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C++ Basics (Part 1)

Structure of a C++ Program

• Sequence of statements, typically grouped into functions
  • function: a subprogram. a section of a program performing a specific task
  • Every function body is defined inside a block (see below)
• For a C++ executable, exactly one function called main()
• Can consist of multiple files and typically use libraries
• Statement -- smallest complete executable unit of a program
  • Declaration statement
  • Execution statement
  • Compound statement -- any set of statements enclosed in set braces { } (often called a block)
  • Simple C++ statments end with a semi-colon. (A block does not typically need a semi-colon after it, except in special circumstances).
• Library
  • usually pre-compiled code available to the programmer to perform common tasks
  • Compilers come with many libraries. Some are standard for all compilers, and some may be system specific
  • Two parts
    • Interface: header file, which contains names and declarations of items available for use
    • Implementation: pre-compiled definitions, or implementation code. In a separate file, location known to compiler
  • Use the #include directive to make a library part of a program (satisfies declare-before-use rule)

Building and Running a C++ Program

Starts with source code, like the first sample program

1. Pre-processing
   • The #include directive is an example of a pre-processor directive (anything starting with #).
   • #include <iostream> tells the preprocessor to copy the standard I/O stream library header file into the program
2. Compiling
   • Syntax checking, translation of source code into object code (i.e. machine language). Not yet an executable program.
3. Linking
   • Puts together any object code files that make up a program, as well as attaching pre-compiled library implementation code (like the standard I/O library implementation, in this example)
   • End result is a final target -- like an executable program
4. Run it!

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Variables

Variables are used to store data. Every C++ variable has a:

• Name -- chosen by the programmer (aka identifier)
• Type -- specified in the declaration of the variable
• Size -- determined by the type
• Value -- the data stored in the variable's memory location
• Address -- the starting location in memory where the value is stored

When a program is compiled, a symbol table is created, mapping each variable name to its type, size, and address

Identifiers

Identifiers are the names for things (variables, functions, etc) in the language. Some identifiers are built-in, and others can be created by the programmer.

• User-defined identifiers can consist of letters, digits, and underscores
• Must start with a non-digit
• Identifiers are case sensitive (count and Count are different variables)
• Reserved words (keywords) cannot be used as identifiers
  • A list of reserved keywords in C++ can be found on page 43 in the textbook

Style-conventions (for indentifiers)

• Don't re-use common identifiers from standard libraries (like cout, cin)
• Start names with a letter, not an underscore. System identifiers and symbols in preprocessor directives often start with the underscore.
• Pick meaningful identifiers -- self-documenting

    numStudents, firstName                // good
    a, ns, fn                             // bad
  

• a couple common conventions for multiple word identifiers
  1. numberOfMathStudents
  2. number_of_math_students

Declaring Variables

• Declare Before Use: Variables must be declared before they can be used in any other statements
• Declaration format: typeName variableName1, variableName2, ...;

   int numStudents;       // variable of type integer
   double weight;         // variable of type double
   char letter;           // variable of type character
  
   // Examples of multiple variables of the same type in single
   //  declaration statements
  
   int test1, test2, finalExam;
   double average, gpa;
  

Atomic Data Types

Atomic data types are the built-in types defined by the C++ language

• bool - has two possible values, true or false
• integer types
  • char - 1 byte on most systems
    • typically used for representing characters
    • stored with an integer code underneath (ASCII on most computers today)
  • short - (usually at least 2 bytes)
  • int - (4 bytes on most systems)
  • long - (usually 4 or more bytes)
  • The integer types have regular and unsigned versions
    • The keyword unsigned used as a type by itself is a shortcut for unsigned int
• floating point types - for storage of decimal numbers (i.e. a fractional part after the decimal)
  • float
  • double
  • long double

Sizes

• Sizes for these types are implementation dependent
• numerical types above listed in smallest to largest order
• can use built-in sizeof() function to find out sizes of types or variables for a system
• For most computers today: char typically 1 byte; int typically 4 bytes

This program will print the number of bytes used by each type - sizes may vary from system to system
 

Initializing Variables

• To declare a variable is to tell the compiler it exists, and to reserve memory for it
• To initialize a variable is to load a value into it for the first time
• If a variable has not been initialized, it contains whatever bits are already in memory at the variable's location (i.e. a garbage value)
• One common way to initialize variables is with an assignment statement. Examples:

   int numStudents;
   double weight;
   char letter;
  
   numStudents = 10;
   weight = 160.35;
   letter = 'A'; 
  

• Variables of built-in types can be declared and initialized on the same line, as well

   int numStudents = 10;
   double weight = 160.35;
   char letter = 'A';
  
   int test1 = 96, test2 = 83, finalExam = 91;
   double x = 1.2, y = 2.4, z = 12.9;
  

• An alternate form of initializing and declaring at once:

   int numStudents(10);		// these are equivalent to the ones above
   double weight(160.35);
   char letter('A');
  
   int test1(96), test2(83), finalExam(91);
   double x(1.2), y(2.4), z(12.9);
  

More Declaration Examples, using various types
 

Constant Variables

(Woohoo! An oxymoron!)

• A variable can be declared to be constant. This means it cannot change once it's declared and initialized
• Use the keyword const
• MUST declare and initialize on the same line

   const int SIZE = 10;
   const double PI = 3.1415;
  
   // this one is illegal, because it's not
   //  initialized on the same line
   const int LIMIT;			// BAD!!!
   LIMIT = 20;		
  

• A common convention is to name constants with all-caps (not required)

Symbolic Constants (an alternative)

• A symbolic constant is created with a preprocessor directive, #define. (This directive is also used to create macros).
• Examples:

    #define PI 3.14159
    #define DOLLAR '$'
    #define MAXSTUDENTS 100
  

• The preprocessor replaces all occurrences of the symbol in code with the value following it. (like find/replace in MS Word). This happens before the actual compilation stage begins

Questions to consider

• What's the difference between these?

    #define SIZE 10
    const int SIZE = 10;
  

• Why use a constant variable (or symbolic constant)? i.e. what's the advantage?
• When is use of a constant a good idea? When is it not worthwhile?

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Other Basic Code Elements

Literals

Literals are also constants. They are literal values written in code.

• integer literal -- an actual integer number written in code (4, -10, 18)
  • If an integer literal is written with a leading 0, it's interpreted as an octal value (base 8).
  • If an integer literal is written with a leading 0x, it's interpreted as a hexadecimal value (base 16)
  • Example:

       int x = 26;			// integer value 26
       int y = 032;			// octal 32 = decimal value 26
       int z = 0x1A;		// hex 1A = decimal value 26
     

• floating point literal -- an actual decimal number written in code (4.5, -12.9, 5.0)
  • Note: these are interpreted as type double by standard C++ compilers
  • Can also be written in exponential (scientific) notation: (3.12e5, 1.23e-10)
• character literal -- a character in single quotes: ('F', 'a', '\n')
• string literal -- a string in double quotes: ("Hello", "Bye", "Wow!\n")

Escape Sequences

• String and character literals can contain special escape sequences
• They represent single characters that cannot be represented with a single character from the keyboard in your code
• The backslash \ is the indicator of an escape sequence. The backslash and the next character are together considered ONE item (one char)
• Some common escape sequences are listed in the table below
   

  Escape Sequence	Meaning
  \n	newline
  \t	tab
  \r	carriage return
  \a	alert sound
  \"	double quote
  \'	single quote
  \\	backslash

 

Comments

Comments are for documenting programs. They are ignored by the compiler.

• Block style (like C)

      /* This is a comment.
         It can span multiple lines  */ 
  

• Line comments -- use the double-slash //

      int x;    // This is a comment 
      x = 3;    // This is a comment 
  

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Input and Output with streams in C++:

In C++ we use do I/O with "stream objects", which are tied to various input/output devices.

Some pre-defined stream objects

These stream objects are defined in the iostream library

• cout -- standard output stream
  • Of class type ostream (to be discussed later)
  • Usually defaults to the monitor
• cin -- standard input stream
  • Of class type istream (to be discussed later)
  • Usually defaults to the keyboard
• cerr -- standard error stream
  • Of class type ostream
  • Usually defaults to the monitor, but allows error messages to be directed elsewhere (like a log file) than normal output

To use these streams, we need to include the iostream library into our programs.

 #include <iostream>
 using namespace std;

The using statement tells the compiler that all uses of these names (cout, cin, etc) will come from the "standard" namespace.

Using Input and Output Streams

• output streams are frequently used with the insertion operator <<
  • Format: outputStreamDestination << itemToBePrinted
  • The right side of the insertion operator can be a variable, a constant, a value, or the result of a computation or operation
  • Examples:

     cout << "Hello World";		// print a string literal
     cout << 'a';			// print a character literal
     cout << numStudents;		// print contents of a variable
     cout << x + y - z;		// print result of a computation
     cerr << "Error occurred";	// string literal printed to standard error
     

  • When printing multiple items, the insertion operator can be "cascaded". Place another operator after an output item to insert a new output item:

     cout << "Average = " << avg << '\n';
     cout << var1 << '\t' << var2 << '\t' << var3;
     

• input streams are frequently used with the extraction operator >>
  • Format: inputStreamSource >> locationToStoreData
  • The right side of the extraction operator MUST be a memory location. For now, this means a single variable!
  • By default, all built-in versions of the extraction operator will ignore any leading "white-space" characters (spaces, tabs, newlines, etc)
  • Examples:

     int numStudents;
     cin >> numStudents;	// read an integer
    
     double weight;
     cin >> weight;		// read a double
    
     cin >> '\n';		// ILLEGAL.  Right side must be a variable
     cin >> x + y;		// ILLEGAL.  x + y is a computation, not a variable
     

  • The extraction operator can be cascaded, as well:

     int x, y;
     double a;
     
     cin >> x >> y >> a;	// read two integers and a double from input
     

Some special formatting for decimal numbers

• By default, decimal (floating-point) numbers will usually print out only as far as needed, up to a certain preset number of decimal places (before rounding the printed result)

    double x = 4.5, y = 12.666666666666, z = 5.0;
  
    cout << x;		// will likely print 4.5
    cout << y;		// will likely print 12.6667
    cout << z;		// will likely print 5
  

• A special "magic formula" for controlling how many decimal places are printed:

    cout.setf(ios::fixed);	// specifies fixed point notation
    cout.setf(ios::showpoint);	// so that decimal point will always be shown
    cout.precision(2);		// sets floating point types to print to
  				//  2 decimal places (or use your desired number)
  

  Any cout statements following these will output floating-point values in the usual notation, to 2 decimal places.

    double x = 4.5, y = 12.666666666666, z = 5.0;
  
    cout << x;		// prints 4.50
    cout << y;		// prints 12.67
    cout << z;		// prints 5.00
  

• Here's an alternate way to set the "fixed" and "showpoint" flags

    cout << fixed;        // uses the "fixed" stream manipulator
    cout << showpoint;    // uses the "showpoint" stream manipulator
  

  These statements use what are called stream manipulators, which are symbols defined in the iostream library as shortcuts for setting those particular formatting flags
   
• An alternative to fixed point notation is scientific notation:

    cout.setf(ios::scientific);	// float types formatted in exponential notation
  

Some simple program examples

• Hello World
• Hello World 2
• Hello World 3 - this is how it would be written if the using statement was left out
• A sample program with cout and cin
• Another version of the above sample program
• Example illustrating I/O and formatting of floating-point values in fixed point notation
• Example illustrating formatting of floating-point values in scientific notation