Wyo C++ - Ch. 7 Notes

Objective #1: Understand how decisions are made in programs.

• Everything that happens inside of a computer is a result of the operation of its circuits. A circuit quite simply allows one out of two choices to be made depending on its inputs. Within all non-trivial computer programs there are many decision that in essence are like the hardware circuits inside of the computer. Practically all computer programs, when modeled with a flowcharts, demonstrate that branching occurs within their algorithms. This gives the user a more interactive experience and it allows the programs to actually solve problems.

Objective #2: Understand how true and false is represented in C++.

• When decisions are made in a computer program, they are simply the result of a computation in which the final result is either true or false. Computer programs do not have intuition or "gut feelings"; they simply make thousands of yes-or-no-like decisions at blinding speeds. The computer programmer must set up these decisions within the algorithm of a computer program to achieve the desired results.

Objective #3: Use relational operators.

• Relational operators provide the tools with which programs make decisions with true and false evaluations.
  
  Relational operators:
  

==      equal to     NOTE: this is two equals symbols next to each other, not to be confused with the assignment operator, =
>        greater than
<        less than
>=      greater than or equal to
<=      less than or equal to
!=       not equal to

Objective #4: Use logical operators.

• When complex decisions must be coded into an algorithm, it may be necessary to "chain together" a few relational expressions (that use relational operators).   This is done with logical operators (also called Boolean operators.)
  
  Logical operators:
  

  &&        is the logical AND operator
  ||            is the logical OR             operator
  !            is the logical NOT            operator
         
  

• Use these truth tables to determine the results of the logical operators
  
  

  AND				OR				NOT
  A	B	A && B		A	B	A || B		A	!A
  0	0	0		0	0	0		0	1
  0	1	0		0	1	1		1	0
  1	0	0		1	0	1
  1	1	1		1	1	1

  
  
• Logical operators may be mixed within evaluation statements but you must abide by their order of operations:   
  
  NOT operator (!), AND operator (&&), OR operator (||)
  
  You must remember that the && operation is always performed before the || operation because && is similar to multiplication in normal arithmetic while || is similar to addition.
  
  
• Short-circuit evaluation in C++ allows the compiler to quickly evaluate a logical expression in some cases without having to completely evaluate each part of the expression.
  
  
  • Example:    inRange = ( i > 0 && i < 11);    
    
    if i is not greater than zero, the program doesn't even worry about checking whether i < 11 or not (since the whole right-hand expression can never be TRUE if i <= 0 ).
    
    
• The following control expressions (as we will use within if statements below) have the resulting value of TRUE assuming that the integer variables a, b, & c have the values a = 1, b = 2, & c = 3.
  
  (a < 2)
  (a + 1 == b)
  (a <= b && b < c)
  (b < a || b < c)
  (0 > b || b > 0 && b > 1)
  (a > 0)
  (a)
  (-a)
  (a = 3)
  
  Note that in the control expression (a = 3) where the assignment operator is sometimes accidentally used instead of the "double equals" relational operator, C++ evaluates the control expression as TRUE even if the variable a is previously assigned some other value.
  
  
• The control expression (a) where the variable a was previously assigned the value 1 is TRUE since C++ considers any control expression that evaluates to a nonzero value to be TRUE. That is even if a was assigned the value -3, the control expression (a) would evaluate to TRUE.
  
  The following control expressions evaluate to TRUE where a = 1, b = 2, and c = 3:
  
  (b)
  (c + a)
  (2 * b)
  (-30 + c)
  (0 - b)
  
  While the following control expressions evaluate to FALSE where a = 1, b = 2, and c = 3:
  
  (a - 1)
  (!a)
  (0 * c)
  (c - a + b)
  
  Note that the ! symbol (the logical NOT operator) changes a TRUE to a FALSE.
  
  The following control expressions have the resulting value of FALSE assuming that the integer variables a, b, & c have the values a = 1, b = 2, & c = 3.
  
  (a > 1)
  (b == 1)
  (a > 0 && a < -9)
  (c % 3)
  (a > 0 + 4)
  

Objective #5: Use the if structure.

• Practically all computer languages have decision (aka selection) structures. In C++, the if structure is a one-way selection structure. Example,

  if (i == 3)
  {
         cout << "The value of i is 3"; 
  }

• All if structures use a control expression to determine if the code in the braces is to be executed. In the example above,
  (number == 3) is the control expression. Note the use of the "double equals" relational operator == and not the assignment operator =.
  
  
• Be sure to place the semicolon at the end of each statement within the braces, which can contain many executable statements.
  
  
• Realize that you must use the AND operator (&&) to form a compound relational expression. The the following syntax is invalid:
  
  if (0 < number < 10)
  {
        cout << number << " is greater than 0 but less than 10." << endl;
  }
  
  Rather the following syntax must be used:
  
  if (0 < number && number < 10)
  {
        cout << number << " is greater than 0 but less than 10." << endl;
  }
  
  
• According to our Coding Standards, you must use curly braces around the body of an if structure even if there is only one statement.
  
• Avoid the following common mistakes that are made with if statements:
  
  • Use the "double equals" symbol (i.e. comparison operator) rather than the assignment operator in control expressions:
    
    int num = 0:
    
    if (num == 5)
    {
       cout << "hello world" << endl;
    }
    
    which would not display "hello world" rather than:
    
    int num = 0;
    
    if (num = 5)
    {
       cout << "hello world" << endl;
    }
    
    which would assign the value 5 to the variable num and then display "hello world"
    
    
  • Avoid using an unnecessary semicolon after a control expression:
    
    if (num > 0);
    {
       cout << "num is positive" << endl;
    }
    
    which would cause the phrase "num is positive" to be displayed even if num is negative.
    
  • To be a safe programmer that uses good style, always use curly braces around the body of an if statement, even if the body of the if statement only contains one statement. In the following code segment
    
    if (num > 0)
       cout << "num is positive" << endl;
       cout << "num is not zero" << endl;
    
    the body of the if statement is only considered to be the first cout statement. The second statement would execute even if num is equal to zero. The indentation of the second statement has no effect on the way the compiler interprets this code.

Objective #6: Use the if/else structure.

• The if/else structure is considered to be a two-way selection structure, since either the block of code after the "if" part will be executed or the block of code after the "else" part will be executed.
  
  
• The "if" part is executed if the control expression is TRUE while the "else" part is executed if the "if" part is false, guaranteeing one part of the expression to be executed or the other. Do not place a control expression after the else keyword.
  
  
• Example:

  if ( i < 0)
  {
       cout << "The number is negative.";
  }
  else
  {
       cout << "The number is zero or positive.";
  }

• Don't forget to include boundary cases when you code control expressions. That is, be sure to use <= or >= instead of < or >, when necessary.
  
  
• You can use an if/else if statement as well. The else clause is optional just as it is with an if statement. Example:
  
  if (number > 10)
  {
          cout << number << " is greater than 10." << endl;
  }
  else if (number <= 9 && number >= 6)
  {
          cout << number << " is greater than 5 and less than 10." << endl;
  }
  else
  {
          cout << number << "must be less than 6." << endl;
  }

Objective #7: Use nested if structures.

• If structures and if/else structures can be nested within one another in order to model complex decision structures. Be sure to use the braces and semicolons properly when coding such structures. Also, be sure to rigorously check the logic of your algorithm since it is quite easy to overlook possible errors.
  
  
• Example:
  
  The if statement that tests for divisibility by 5 is located inside of the if statement that tests for divisibility by 3 therefore it is considered to be a nested if statement.
  
  if (number % 3 == 0)
  {
          cout << number << " is divisible by 3." << endl;
  
          if (number % 5 == 0)
          {
                  cout << number << " is divisible by 3 and 5." << endl;
          }
  }

Objective #8: Use the switch structure.

• The switch structure is a multiple-selection structure and it allows you to model even more complicated decision statements than a two-way if/else structure allows. It chooses one of the "cases" depending on the result of the control expression.
  
  
• Only integer or character data types may be used as control expressions in switch statements.
  
  
• Example:

  switch(characterEntered)
  {
        case 'A':
              cout << "You entered an A";
              break;
  
        case 'B':
              cout << "You entered a B";
              break;
  
        default:
              cout << "Illegal entry";
              break;
   }

• Example where the variable menuChoice is an int variable. Note that single quotes are NOT used around the integer values in each case.
  
  switch (menuChoice)
  {
  case 1:
       cout << "You entered menu choice #1";
       break;
  case 2:
       cout << "You entered menu choice #2";
       break;
  case 3:
       cout << "You entered menu choice #3";
       break;
  default:
       cout << "You failed to enter a valid menu choice";
       break;
  }
  
  
• The default case, if present, will result if neither of the prior cases apply. Also, note the use of the break statement, which is necessary to make sure that the program evaluates the next executable statement AFTER the switch structure. For example, in the following example, the output would be "You entered a B", even if characterEntered = 'A':

switch(characterEntered)
{
   case 'A':
   case 'B':
       cout << "You entered a B";
       break;

   default:
       cout << "Illegal entry";
       break;
}

However, in the following example, the would be no output if characterEntered = 'A':

switch(characterEntered)
{
   case 'A':
       break;
   case 'B':
       cout << "You entered a B";
       break;

   default:
       cout << "Illegal entry";
       break;
}

• The break statement must be used within each case if you do not want following cases to evaluate once one case is found. When the break statement is executed within a switch, C++ will execute the next statement outside of the switch statement. However, sometimes it is desirable not to use the break statement in a particular case. Example:
  
  switch (donationLevel)
  {
  case 1:
          cout << "You donated over $1,000, therefore you will receive a complimentary Wyomissing pocket watch." << endl;
  case 2:
          cout << "You donated over $500, therefore you can have a complimentary Wyomissing 14K gold pen & pencil set. " << endl;
  case 3:
          cout << "You donated over $250, therefore you will receive a complimentary Wyomissing necktie." << endl;
  case 4:
          cout << "You donated over $100, therefore you will receive a complimentary Wyomissing bumper sticker. " << endl;
          break;
  default:
          cout << "You are cheap and ungenerous." << endl;
          break;
  }
  
  
• You can save redundancy in your code by placing the cases next to each other as in the following example.
  
  switch (number)
  {
  case 1:
  case 3:
  case 5:
  case 7:
  case 9:
       cout << number << " is an even number." << endl;
       break;
  case 2:
  case 4:
  case 6:
  case 8:
       cout << number << " is an odd number. " << endl;
       break;
  default:
       cout << number << " is not a value between or including 1 and 9." << endl;
       break;
  }