Iteration Loops. The Do While Loop (Continued) Looping Subtasks

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Gyles Fitzgerald
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1 Iteration s The Do While (Continued) ing Subtasks This section in your book is from pages 2 to. You might think that, since this in the Do While loop section, it only refers to those kinds of loops. This is not true. You can do these types of things for all types of loops. Count controlled loops A counter-controlled loop executes a specific number of times, according to the counter. It keeps track of how many times a loop executes. The programmer can set this number for the counter within the loop or can ask the user to enter how many times to run the loop. The loop counting variable is usually set to at the start of the loop, but any value can be used. Although counting usually increments, goes up, i.e.,, 2,, etc., you can also decrement, go downwards, i.e.,, 2,. A counting iterative control statement has a variable, called the loop variable, in which the count value is maintained. This includes some means of specifying the initial and terminal values of the loop variable and the difference between sequential loop variable values, often called the step-size. The loop variable, initial and terminal value, and the stepsize are the known as the loop parameters. You will prime the loop by setting a value to a variable that will change within the loop. To prime the loop means initializing a variable that will be used as the counter. This variable will be declared and initialized before the loop body starts. You might have something like this: Dim intcount As Integer = Or like this Dim intcount As Integer = This variable (intcount) is called a counter. It counts the number of iterations and you can use that number to do something else later in your program code. The line that increments the loop variable is usually the last line of the loop body. If you do not change this value, you will end up with an infinite loop. An example of a program that would use a counter would be a registration program. Each time a student registers for a class, you will add to the count. At the end of the loop, after all of the students have registered, you will know the number of students that are in the class. Some examples of count controlled while loops. Try tracing them. Pretest and Posttest s s can be tested at the top of a loop or at the bottom of the loop. If the test is at the top of a loop, it is called a pretest loop. The condition must be True for the loop body to execute. If the condition is False, the loop body will not execute. The program will drop down to any statements below the loop. CSC The Do While (Continued) Page of 7

2 In a Posttest loop, the body of the loop is above the loop test. This means that these statements will execute first, no matter what the condition tests is. The loop body will execute and the condition is tested. If the condition is True, the loop body will execute again. If the condition is False, the loop body will not execute again. The program will drop down to any statements below the loop. Example #. This is a pretest loop. The loop test is at the top of the loop. If the condition tests is False, the loop body will not execute. Dim intcounter As Integer = counter. ' The initial values =. This is a precondition. This is the Do while intcounter <=5 lblshowresults.text &= "The number is " & intcounter & ControlChars.CrLf intcounter = intcounter + ' The step-size is. Notes: intcounter = intcounter + can also be written as intcounter +=. I used lblshowresults.text &= so that the program would print the line in the label and then append the next line into the box. Each time the loop iterates, this line will print under the one above it using the new value for intcounter. If I used the = operator, each line would overwrite the previous one and this program would only print the last line. Variable value Before loop starts. After each iteration of the body, the value is replaced with the new # intcounter is now not <=5, so the loop stops. The is the terminal value, the post condition. CSC The Do While (Continued) Page 2 of 7

3 The number is The number is The number is 2 The number is The number is 4 The number is 5 ' Notice that this loop does not print the. Example #2. Changing the step-wise value. This is another pretest loop. Dim intcounter As Integer = Do while intcounter <= lblshowresults.text &= "The number is " & intcounter & ControlChars.CrLf intcounter = intcounter + ' The step-size is. Variable value 9 2 intcounter is now not <=, so the loop stops. The 2 is the terminal value. The number is The number is The number is The number is 9 Notice that this loop does not print the 2. CSC The Do While (Continued) Page of 7

4 Example #. Using a posttest loop. The loop test is at the bottom of the loop. The loop body will execute at least once. After those statements execute, the condition is tested. If it is False, the loop body will not execute again. Dim intcounter As Integer = Do lblshowresults.text &= "The number is " & intcounter & ControlChars.CrLf intcounter += ' The step-size is. while intcounter <= Variable value intcounter is now not <=, so the loop stops. The is the terminal value. The number is ' This will print even if the loop tests False. It prints before the test. Notes: Count-controlled loops should always use whole numbers. In most cases counting can go downwards instead of upwards and step sizes other than can be used. The most common loop errors are unintended infinite loops and off-by-one errors in counting loops. Sooner or later everyone writes an unintentional infinite loop To get out of an unintended infinite loop enter ^C (control+c) s should be tested thoroughly, especially at the boundaries of the loop test, to check for off-by-one and other possible errors. Keeping a ning Total To keep a running total means that you will be summing or adding values to a variable. The variable that will be used to sum will need to be initialized to be before the loop starts. This variable is called an accumulator. Using our student program, we could have a variable named dblpayment starting at zero. As each student registers for the class, he or she pays for that class. The summing variable, dblpayment, would add each payment to itself each time the loop iterates. At the end of the loop, you will know the total amount paid for the class. CSC The Do While (Continued) Page 4 of 7

5 Example #4. I have added another variable to the program and will show the output of it. Dim intcounter As Integer = Dim intsum As Integer = ' This is the accumulator. Do while intcounter <=5 lblshowresults.text = "The counter is " & intcounter & " and the sum of the numbers is " & intsum & ControlChars.CrLf intcounter +=' intsum = intsum + intcounter ' This is the accumulator. intcounter intsum 5 Stop loop. Condition is False. The counter is and the sum of the numbers is The counter is and the sum of the numbers is The counter is 2 and the sum of the numbers is The counter is and the sum of the numbers is The counter is 4 and the sum of the numbers is The counter is 5 and the sum of the numbers is 5 CSC The Do While (Continued) Page 5 of 7

6 Fun Stuff! Example #5. This code will use the Asc method and the Chr method to convert a character to its ASCII value and back again. Dim intcodeint As Integer = Asc("F") ' A capital F has the ASCII value Of 7 Dim intnumber = 75 Do While intcodeint < intnumber lblshowresults.text &= "The character is " & Chr(intCodeInt) & ControlChars.CrLf intcodeint += intcodeint 7 intnumber Stop loop. Condition is False. The character is F The character is G The character is H The character is I The character is J CSC The Do While (Continued) Page of 7

7 Letting the User Control the All this means is that you have the user enter some number. That number will become the number of times the loop will execute. Since the user needs to enter something to have this type of loop stop, you need to have as the last line of the body of the loop a way to read the new value into the condition variable. If you do not, the loop will be infinite. Example #. This is a pretest loop. The loop test is at the top of the loop. If the condition tests is False, the loop body will not execute. Dim intmyinteger As Integer intmyinteger = CInt(txtEnterNumber.Text) Do While intmyinteger >= lblshowresults2.text &= CStr(intMyInteger) & ControlChars.CrLf intmyinteger = intmyinteger - ' You can decrease, too Assume that the user enters 5 as the number CSC The Do While (Continued) Page 7 of 7

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