Wyo C++ - Ch. 4 Notes

Objective #1: Understand the different variable types used in C++ and how they differ from constants.

• Variables are used to store values in virtually every computer program.
  
  
• Since values can either be whole numbers, decimal numbers, letters (i.e. characters), or whole words (i.e. string values), you need to first choose an appropriate data type when you use a variable. For example, if you want to store coffee in a container, you would use a styrofoam cup and not a Dixie cup. Otherwise, you would burn your hands. Likewise, if you want to store chicken soup in a container, you would use a bowl and not a plate. Otherwise, the soup would spill off of the edge of the plate.
  
  
• Most computer languages have a select number of different data types.
  
  
• You must select the proper data type for each variable that you use in a program in order to program efficiently.
  
  
  • this decreases memory (RAM) usage
  • this increases the speed of your program
    
    
• The following data types are common in C++ and will be used in this course:
  
  
  • To store whole numbers in a variable, we use a variable of the int data type.
    • An int variable uses 4 bytes of memory.
    • An int variable can store a number as low as -2,147,483,648.
    • An int variable can store a number as high as  2,147,483,647.
      (Ignore our textbook which may say that the int data type uses only 2 bytes of memory and can only store values up to 32,767. The textbook is slightly out-of-date in this regard.)
      
      
  • To store decimal numbers in a variable, we use a variable of the double data type.
    • A double variable uses 8 bytes of memory.
    • A double variable can store a number as low as -1.7 x 10³⁰⁸.
    • A double variable can store a number as high as 1.7 x 10³⁰⁸.
    • A double variable can store a number with up to 15 digits of precision (significant digits.)
      
      
  • To store a letter or a single character (such as #, $, *, etc.), we use a variable of the char data type.
    • A char variable only uses 1 byte of memory.
    • A char variable can only hold one letter, digit, or character.
      
      
  • To store a word or phrase (string value), we use a variable that is a string. Actually, we will call this an apstring later in the course. Technically string is not a data type but rather an object. We will study apstring objects in more detail in Ch. 6.
    
    
• There are other data types that are used in C++ but we will not use them too often in this course. These include unsigned char, short, unsigned int, long, and unsigned long for whole numbers and float and long double for decimal values. The data type bool is useful to store true and false values but we will simply use an int variable with either a 1 value (to represent true) or a 0 value (to represent false) if necessary in this course.

Objective #2: Be able to write declaration statements that declare variables. Also be able to write initialization statements that initialize variables.

• Variables must be declared before they are used in C++. Get into the habit of doing this at the top of your program.
  
  Examples of declaration statements with explanatory inline comments:
  

  char grade;                              // a students semester grade
  int numStudents;                     // number of students in our class
  double price;                           // price of item
  apstring userName;               // user's name

• Variable names are technically known as identifiers . You can choose your own variable names but you must be careful to use valid ones. Otherwise, the compiler will be confused and errors will result. Identifiers are really just tags for memory addresses similar to how the Web page address www.microsoft.com is a tag for an IP address. When choosing your variable names:

• do not use keywords that are defined in the programming language
• do not include spaces or other disallowed characters
• do not use more than 31 characters
• begin the identifier with a letter
  
  
• Use a conventional method of making your variables easy to read at a quick glance. For this class, we will begin variable identifiers with lowercase letters (eg. grade). However, if you wish to use more than one word within the identifier, you must capitalize the following words or parts of words (eg. semesterGrade, billGatesWealth). Please do not separate successive words with underscore characters ( _ ) as you may have seen in the textbook or other programming courses (eg. semester_grade, bill_gates_wealth).
  
  
• You should usually initialize your variables at the same time that you declare them. This is done with a declaration statement that is also an initialization statement. C++ does not automatically initialize all variables to the value 0 like some other programming languages. If you do not initialize a variable to a certain value, the variable will have an indeterminate value (also known as "garbage value"). Garbage values can corrupt the logic of your program and cause errors when you least expect.
  
  Examples:

  int numberOfPizzas = 3;          
  double monthlyCarPayment = 685;
  char letterGrade = 'A';
  apstring firstName = "Bill";
  
  Note that char values must be surrounded with single quotes and string values must be surrounded with double quotes.

• true initialization occurs within variable declaration statements only. An assignment statement that assigns the value of zero to a variable towards the top of a program technically is not considered initialization. In the following example, initialization technically does not occur:
  
  int numPizzas;
  numPizzas = 10;
  
  
• For this course, please do not initialize multiple variables in a single statement with commas even though your textbook and other C++ programmers may do so. Rather, you should use multiple statements for clarity and the preferred style for this course.
  
  Instead of
  
  int num = 0, salary = 0;
  
  or
  
  int num = 0,
      salary = 0;
  
  please use
  
  int num = 0;
  int salary = 0;
  
  
• As an exercise, write a program that prints out the indeterminant value (garbage value) of an uninitialized variable.

Objective #3: Use constants.

• Sometimes you need to use the same value many times throughout a program. In this case, it is proper to use a constant rather than a variable.
  
  
• For example, if you need to use the number 12 many times because of the fact that there are 12 items in a dozen then it would be wise and efficient to use a constant named DOZEN that is initialized to the value 12 rather than to type the number 12 itself throughout your program. Using the constant named DOZEN makes your program easier for others to understand. It also makes the program easy to upgrade down the road. For example, if the government were to change the standard that there are 13 in a dozen rather than 12, then it would be easier to update your program if you had used the constant DOZEN. I admit this is a far-fetched example but it can definitely be more efficient to use constants in such situations.
  
  
• Many programmers use the convention of having all uppercase letters in constant identifiers. Use the underscore character ( _ ) between consecutive words. This allows other programmers to be able to "pick out" your constants at a quick glance.
  
  
• Examples:
  • const double PI = 3.14159;
  • const double PA_SALES_TAX = 0.06;
  • const int SPEED_OF_LIGHT = 299792458; // commas can't be used here

Objective #4: List and explain the 5 steps of the Programming Process. (This objective is from Ch. 2 of our textbook.)

• The 5 steps of the Programming Process are:
  
  
  1. Define the problem and understand the given specifications. Create a test plan during this step.
  2. Develop an algorithm by writing pseudocode.
  3. Code the program by writing it out on paper first.
  4. Test and debug the program.
  5. Document and maintain the program.
     
     
• We will discuss and use the 5 steps of the Programming Process in our first programming assignment. You must memorize these steps however at this time.

Objective #5: Explain and appreciate the importance of using good C++ coding style.

• comments can be inline (using // ) or block ( using /*....*/ ). However, you should only use the old-fashioned C-style comments (/*...*/) when commenting out blocks of code as you debug a program. Use // comments in your programming assignments before submitting them for a grade.
• For this course, you will be required to follow our Coding Standards. Otherwise, you will lose points on assignments even if a program happens to execute successfully.
• Since it is extremely rare for a programmer to work alone, his or her code must be read by coworkers from time to time. It is important to observe common coding standards so that others can read & understand your code with minimum difficulty. Also, others will need to integrate your code into their program, so following consistent coding standards will help avoid syntax and lexical errors.