CMPSC 201

Ch. 5 Notes

Objective #1: Explain the importance of loops in programs.

• In order to write a nontrivial computer program, you almost always need to repeat sets of statements over and over again. Repetition (or iteration) structures are used to make this occur. Loops are an example of a repetition structures in most programming languages. There are three kinds of loops in C++: for loops, while loops and do/while loops.
  
  
• It is important to be aware of how many iterations a given loop is expected to perform. Each pass of the loop is an iteration. If a loop does not iterate a finite number of times, it is considered to be an infinite loop. Rarely would a programmer ever intentionally incorporate an infinite loop into a program, however it is easy to do so by mistake.

Objective #2: Use for loops.

• A for loop usually executes a specific number of iterations. Use a for loop when you know exactly how many times a set of statements must be repeated. In this situation, a for loop is called a fixed-count loop. However, there are times when a user could be allowed to input data and to use that data to control a for loop. In those situations, the for loop is a variable condition loop. As we will see, it is usually a better choice to use a while or do while loop in variable condition situations and to use a for loop in fixed-count situations.
• A for loop is often called a determinate or definite loop because the programmer knows exactly how many times it will iterate in most fixed-count situations. You can mathematically determine the number of iterations by deskchecking the logic of the loop.
• The for loop is also an example of a pretest (or entrance-controlled) loop since its control expression is tested at before its first iteration. Therefore, if the control expression evaluates to FALSE, the for loop will never iterate even once.
• After the keyword for a set of parentheses is necessary with three expressions as parameters.

  for (initializing expression; control expression; step expression)
  {
         // one or more statements
  }
  
  

  • The initializing expression (sometimes called the initializing list) sets the counter variable for the loop to its initial value.
  • The control expression ends the loop at the specified moment.
  • The step expression (sometimes called altering list) changes the counter variable,effectively determining the number of iterations. The counter variable often increments by one, but it may increment, decrement, or count in other ways.
    
    
• Example:

  for (num = 1; num <= 3; num++ )       // the counter variable num is initialized to the value 1
  {                                              // the loop will iterate while num is less than or equal to 3
         cout << num;                   // the counter variable num increments by 1 on each iteration         
         cout << endl;             
  }                                              // both statements are executed in the body of the loop on each iteration                                          

• It is possible to have variable increase by a value other than one. For example the following loop would iterate 4 times with the variable num taking on the values 1, 4, 7, and 10. The step expression adds 3 to the value of num on each iteration.
  
  for (num = 1; num <= 10; num = num + 3)
  
  It is a common error though for students to use
  
  for (num = 1; num <= 9 ; num + 3)
  
  which causes a compile error.
  
  
• It is possible to initialize the loop variable within the initializing expression. But I recommend against CMPSC 201 students initializing loop variables within the loop and may deduct points for doing so. Students are required to declare loop variables at the top of the function (e.g. main function) where they are used.

  for (int num = 1; num <= 9; num = num + 3)
  {
             cout << num << endl;
  }

• If an if statement only contains one body statement, the compiler doesn't require the use of curly braces. But I recommend against doing so for CMPSC 201 students. That is, for this course, I require students to use curly braces to always be used with for, while, and do/while loops as well as if statements.
  
  Do not use the following syntax without curly braces:

  for (num = 0; num < 10; num++)
         cout << num << endl;

• I also require that you place blank lines above and below any for, while, and do/while loops so that your loops stand out and are easier to read by fellow programmers.

Objective #3: Use while loops.

• A while loop does not necessarily iterate a specified number of times. Rather, as long as its control expression is TRUE, a while loop will continue to iterate. This type of loop is very useful in when used as a variable condition loop, particularly when the user is given the opportunity to interact with the program.
  
  
• A while loop is considered to be an indeterminate or indefinite loop because usually only at run-time can it be determined how many times it will iterate.
• Since its control expression is evaluated before its first iteration, it is considered to be a pretest loop.
  
  
• After the keyword, while, a control expression is necessary.

  while (control expression)
  {
          // one or more statements
  }
  
  The control expression must evaluate to TRUE in order for the while loop to iterate even once.
  
  

• Example:

  while (num > 100 )        // the control expression is TRUE if the num is greater than 100
  {
          cout << num;                
          num = num / 2;      // the variable num is reassigned a new value during each iteration
  }    

Objective #4: Use do while loops.

• As with a while loop, a do while loop does not necessarily iterate a specified number of times. However, it is guaranteed that a do while loop will iterate at least one time because its control expression is placed at the end of the loop. This loop is useful when you want to guarantee at least one iteration of the loop. Because the control expression is at the end of the loop, the do while is called as a posttest, exit-controlled, or bottom checking loop.
  
  
• Like a while loop, a do while loop is a better choice for variable condition situations rather than fixed-count ones. Therefore, it is often considered to be an indeterminate or indefinite loop.

• The do keyword is placed on a line of code by itself at the top of the loop. A block of statements follows it with a control expression after the keyword while at the bottom of the loop.
  
  do
  {
            // body statements would be placed here
  }while (control expression);
  
  The control expression must evaluate to TRUE in order for the do while loop to iterate after the first time.
  
  
• Example:
  
  do
  {
       cout << "Enter a number (0 to quit): ";
       cin >> num;
       sum = sum + num;
       cout << "Current sum is " << sum << '\n';
   } while (num != 0 );
  
  

• Don't forget to include the required semicolon after the control expression.
  

• Often, sentinels are used with while or do while loops. A sentinel is a reserved value that is the user enters to indicate that the loop should stop iterating. For example, you may ask the user to enter his exam scores (numbers between 0 and 100) and to enter the special value of -99 to indicate that he/she is finished entering scores. Of course, the -99 is not counted nor meant to be an actual exam score. The code might look like this where the sentinel is -99:
  
  do
  {
       cout << "Enter an exam score (-99 to quit): ";
       cin >> score;
  
       if (score != -99)
       {
             numExams++;
             total = total + score;
             cout << "Current exam average is " << total / numExams << endl;
        }
  
  }while (num != -99 );
  

Objective #5: Use the break and continue statements.

• A break statement is used to stop the execution of a loop or switch structure immediately and to continue by executing the statement that comes directly after the loop or switch structure. Only use the break statement when it is not practical to control the execution of a loop with its control expression. That is, only use a break statement when absolutely necessary.
  
  
• A continue statement is used to stop the execution of the statements in the loop's body on that particular iteration and to continue by starting the next iteration of the loop.

Objective #6: Use nested loops.

• A loop of any kind may be placed in another loop (of any kind). One must be sure though to entirely encapsulate the inner loop inside of the outer loop, otherwise an error is sure to occur. Two loops are considered to be nested loops if one is enclosed within the other.
  
  
• Example:

  for (row = 1; row <= 2; row++ )
  {
      cout << "row is " << row << endl;
  
      for (column = 1; column <= 3; column++ )
      {
                cout << "column is " << column << endl;
      }

  }

• Another example displays a multiplication table:
  
  

  	for (firstNum = 0; firstNum < 6; firstNum++)
  	{
  		
  		for (secondNum = 0; secondNum < 5; secondNum++)
  		{
  			cout << setw(2) << firstNum * secondNum << " ";
  		}
  
  		cout << endl;
  	}
  

  Observe the use of the setw manipulator to display neat columns. Also observe the use of the cout << endl; statement which is found inside of the outer loop but outside of the inner loop. This statement causes separate rows to display. Finally observe the single blank space that is displayed between values in each row.