Advanced Excel Topics – Loops 259 Lecture 11 Spring 2017 Advanced Excel Topics – Loops

Topics Loops Zip() Unzip() FOR…NEXT DO…WHILE DO…UNTIL “Infinite” Worksheet Functions Unzip()

Loops Loops are used to repeat portions of VBA code over and over again. Loops can be thought of as a type of “Conditional Statement”. Most loops can be classified under the following four categories: For Next Do While Do Until Infinite

FOR…NEXT Loop Used when the exact number of cycles can be pre-determined.

FOR…NEXT Loop Syntax: For counter = start To end 'Do something here Next counter counter is a variable that identifies which step of the loop we are on. start is the starting value of the counter. end is the ending value of the counter.

FOR…NEXT Loop Example 1: Roll a die 12 times keeping track of the total. Total = 0 For N = 1 To 12 Roll = Int(Rnd()*6)+1 Total = Total + Roll Next N

FOR…NEXT Loop Example 2: Compute the sum of the first 15 positive even integers. Two possible ways to do this in VBA! Total = 0 For N = 2 To 30 Step 2 Total = Total + N Next N Total = 0 For N = 1 To 15 Total = Total + 2*N Next N

DO…WHILE Loop Used to continue looping while a condition is TRUE . The condition is checked at the beginning of each cycle of the loop. This type of loop can be used when the exact number of cycles cannot be pre-determined. It is possible for this type of loop to perform zero cycles.

DO…WHILE Loop Syntax: Do While (Expression) ‘Code to Execute Loop (Expression) is any logical expression that evaluates to TRUE or FALSE.

DO…WHILE Loop Example 3: Keep rolling a die while the total is less than 200, keeping track of the number of rolls required to accomplish this task. Show in flowchart form. Num_Rolls = 0 Total = 0 Do While (Total < 200) Roll = Int(Rnd()*6)+1 Total = Total + Roll Num_Rolls = Num_Rolls + 1 Loop

DO…UNTIL Loop Used to continue looping until a condition is TRUE. The condition is checked at the end of each cycle of the loop. This type of loop can also be used when the exact number of cycles cannot be pre-determined. Since the condition is checked at the end of the cycle, this type of loop will always perform at least one cycle.

DO…UNTIL Loop Syntax: Do ‘Code to Execute Loop Until (Expression) (Expression) is any logical expression that evaluates to TRUE or FALSE.

DO…UNTIL Loop Example 4: Keep rolling a die until the total exceeds 300, keeping track of the number of rolls required to accomplish this task. Show in flowchart form. Num_Rolls = 0 Total = 0 Do Roll = Int(Rnd()*6)+1 Total = Total + Roll Num_Rolls = Num_Rolls + 1 Loop Until (Total>300)

“Infinite” Loop Usually created by “ACCIDENT” but sometimes created on purpose. Infinite loops can be dangerous. To exit from an “infinite loop”, keep pressing the ESC key and “hope” that you saved the work that you have done. Also try pressing CTRL+BrEAK to stop an infinite loop. The “tighter” the loop is, the harder it is to recover from. A “tight loop” heavily uses I/O or processing resources, failing to adequately share them with other programs running in the operating system.

“Infinite” Loop Example 5: Create an infinite loop by accident by making a simple “logic error”. Note: The DoEvents function surrenders execution of VBA code so that the operating system can process other events. The DoEvents function passes control from the application to the operating system. Num_Rolls = 0 Sum = 0 Do Num_Rolls = Num_Rolls - 1 Roll = Int(Rnd()*6)+1 Sum = Sum + Roll DoEvents Loop Until (Num_Rolls > 15)

Putting it All Together! The next two examples show how we can create a user defined function that combines conditional statements with loops! Zip() Unzip()

Zip() Function Zip(String1 As String, String2 As String) As String ‘combines two strings taking alternating characters from String1 and String2 Dim T As String, K As Integer, Ch1 As String, Ch2 As String T = “” For K = 1 To Max(Len(String1),Len(String2)) ‘get character from String1 if possible If K <= Len(String1) Then Ch1 = Mid(String1,K,1) Else Ch1 = “” End If ‘Get character from String2 if possible If K <= Len(String2) Then Ch2 = Mid(String2,K,1) Ch2 = “” ‘String Accumulator T = T + Ch1 + Ch2 Next K Zip = T End Function Example 6: Create a user defined function Zip(String1,String2) that will combine two strings taking alternating characters from String1 and String2. For example, Zip(“MNMIER!”,”YAESAL”) should return the string “MYNAMEISEARL!”

Worksheet Functions If a “built-in” Excel function does not coincide with a “built-in” VBA function, the Excel function can be used in VBA by calling the Excel function as a Worksheet Function. In the VBA code for Zip(), the function “Max()” is undefined. To get the Zip() user defined function to work, change “Max” to “Application.WorksheetFunction.Max”.

Unzip() Example 2:Create a user defined function UnZip(String1,1) and UnZip(String1,2) that will return the odd numbered characters and even numbered characters of a string, String1, respectively. For example, Unzip(“MYNAMEISEARL!”,1) should return the string “MNMIER!” and Unzip(“MYNAMEISEARL!”,2) should return the string “YAESAL” Function UnZip(String1 As String, N As String) As String ‘Returns the “odd” numbered characters ‘from String1, if N=1 ‘Returns the “even” numbered characters ‘from String1, if N=2 Dim T As String, Y As Integer, Ch As String T = “” Do While N <= Len(String1) T = T + Mid(String1,N,1) N = N + 2 Loop UnZip = T End Function

References Loops Notes – John Albers http://www.databison.com/vba-for-loop-for-next-and-for-each-in-next/ http://www.vb6.us/tutorials/understanding-do-and-while-loops http://www.definitions.net/definition/TIGHT%20LOOP http://support.microsoft.com/kb/118468/en-us