cs110 lec4 week2

• The Basics
• C++ syntax and semantics

  • Basic output
• C++ program structure

  • Variables and Constants
  • Identifiers
  • Assignment/Declaration
  • Memory allocation
  • Basic overview of datatypes
• Writing C++ Code

  1. You've seen a few examples of C++ code

  2. Now, we'll start to dig into what the code in the simplest examples actually mean

  3. Lots of specifics when writing code, lots of rules you must follow
• Let's Begin with Output

  1. How to print something to the screen

  2. 'cout' is the "output buffer"

  3. The insertion operator << is used with cout

  4. We put anything we want to display on the screen to the right of the insertion operator
• Example Output Code

  • cout << "Hello World";
  • Screen will display: Hello World
• Output Statements


  Syntax: cout << Expression << Expression . . . ;
• Output Statements


  • cout << "The answer is " ;
  • cout << 3 * 4 ;
  • cout << "The answer is " << 3 * 4 ;
• What does this statement do?

  cout << "Hello" << "World";

  Answer: the output is HelloWorld

  ➢ Be aware of “whitespace”
• Output manipulation


  • cout << endl; (ends the line)
  • cout << "This \"thing\" "; (\" produces a quotation mark)
• Hello World


  • #include <iostream>

  using namespace std:

  void main()

  {

  cout << "Hello World!" << endl;

  }
• Basic code boilerplate


  #include <iostream>

  using namespace std;

  void main()

  {

  // code goes here

  }
• Boilerplate


  • Some things are needed for all programs
  • In most cases these things will be provided for you, on tests etc
  • We should have an idea of why they exist and why they are needed
• Structure of a C++ program: Headers

  #include <iostream>

  This lets us use bits of code someone else wrote.

  iostream is called a header file

  iostream is part of the standard library that comes with C++

  We will be using two functions from iostream a lot:

  cin — reads input from the terminal

  cout — prints output to the terminal
• Structure of a C++ program: Namespace


  using namespace std;

  This tells the compiler to look in the standard library for the functions we call

  If we didn't do this, every time we used a function from std, such as "cout", we would have to write it as "std::cout"
• Structure of a C++ program: main function


  void main()

  {

  // code goes here

  }

  All C++ programs begin at the main function (which is a subroutine)

  { and } begin and end the code for the "main" function
• Structure of a C++ program: Comments

  // code goes here

  The highlighted text is called a "comment"

  The "//" at the beginning of the line tells the compiler to ignore everything after it until there is a new line.
• Extending the Simple C++ Program

  • #include <iostream>

  using namespace std;

  void main()

  {

  cout << "Programming is fun!" << endl;

  cout << "Fundamentals First" << endl;

  cout << "Problem Driven" << endl;

  }
• Computing with Numbers

  • #include <iostream>

  using namespace std;

  void main()

  {

  cout << "(10.5 + 2 * 3) / (45 - 3.5) = ";

  cout << (10.5 + 2 * 3) / (45 - 3.5) << endl;

  }
• Identifiers


  Identifiers are used to identify C++ data objects (variables, constants, and functions)

  Identifiers have to follow certain rules:

  • They may only include letters, digits, and underscores (_)
  • They must start with a letter or underscore — they cannot start with a digit
  • They cannot be the same as a reserved word
  • They can be any length, but some C++ compilers will place limits on their length. For compatibility, make sure identifiers are no more than 31 characters.

  Identifiers are case-sensitive.
• Identifiers

  • VALID: age_of_dog, taxRateY2K, PrintHeading, ageOfHorse

  NOT VALID: age#, 2000TaxRate Age-Of-Cat
• Choosing identifiers: 3 bad ways
• More About Identifiers

  Some C++ compilers recognize only the first 32 characters of an identifier

  If so, these identifiers are the same: age_Of_This_Old_Rhinoceros_At_My_Zoo, age_Of_This_Old_Rhinoceros_At_My_Safari

  Consider these: Age_Of_This_Old_Rhinoceros_At_My_Zoo, age_Of_This_Old_Rhinoceros_At_My_Zoo
• Identifier Style: Capitalization

  thisIsVariableStyle (start with lower-case letter, capitalize each word except first)

  ThisIsFunctionStyle (Start with upper-case letter, capitalize each word)

  NAMED_CONSTANT_STYLE (All capital letters, words separated by underscores)
• Activity: for each identifier, is it…

  Legal?

  Descriptive?

  • char a; (legal, not descriptive)

  float 200taxRate (not legal, not descriptive)

  int age; (legal, descriptive)

  int student#; (not legal, not descriptive)

  int age_of_dog; (legal, descriptive)

  int Age-Of-Cat; (not legal, descriptive)

  float payRate2000; (legal, descriptive)

  string Name; (legal, descriptive)

  const int X=2; (legal, not descriptive)
• Variables

  A variable is a location in memory which we can refer to by an identifier, and in which a data value that can be changed is stored.

  Declaring a variable means specifying both its name and its data type.
• Variables vs. Identifiers

  An identifier is the name we give to a variable.

  A variable is the actual location in memory where the data is stored.
• Declaration, Initialization, & Assignment

  Declaration: Telling the computer, "Hey computer, this thing exists. Please reserve some memory for it, and call it by this identifier."

  Assignment: When you assign a value to a variable you are putting a value into the variable.

  Initialization: When you assign a value to a variable for the first time.
• Declaration, Initialization, & Assignment

  • int foo; // declaring an integer with identifier foo

  foo = 5; //assigning the value 5 to "foo"

  int foo = 5; // declares and initializes in a single statement
• What Does a Variable Declaration Do?

  A variable declaration tells the compiler to allocate enough memory to hold a value of this data type, and to associate the identifier with this location.
• Data Types

  A Data Type is a specific set of data values along with a set of operations on those values.

  e.g. integer: values = positive and negative whole numbers (within a limit), operations = standard math operations
• C++ Data Types

  • Integral Types (int, short, long)

  Floating Types (float, double, long double)

  Character Types (char)
• Variables vs. Constants

  Variables: The value can be changed

  Constants: The value can't be changed, and must be initialized when declared

  const datatype IDENTIFIER = value;
• Why use constants?

  • Ensures the value won't be changed by human error

  Compilers can optimize and speed up the program if data is stored in constants rather than variables