Counter Controlled Repetition Structures

Today we will discuss some Counter controlled repetition structures.

Counter-controlled repetition requires

• Name of control variable/loop counter
•  Initial value of control variable
• Condition to test for final value of control variable
• Increment/decrement operator to modify control variable when looping

For Repetition Structure:

General format when using for loops is:

                              for ( initialization; LoopContinuationTest;      increment )

                               statement

         

         note that there is no semicolon after the increment statement

       For example:

                           

                               for( int counter = 1; counter <= 10; counter++ )

                              cout << counter << endl;

     

     the above statement prints integers from one to ten

     

for loops can usually be rewritten as while loops

                                         initialization;

                 while ( loopContinuationTest){ 

                 statement

                 increment;}

  

     For multiple variables, use comma-separated lists

                                for (int i = 0, j = 0;  j + i <= 10; j++, i++)

             cout << j + i << endl;

General format when using for loop is:

Consider a program to calculate compound interest:

A person invests $1000.00 in a savings account yielding 5 percent interest. Assuming that all interest is left on deposit in the account, calculate and print the amount of money in the account at the end of each year for 10 years. Use the following formula for determining these amounts:

  a = p(1+r)

where p is the original amount invested (i.e., the principal),

          r is the annual interest rate,

         n is the number of years and

         a is the amount on deposit at the end of the nth year

While Repetition Structure:

In this repetition structure action is repeated while some condition remains true.

Pseudo-code

                          while there are more items on my shopping list

                          Purchase next item and cross it off my list

while loop repeated until condition becomes false, for example;

           int product = 2;

           while ( product <= 1000 )

           product = product * 2;

A flowchart for while structure is:

The code example of while loop is:

Formulating Algorithms (Counter-Controlled Repetition):

Counter-controlled repetition is a ‘Definite Repetition’, each loop is repeated until the counter reaches a certain value, the number of repetitions is known;

for example;

 A class of ten students took a quiz. The grades (integers in the range 0 to 100) for this quiz are available to you. Determine the class average on the quiz. 

Pseudocode example:

Set total to zero

Set grade counter to one

While grade counter is less than or equal to ten
  Input the next grade
  Add the grade into the total
  Add one to the grade counter

Set the class average to the total divided by ten
Print the class average

C++ code for this example is:

Formulating Algorithms (Sentinel-Controlled Repetition):

Sentinel controlled repetition is an ‘Indefinite Repetition’, Suppose problem becomes:

         

         Develop a class-averaging program that will process an arbitrary number of grades each time the program is run

        In this case:

•  Unknown number of students
•  How will program know when to end?

A sentinel value:

•  Indicates “end of data entry”
• Loop ends when sentinel value is input
• Sentinel chosen so it cannot be confused with regular input
• -1 in this case

A code for this problem is shown below:

Nested Control Structures:

Problem statement:

  A college has a list of test results (1 = pass, 2 = fail) for 10 students.  Write a program that analyzes the results.  If more than 8 students pass, print "Raise Tuition".

Notice that:

Program processes 10 results ie Fixed number, use counter-controlled loop

Two counters can be used, one counts number that passed and another counts number that fail

Each test result is 1 or 2, so if not 1, assume 2

Top level outline:  

      Analyze exam results and decide if tuition should be raised

First refinement

              Initialize variables

              Input the ten quiz grades and count passes and failures

              Print a summary of the exam results and decide if tuition should be raised

Further Refinement:

Initialize variables

to

Initialize passes to zero

Initialize failures to zero

Initialize student counter to one 

                  
                 Input the ten quiz grades and count passes and failures

         to

         While student counter is less than or equal to ten
   Input the next exam result

         If the student passed

        Add one to passes
 Else
 Add one to failures

       Add one to student counter 

              Print a summary of the exam results and decide if tuition should be raised

         to

       Print the number of passes

       Print the number of failures

       If more than eight students passed
 Print “Raise tuition”