CS 106, Winter 2009 Class 14, Section 4

1. CS 106, Winter 2009Class 14, Section 4 Slides by: Dr. Cynthia A. Brown, cbrown@cs.pdx.edu Instructor section 4: Dr. Herbert G. Mayer, herb@cs.pdx.edu 1

2. Assignment 5 • In Assignment 5 you will practice with procedures and functions • The assignment statement and examples are on the website. If you compare the code for Ice Cream Store version 2 and 3, you will be changing the Plane Ticket application in the same way, so that the end result does the same thing as the original but uses general procedures and functions.

3. Chapter 6: Repetition • Repetition of the same or similar actions is often needed in process design • Three styles of repetition: • Repeat for a fixed number of times • Repeat as long as (while) a certain condition is true • Repeat until a certain condition is true

4. Examples • Keep ringing up items as long as (while) the customer has more • Keep adding numbers until you get to the end of the list • Cut a paycheck for each employee in the roster

5. Printing a Multiplication Table • Our job: input a number between 1 and 12 in a text box • Print a multiplication table for this number in a list box • This is possible but painful with our current set of tools

6. Possible code… num = Cint(txtInput.Text) strNum = txtInput.Text lstMult.Items.Clear() lstMult.Items.Add(strNum&“ x 1 = “ &CStr(num * 1) ) lstMult.Items.Add(strNum&“ x 2 = “ &CStr(num * 2) ) lstMult.Items.Add(strNum&“ x 3 = “ &CStr(num * 3) ) … lstMult.Items.Add(strNum&“ x 12 = “ &CStr(num * 12) )

7. Ugh! • This is clumsy and unbearably repetitive • If we wanted to change the upper limit in some way (say do up to 8 *8, or 10*10, instead of going to 12 each time), we would need even uglier code • We couldn’t even do some very natural tasks • Luckily, VB has some nice repetition constructs

8. For loop • Repetitions are called loops in VB • A For loop is used when we can determine the number of repetitions before starting the loop num = Cint(txtInput.Text) strNum = txtInput.Text lstMult.Items.Clear() Forj = I To 12 lstMult.Items.Add(strNum&“ x “ &CStr(j) &“ = “ & _ CStr(num * j)) Next

9. For loop version 2 • Let’s modify the previous example so it prints a multiplication table up to N * N, instead of going to 12. num = Cint(txtInput.Text) strNum = txtInput.Text lstMult.Items.Clear() Forj = I To num ‘the limits can be variables or expressions lstMult.Items.Add(strNum&“ x “ &CStr(j) &“ = “ & _ CStr(num * j)) Next

10. For Loop Syntax cVar = sVal ForcVar = sValToeVal statements Next Here cVar is the control variable, sVal is the start value for cVar, and eVal is the end value for cVar yes cVar>eVal? no Execute loop statements Increment cVar

11. Let’s step through this with an example…

12. The Do-While Concept • Sometimes we can’t tell in advance how many times a loop will need to execute • Or, it might just be clearer to use a logic that checks as we go along • In that case we can use a Do loop instead of a For loop

13. Do-While Example • Let’s try the same program we did with the For loop, but this time with a Do-While loop num = Cint(txtInput.Text) strNum = txtInput.Text lstMult.Items.Clear() j = 1 Do While j<= 12 lstMult.Items.Add(strNum&“ x “ &CStr(j) &“ = “ & _ CStr(num * j)) j = j + 1 ‘what would happen if I forgot this line? Loop

14. Do-While Example • Here’s the n by n version num = Cint(txtInput.Text) strNum = txtInput.Text lstMult.Items.Clear() j = 1 Do While j<= num ‘note the upper bound is now a variable lstMult.Items.Add(strNum&“ x “ &CStr(j) &“ = “ & _ CStr(num * j)) j = j + 1 Loop • What happens if num = 0?

15. Do Loop Syntax (While) No Do While condition statement(s) Loop (Loop statements may never be done.) Condition true? Yes Execute statements within the loop. Execute statements that follow the loop.

16. The Do-Until Variation • Instead of testing at the beginning of the loop, we can test at the end. • This is useful because, many times, we want to do the loop code at least once, no matter what. • We’ll mostly focus on the Do-While and For loops.

17. Do-Until Example • Here’s the multiplication example using a Do-until loop num = Cint(txtInput.Text) strNum = txtInput.Text lstMult.Items.Clear() j = 1 Do lstMult.Items.Add(strNum&“ x “ &CStr(j) &“ = “ & _ CStr(num * j)) j = j + 1 Loop Untilj> 12

18. Do Loop Syntax (Until) Do statement(s) Loop Until condition (Loop statements are always done at least once.) No Execute statements within the loop. Condition true? Yes Execute statements that follow the loop.

19. BREAK 10 minutes

20. Nested Loops • Remember how if statements can be “nested”: you can have an if inside another if • We can also put whatever we want inside a loop, including another loop • If we do this, the inner loop is executed completely, for all of its repetitions, each time the outer loop is executed once.

21. Example: Complete Multiplication Table Dimj,kAs Integer DimtableLineAs String Forj= 1 To 12 tableLine = “” Fork= 1 To 12 tableLine = tableLine&CStr(j) &“ x “ _ &CStr(k) &“ = “ &CStr(j*k) &“ “ Next‘k lstMult.Items.Add(tableLine) Next‘j