LSP 121. LSP 121 Math and Tech Literacy II. Topics. Loops. Loops. Loops

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1 LSP 121 Math and Tech Literacy II Greg Brewster DePaul University Topics The FOR loop Repeats a set of statements a fixed number of times The WHILE loop Repeats a set of statements until a condition is false Strategies for Writing VB Programs Write pseudo code first, then translate to VB statements, then test How to continue after Excel error messages Commenting out statements while testing Adding MsgBox statements to check variable or cell values while testing provide a way to repeatedly execute VB statements in a subroutine (subroutine = macro). We will look at 2 types of FOR loops Repeat a set of statements a fixed number of times. DO WHILE loops Repeat a set of statements until a condition becomes false. Greg Brewster, DePaul University Page 1

2 The FOR Loop A basic FOR loop looks like this: FOR <loop-var> = 1 to <number> <statement1> <statement2> NEXT <loop-var> This causes the <statements> between FOR and NEXT to be repeated <number> times. <number> can be a constant value or a variable. The <loop-var> is a variable whose value will be equal to 1 on first loop repetition, 2 on 2 nd repetition, 3 on 3 rd repetition, etc. FOR loop example Here is a subroutine that pops up 10 MsgBox windows displaying values 1 through 10 Sub Show_Loop() FOR my_loop = 1 to 10 MsgBox Loop variable is & my_loop NEXT my_loop End Sub FOR loop example Here is a subroutine that adds together 20 die rolls and displays the total. Sub Add_Rolls() roll_total = 0 FOR roll_loop = 1 to 20 die1 = roll_die() roll_total = roll_total + die1 NEXT roll_loop MsgBox Sum of 20 die rolls is & roll_total End Sub Greg Brewster, DePaul University Page 2

3 The DO WHILE Loop The DO WHILE loop keeps executing statements while a condition is true. DO WHILE <condition> <statement1> <statement2> LOOP If <condition> is true, then <statements> are executed. If <condition> is still true, then <statements> are executed again. Repeat until <condition> is false DO WHILE loop example Here is a subroutine that pops up 10 MsgBox windows displaying values 1 through 10 Sub Show_Loop2() my_var = 0 DO WHILE (my_var < 10) my_var = my_var + 1 MsgBox Value is & my_var LOOP End Sub DO WHILE loop example This subroutine that keeps asking user to enter a value until he/she types a value between 1 and 6. Sub Input_Loop() my_var = InputBox(prompt:= Enter value from 1 to 6 ) DO WHILE ((my_var < 1) OR (my_var > 6)) MsgBox That value was not between 1 and 6. my_var = InputBox(prompt:= Enter value from 1 to 6 ) LOOP MsgBox The value you entered was: & my_var End Sub Greg Brewster, DePaul University Page 3

4 Using Cells(x,y) There are 2 ways to refer to an Excel worksheet cell within a VB subroutine. VB uses the Range keyword to refer to cells in the traditional Excel way, with a letter for the column and number for the row (like cell C5, cell D8, etc.) Range( C5 ).Value refers to value in cell C5. You can also refer to a cell by its row number and column number, using the Cells keyword, where Cells(x,y) refers to the cell in row x, column y. Cells(5, 3).Value also refers to value in cell C5 (which is in 5 th row, 3 rd column). Range names for cells Cells(x,y) names for cells Greg Brewster, DePaul University Page 4

5 Greg Brewster, DePaul University Page 5 and Cells(x,y) Combining and the Cells(x,y) naming we can create useful macros for setting values of many cells in the worksheet. and Cells(x,y) Combining and the Cells(x,y) naming we can create useful macros for setting values of many cells in the worksheet. Example: Numbering Rows This VB subroutine puts row numbers in the left column of multiple rows. Sub number_rows() Num_rows = InputBox(prompt:= How many rows? ) For loop_var = 1 to Num_rows Cells(loop_var, 1).Value = loop_var Next loop_var End sub

6 Example: Multiplication Table This VB subroutine displays a square multiplication table (nested for loops). Sub mult_table() Num_rows = InputBox(prompt:= How many rows/cols? ) For var1 = 1 to Num_rows For var2 = 1 to Num_rows Cells(var1, var2).value = var1 * var2 Next var2 Next var1 End sub Strategies for VB Strategies for Writing VB Programs Write pseudo code first, then translate to VB statements, then test How to continue after Excel error messages Commenting out statements while testing Adding MsgBox statements to check variable or cell values while testing Strategies for VB Don t just start typing in the Visual Basic Editor. Write your entire program out first in English psuedo code. Decide how many subroutines you will write and what each one will do. Each pseudo code statement should do one simple operation, like move a data value, input a value, perform a calculation, display a value. Pseudo code statements can then be grouped into sets of statements in IF or Loop constructs Greg Brewster, DePaul University Page 6

7 Strategies for VB After all details of each pseudo code statement in each subroutine are written down on paper, then start typing Visual Basic statements in the Visual Basic Editor. Type the Sub name() line first. VB will add the End Sub in for you. Then, as you type in each VB statement, test it to make sure it works before you go on to type the next one. Strategies for VB As you type in Visual Basic statements, make sure that they match exactly with the way they looked in the PPT examples. In the PPT slides, capitalized words should be included unchanged, while words in brackets ( < > ) should be taken out and replaced by your own choice. For example: FOR <var> = 1 TO <num> in the PPT slides means you must type FOR, then you put in your own variable name for <var>, then you must type = 1 TO, and then you put in your own number for <num>. Strategies for VB As you type in Visual Basic IF and Loop constructs, it is helpful to indent the statements inside these constructs to make it clear where the IF or Loop begins and ends. But this is not required. Greg Brewster, DePaul University Page 7

8 Testing VB Statements How do you test individual VB statements? First, make sure that there are no formatting errors in your statement. If your statement has format errors, the VBE will show statement in Red. Go back to PPT slide examples of the same statement to make sure that you have all punctuation marks, parentheses, etc. exactly the same as the example. VB may try to show you the first character where the error occurs. Try to see what this should be changed to make it right. Then, run the macro to see whether that one statement works correctly. You can usually add a MsgBox statement to display results and see whether statements executed correctly. Then comment out the MsgBox after you are done testing. Testing VB Statements As you are testing statements, you can comment out other statements by putting a single-quote ( ) at the beginning of all statements that you do not want to be executed. Remember, the single-quote at the start of a statement causes VB to ignore that statement because it believes it is a comment for humans only. When you want the statement to execute again, simply remove the single-quote at the start of the statement and it will execute again. You can use commenting out to separately test different parts of your subroutines without re-typing. Excel VB Errors When Excel finds problems, it pops up an Error Box with an error statement and highlights the statement that caused the problem. Sometimes the error box message may help you to find the problem. Other times it won t. But focus on the place the error occurred. After the error occurs, if your Subroutine name is highlighted in Yellow, then VB is in Debug Mode and you need to press F5 key (or Run->Continue) after fixing the error to continue execution. If you are really stuck on an error, then send me a copy of your subroutine in and I ll help. Greg Brewster, DePaul University Page 8

9 Adding MsgBox statements Adding extra MsgBox statements to your subroutine is a great way to verify the values of variables in the middle of execution and see that everything is working right. Once you are sure that everything is correct, you can remove the MsgBox Adding MsgBox statements Adding extra MsgBox statements to your subroutine is a great way to verify the values of variables in the middle of execution and see that everything is working right. Once you are sure that everything is correct, you can remove the MsgBox Excel 2003? If you do not have Excel 2007 available on your personal computer, you can still do VB programming using Excel The Record Macro, Macros and Visual Basic options are under the Tools menu in Excel 2003 rather than the Developer ribbon. Otherwise, everything works pretty much the same. Greg Brewster, DePaul University Page 9

10 Greg Brewster, DePaul University Page 10 In-Class Example Let s work through the pseudo code and VB statements for the following example: Create a subroutine to be assigned to the button shown on the next page so that clicking the Flip Coin button shows a new coin toss (H or T) and keeps track of how many heads and tails have been tossed so far. In-Class Example Heads so Far Coin Toss H Tails so Far 0 0