Loops

1. Loops ISYS 350

2. Two Types of Loops • while loop • for loop

3. The while Loop • The while loop causes a statement or set of statements to repeat as long as a Boolean expression is true • The simple logic is: While a Boolean expression is true, do some task • A while loop has two parts: • A Boolean expression that is tested for a true or false value • A statement or set of statements that is repeated a long as the Boolean expression is true Boolean Expression True Statement(s) False

4. Structure of a while Loop • In C#, the generic format of a while loop is: while (BooleanExpression) { Statements; } • The first line is called the while clause • Statements inside the curly braces are the body of the loop • When a while loop executes, the Boolean expression is tested. If true, the statements are executed • Each time the loop executes its statement or statements, we say the loop is iterating, or performing an iteration

5. Example: An Infinite Loop while (1 > 0) { MessageBox.Show("Looping"); }

6. An Infinite Loop that plays beep sound To play beep, we need System.Media namespace: using System.Media; while (1 > 0) // while (true) { SystemSounds.Beep.Play(); }

7. Using a Counter to Control the Loop int counter = 0; while (counter<5) { MessageBox.Show(counter.ToString()); ++counter; //counter++; } int counter = 1; while (counter<=5) { MessageBox.Show(counter.ToString()); ++counter; //counter++; }

8. Using a Flag Boolean continueFlag = true; while (continueFlag) { SystemSounds.Beep.Play(); if (MessageBox.Show("Do you want to continuue? ", "Continue?", MessageBoxButtons.YesNo) == DialogResult.No) continueFlag = false; }

9. Flag Example bool myFlag = true; int counter2 = 1; while (myFlag) { MessageBox.Show(counter2.ToString()); ++counter2; if (counter2 > 5) {myFlag = false;} }

10. Accumulator Find the sum of all numbers between 1 and N. int N, Sum, Counter = 1; N = int.Parse(textBox1.Text); Sum = 0; while (Counter <= N) { Sum += Counter; ++Counter; } MessageBox.Show(Sum.ToString());

11. While Loop ExampleN Factorial, N! int N, NFact, Counter = 1; N = int.Parse(textBox1.Text); NFact = 1; while (Counter <= N) { NFact *= Counter; ++Counter; } MessageBox.Show(NFact.ToString());

12. Enter present value and rate in textboxes, then display future values for years from 5 to 40 with a step of 5 in a ListBox double pv, rate, year, fv; pv = double.Parse(textBox1.Text); rate = double.Parse(textBox2.Text); year = 5; while (year <= 40) { fv = pv * Math.Pow(1 + rate, year); listBox1.Items.Add("Year = " + year.ToString() + " Future valuet is: " + fv.ToString("c")); year += 5; }

13. Enter present value and rate in textboxes, then display future values with user specified starting year, ending year and step value in a ListBox

14. Code Example double pv, rate, year, startYr, endYr,stepValue,fv; pv = double.Parse(textBox1.Text); rate = double.Parse(textBox2.Text); startYr = double.Parse(textBox3.Text); endYr = double.Parse(textBox4.Text); stepValue = double.Parse(textBox5.Text); year = startYr; while (year <= endYr) { fv = pv * Math.Pow(1 + rate, year); listBox1.Items.Add("Year = " + year.ToString() + " Future valuet is: " + fv.ToString("c")); year += stepValue; }

15. Enter pv and year in textboxes, then display fv for rates from 5% to 10% with a step of 0.5% in a ListBox double pv, rate, year, fv; pv = double.Parse(textBox1.Text); year = double.Parse(textBox2.Text); rate = 0.05; while (rate <= .1) { fv = pv * Math.Pow(1 + rate, year); listBox1.Items.Add("Rate = " + rate.ToString("p") + " Future valuet is: " + fv.ToString("c")); rate += .005; } Note: A double may not exactly equal to .1 while (rate < .105)

16. The for Loop • The for loop is specially designed for situations requiring a counter variable to control the number of times that a loop iterates • You must specify three actions: • Initialization: a one-time expression that defines the initial value of the counter • Test: A Boolean expression to be tested. If true, the loop iterates. • Update: increase or decrease the value of the counter • A generic form is: for (initializationExpress; testExpression; updateExpression) { } • The for loop is a pretest loop

17. Sample Code int count; for (count = 1; count <= 5; count++) { MessageBox.Show(“Hello”); } • The initialization expression assign 1 to the count variable • The expression count <=5 is tested. If true, continue to display the message. • The update expression add 1 to the count variable • Start the loop over // declare count variable in initialization expression for (int count = 1; count <= 5; count++) { MessageBox.Show(“Hello”); }

18. Other Forms of Update Expression • In the update expression, the counter variable is typically incremented by 1. But, this is not a requirement. //increment by 10 for (int count = 0; count <=100; count += 10) { MessageBox.Show(count.ToString());} • You can decrement the counter variable to make it count backward //counting backward for (int count = 10; count >=0; count--) { MessageBox.Show(count.ToString());}

19. Accumulator Find the sum of all numbers between 1 and N. int N, Sum, Counter; N = int.Parse(textBox1.Text); Sum = 0; for (Counter = 1; Counter <= N;Counter++ ) { Sum += Counter; } MessageBox.Show(Sum.ToString());

20. Enter present value and rate in textboxes, then display future values for years from 5 to 40 with a step of 5 in a ListBox double pv, rate, year, fv; pv = double.Parse(textBox1.Text); rate = double.Parse(textBox2.Text); for (year=5;year<=40;year+=5) { fv = pv * Math.Pow(1 + rate, year); listBox1.Items.Add("Year = " + year.ToString() + " Future valuet is: " + fv.ToString("c")); }

21. Increment smaller than 1:Enter Loan and Term in textboxes, then display monthly pay for rates from 3% to 10% with a step of .5% in a ListBox double pv, rate, year, fv; pv = double.Parse(textBox1.Text); year = double.Parse(textBox2.Text); for (rate=.03;rate<=.1;rate+=0.005) { fv = pv * Math.Pow(1 + rate, year); listBox1.Items.Add("Rate = " + rate.ToString("p") + " Future valuet is: " + fv.ToString("c")); } Note: for (rate = .03; rate < .105; rate += .005)

22. Find the Sum of All Even Numbers between 1 and N int N, Sum, Counter; N = int.Parse(textBox1.Text); Sum = 0; for (Counter = 1; Counter<=N;Counter++ ) { if (Counter % 2 ==0) Sum += Counter; } MessageBox.Show(Sum.ToString());

23. Search a String string myString = textBox1.Text, searchChar = textBox2.Text; int index=0, counter = 0; Boolean found = false; for (index=0;index<myString.Length;index++) { if (myString.Substring(index, 1) == searchChar) { ++counter; found = true; } } if (found) { textBox3.Text = "Found " + counter.ToString(); } else { textBox3.Text = "Not found!"; }

24. Flag Example bool myFlagTrue = true; int counter = 1; for (counter=1;myFlagTrue;counter++) { MessageBox.Show(counter.ToString()); if(counter>=5) { myFlagTrue = false; } }

25. The loop control variable may be declared before the loop or within the for statement Int counter; for (counter = 0; counter <=5; counter++) { MessageBox.Show(counter.ToString());} What is the value of counter after the loop? • Or for (int counter= 0; counter <=5; counter++) { MessageBox.Show(counter.ToString());}

26. Search a String • Useful property and method: • Length: number of characters in a string • Substring(starting index, number of character) • IndexOf()

27. Code Example string myString = textBox1.Text, searchChar = textBox2.Text; int index=0, counter = 0; Boolean found = false; while (index<myString.Length) // while (index<=myString.Length-1) { if (myString.Substring(index,1)==searchChar) { ++counter; found = true; } ++index; } if (found) { textBox3.Text= "Found " + counter.ToString(); } else { textBox3.Text = "Not found!"; }