Loop Structures. Loop Structures. Algorithm to record 5 TV programmes. Recall Structured Programming..3 basic control structures.

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Rodger Osborne
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1 Loop Structures Recall Structured Programming..3 basic control structures Sequence Input -> Process -> Output Selection IF ENDIF SELECT CASE END SELECT Loop Structures DO WHILE LOOP DO LOOP UNTIL FOR NEXT Loop Structures Event Driven Loop By default VB has event-driven loops Not possible with procedural languages Eg. Code runs every time you click on command button Voter application Algorithm to record 5 TV programmes 1. Insert tape 2. If tape not rewound, re-wind it 3. Repeat steps 4-7 for each of 5 programmes 4. Set date 5. Set start time 6. Set stop time 7. Set channel 8. Set video recorder to time mode 1

2 Loop Structures Repeating a set of commands a number of times Recall.. Repeat steps 4-7 for each of 5 programmes 4. Set date 5. Set start time 6. Set stop time 7. Set channel DO WHILE.LOOP Controlled by a Boolean Expression..TRUE or FALSE The loop executes as long as the condition is TRUE If condition FALSE, skip to statement after the loop Start numprogs=0 numprogs <5? Start and end time date & channel add 1 to numprogs Condition is tested every time, => you need to»set initial value»change value within the loop..otherwise get an infinite loop End DO WHILE LOOP Form Action DO WHILE condition. Loop body. LOOP If condition is evaluated as TRUE, the body of the loop is executed. Control branches to the top and retest condition etc If condition FALSE, skip to statement after LOOP. 2

3 DO WHILE LOOP Input num Pseudocode INPUT num DO WHILE num <> 0 OUTPUT num INPUT num LOOP OUTPUT Done Output done Is num <>0? Output num Input num DO WHILE is a pre-test loop the exit test is made before the loop is executed If the test fails first time, the loop is NEVER executed DO WHILE with a counter DO WHILE.. LOOP : counter used to perform an operation a specified number of times Counter is initialised before entering loop, tested at start of loop and incremented within loop Eg. Program accepts four numbers and totals them Pseudocode count = 0 tot = 0 DO WHILE count < 4 INPUT next number ; num tot = tot + num count = count +1 LOOP Output total ; tot Private Sub cmdok_click() Dim sum As Single, num As Single, count As Integer count = 1 sum = 0 Do While count <= 3 num = Val(InputBox("Enter Number" & Str$(count))) sum = sum + num count = count + 1 Loop txtresult.text = "Sum of Numbers is" & Str(sum) 3

4 DO LOOP UNTIL Pseudocode DO INPUT num OUTPUT num LOOP UNTIL num =0 OUTPUT Done Output done DO.. UNTIL is a post-test loop the exit test is made after the loop is executed The loop will be executed at least once Input num OUTPUTnum YES Is num = 0? NO Form Action DO LOOP UNTIL DO.. LOOP UNTIL condition The body of the loop is executed and the condition is evaluated. If TRUE, skip to statement after LOOP UNTIL; if FALSE, loop body repeated. Example Private Sub cmdok_click() Dim sum As Single, num As Single, count As Integer count = 1 sum = 0 Do num = Val(InputBox("Enter Number" & Str(count))) sum = sum + num count = count + 1 Loop Until count > 3 txtresult.text = "Sum of Numbers is" & Str(sum) 4

5 Beware!! A loop coded so that it cannot be exited is called an infinite loop A DO WHILE LOOP infinite loop if condition can never become false A DO LOOP UNTIL infinite loop if condition can never become true Be careful with code FOR NEXT loop Used when you know the number of times the loop will be executed Takes the form FOR Counter = 0 to 10 Step 1 Sum=Sum + Counter NEXT Counter Counter is the lcv : loop control variable 0 is the initial value 10 is the terminal value 1 is the increment (if not specified 1 is assumed) ALL 4 must be numeric What does this Loop do? FOR.. When the FOR statement is FIRST encountered The initial,terminal and (if given) step value expressions are evaluated The loop control variable (lcv) is given its initial value lcv is tested against the terminal value If test TRUE the loop is executed If not TRUE, the loop body is skipped, and control passes to the statement following NEXT. 5

6 NEXT When the NEXT statement is encountered The step value is added to the lcv. Test lcv against terminal value If test is TRUE loop body is executed else loop exited FOR..NEXT Can have positive and negative step values EG. FOR Num = 1 TO 3 STEP 1 Sum=Sum + Num NEXT Num FOR Num = 3 TO 1 STEP -1 Sum=Sum + Num NEXT Num Form Action FOR.. NEXT loop FOR variable = initial TO limit STEP increment.... NEXT variable NOTE: USE indentation for clarity Executes the code between the FOR and NEXT statements a fixed number of times Example 6

7 Private Sub cmdadder_click() Dim Sum As Single, num As Single Sum = 0 For i = 1 To 3 num = Val(InputBox("Enter Number" & Str(i))) Sum = Sum + num Next i txtresult.text = "Sum of Numbers is" & Str(Sum) Beware!!! The initial, terminal and step values cannot be modified in the loop body You can but should NEVER modify the loop control variable in the loop body. It makes execution unpredictable Event Driven Adder Option Explicit Dim sum As Single Private Sub cmdsum_click() Dim num As Single num = Val(txtNum.Text) sum = sum + num txtsum.text = Str(sum) Private Sub Form_Load() sum = 0 7

8 Event Driven Adder Number added to sum each time command button clicked Need to Declare sum in General Declarations Variable Sum has Form level scope Different event procedures on same Form can use it Up to now, variables have had Local scope only Can also have variables that are visible across diff Forms Project Level scope Initialise sum=0 in Form_Load What happens if you initialise sum=0 within command button code? Input Validation Very important for robust code Eg Grade must be in range class size must be positive Use of.. Event Driven Loop DO WHILE LOOP DO LOOP UNTIL Private Sub cmdok_click() Dim amount As Single amount = Val(txtInput.Text) Rem Input validation If amount < 0 Then txtmessage.text = "Invalid entry" Else Rem valid amount.. continue Select Case amount Case Is < 1000 txtmessage.text = "Interest Rate is 3%" Case Is <= 5000 txtmessage.text = "Interest Rate is 4%" Case Else txtmessage.text = "Interest rate is 5%" End Select End If 8

9 Private Sub cmdok_click() Dim amount As Single amount = Val(InputBox("Enter amount you wish to invest")) Rem Input validation Do While amount < 0 txtmessage.text = "Invalid entry" amount = Val(InputBox("Invalid amount..must be >=0")) Loop Rem valid amount.. continue Select Case amount Case Is < 1000 txtmessage.text = "Interest Rate is 3%" Case Is <= 5000 txtmessage.text = "Interest Rate is 4%" Case Else txtmessage.text = "Interest rate is 5%" End Select Nested loops Have already seen nested IF Statements Loop within a loop Each loop has its own set of control variables Used for input validation cf h/o Also used for multi-dimensional input FOR outer = 1 to to 4 FOR inner = 1 to to 2. NEXT inner NEXT outer Nested FOR NEXT loops Each time the outer loop is executed, the inner loop is executed twice When inner loop terminated, control passes to statement after NEXT inner.next outer Outer is incremented by 1 and tested If outer is still <=4, execute statement block which is the inner Loop. Note Each loop has unique loop control variable One loop is entirely contained within the other 9

10 Comparison loop structures Event Driven Loops can often be used FOR NEXT is used in counting loops that execute a pre-specified number of times. Also. lcv automatically incremented and compared (unlike DO loops) less error prone Either type of DO.. LOOP may be used in place of a FOR NEXT DO LOOPs can also be used with sentinel values - this cannot be done with a FOR NEXT loop 10

Repetition Structures

Repetition Structures Chapter 5 Fall 2016, CSUS Introduction to Repetition Structures Chapter 5.1 1 Introduction to Repetition Structures A repetition structure causes a statement or set of statements