt; Example: double radius; // radius of pool int length; // length of diving board cin >> radius >> length; cin >> radius >> length; cout << radius << " " << length << endl; input: 30.7 9 40 6 3.1 output? What happens to the input value 3.1? What (not how) does this code do? __________________________ Example: int x, y, z; cin >> x >> y >> z; cout << x << " " << y << " " << z << endl; cout << z << " " << x << " " << y << endl; input: 40 -8 6 output: 40 -8 6 6 40 -8 Final values: x: 40 y: -8 z: 6 What (not how) does this code do? __________________________ Example: cin >> y >> z >> x; cout << x << " " << y << " " << z << endl; input: 40 -8 6 output: 6 40 -8 Final values? x: 6 y: 40 z: -8 What (not how) does this code do? __________________________ Example: cin >> x >> y >> x; cout << x << " " << y << " " << x << endl; input: 40 -8 6 23 output: 6 -8 6 Final values? x: 6 y: -8 What (not how) does this code do? __________________________ Example: cin >> y >> x >> y; cout << x << " " << y << " " << x << " " << x << endl; input: 40 -8 6 37 output: -8 6 -8 -8 Final values? x: -8 y: 6 What (not how) does this code do?

CS 102/ECE 206 Lecture Notes: Chapter 2, Part 1C++ Programming

Preprocessor Directives

Complete C++ program:

Variables and Constants

Naming Conventions

Data Types

Declaring Program Variables

Symbolic Constants

Assignment Statements

I/O: objects cin and cout

CS 102/ECE 206 Lecture Notes: Chapter 2, Part 1
C++ Programming

`I/O: objects cin and cout`