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IMS1906 Programming in VB.NETWeek 5 – Lecture 1 and 2

The repetition structure (Part I and II)

© Angela CarboneMonash University School of Information Management and Systems

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Lecture Outline

Repetition (looping)• The For Next Loop• The Do While Loop• The Repeat Until Loop

Application Development

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Control Structures

• The Structure Theorem (from Week 1):-any computer program can be built from just three control structures:– Sequence– Selection– Repetition – This Study Guide

• Used to determine (control) the order in which the steps in an algorithm are to be executed

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Repetition Structures/Loops

• Some problems require a sequence of tasks to be processed repeatedly, e.g.:

– apply same processing to a set of different data items

– processing until and target point is reached• Repetition structures (aka loops,

iteration structures) cater for this

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Repetition structures have …

• A block of statements that are to be repeated

• A condition that will determine if the loop processing should stop

• the condition is evaluated once in each iteration

• A variable that will change each time the loop is processed

– tested in the condition

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Repetition Structures

Pre-test Loops

(Do-while)

Post-testLoops

(Repeat-Until)

CountedLoops(For)

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The Pre-Test Loop

• Condition is tested PRIOR to the execution of the statement block

• If the condition is TRUE:– Perform the statement block (perform

one iteration)– Go back to the condition (to check

again)• If the condition is FALSE:

– “exit” from the loop (do not perform any more iterations)

• The loop may potentially not be executed:– if condition fails on first test

number = 0DOWHILEnumber <10

display numbernumber = number + 1

ENDDO

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DOWHILE - Algorithm

DOWHILE a condition evaluates to TRUEstatement block

ENDDO

For example:number = 0DOWHILE number <10

display numbernumber = number + 1

ENDDO

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DO WHILE Loops

• If the condition evaluates to FALSE initially, no iterations are performed

• To ensure at least one iteration, variables tested in condition need to be initialised prior to the loop

• At least one variable tested in the condition must change within the statement block

– otherwise endless loop (“infinite” loop)

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DOWHILE - Problem Example

Fahrenheit - Celsius conversion (Robertson, pp. 51-54)Everyday, a weather station receives 15 temperatures expressed in Fahrenheit. A program is to be written which will accept each Fahrenheit temperature, convert it to Celsius and displaythe converted temperature to the screen. After 15 temperatures have been processed, the words ‘All temperatures processed’ are to be displayedon the screen.

Celsius = (Fahrenheit – 32) x 5/9

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Defining Diagram

INPUT PROCESS OUTPUT

fahrenheit Read fahrenheit

Calculate celsius

Display celsius

celsius

Sometimes called an IPO chart (for Input, Processing, Output)

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DOWHILE - Solution Algorithm

Set temperature_count to zeroDOWHILE temperature_count < 15

Prompt for f_tempGet f_tempCompute c_temp = (f_temp - 32)* 5/9Display c_tempIncrement temperature_count

ENDDODisplay ‘All temperatures processed’ to the screenEND

Initialise the test variable

Test the condition:

Change the test variable

If condition is true, go here

Go

Bac

k If condition is false, go here

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DOWHILE - Desk Check

Input data: test value 1: f_temp = 32, c_temp = (f_temp - 32) * 5/9 = 0Expected outcome: c_temp = 0

test value 2: f_temp = 50c_temp = (f_temp - 32) * 5/9 = 10Expected outcome: c_temp = 10

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DO WHILE – Desk Checking

TRUE …DO WHILE

1Add

DisplayDisplay

0Compute

32Read

TRUEDO WHILE

0Initialize

DOWHILE condition

c_tempf_tempTemp_countStatement

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DO WHILE – Desk Checking …

END.

Display

FALSEDO WHILE

15Add

DisplayDisplay

10Compute

50Read

TRUEDO WHILE

14… Add

DOWHILE condition

c_tempf_tempTemp_countStatement

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Common Traps with Loops

• Forgetting to give initial values to loop counters

• Forgetting to change the condition variables inside the loop

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The Post-Test Loop

• The condition is tested at the END of each iteration of the loop

– At least one iteration will be performed

• If the condition evaluates to FALSE, another iteration will be executed

• If the condition is TRUE, exit from the loop

• Expressed by:– REPEAT … UNTIL loops (for

algorithms)– Do … Loop Until (for Visual Basic

.NET)

number = 0REPEAT

display numbernumber = number + 1

UNTIL number = 10

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REPEAT UNTIL - Algorithm

REPEAT

statement blockUNTIL a condition evaluates TRUE

For example:number = 0REPEAT

Display numberIncrement number by 1

UNTIL number = 10

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• A Repeat Until in an algorithm becomes a DO .. LOOP UNTIL in VB.NET code:

Do…Statements…

Loop Until condition

REPEAT UNTIL – VB.NET Syntax

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Counted loops

• Execute the statement block a pre-determined number of times– Number of iterations known in advance

• A control variable keeps count of the number of repetitions– No need to change this explicitly in code

• May use i, j & k as control variables (historical)– meaningful names better

• Do .. End Do (algorithms)• FOR…NEXT loops (VB.NET)

DO i = 1 to 10display i

END DO

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DO counter = m to n [ step x ]

statement blockEND DO

For example: Another example:DO i = 1 to 10 DO i = 1 to 11 step 2

display i display iEND DO END DO

DO … END DO - Algorithm

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Fahrenheit - Celsius conversion (Robertson, p.52)

Everyday, a weather station receives 15 temperaturesexpressed in Fahrenheit. A program is to be written which will accept each Fahrenheit temperature, convert it to Celsius and displaythe converted temperature to the screen. After 15 temperatures have been processed, the words ‘All temperatures processed’ are to be displayed on the screen.

Counted Loops - Problem Example

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Counted Loops - Solution Algorithm

Fahrenheit_Celsius_conversionDO temperature_count = 1 to 15

Prompt operator for f_tempGet f_tempCompute c_temp = (f_temp - 32)* 5/9Display c_temp

END DODisplay ‘All temperatures processed’ to the screen

END

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FOR NEXT – VB.NET Syntax

For counter_var = m to nstatement block

Next counter_var

For counter_var = m to n step sstatement block

Next counter_var

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Hints for Programming with Loops

• Minimise the steps inside any loop – Keep It Simple!

• Make each loop perform one function • Document (Comment) the loop by:

– Stating the conditions for which the loop executes, or– Explicitly stating the termination conditions

• Enter the loop from only one location• Leave the loop only by satisfying the exit

condition– Structured programming requires this

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Repetition – Summary

• Pre-test loops: Do While …

• Post-test loops: Repeat … Until• Counted loops: For … Next

You have now learnt all the elements of the Structure Theorem.

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Questions/Reading

• Questions– Hammer in the key concepts– Pull out those questions– List the three types of loops

and when they should be used.– Do you know the syntax of

each loop?• Reading

– Unit Guide, Study Guide 5– Zak, Chpt 5– Robertson Chpt 5

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TEST 1 (Next week) Overview

• Test will cover topics weeks 1-5

• Topics include:• Week 1

– Algorithm design – Structure Theorem– Steps in writing a program– Desk checking an algorithm

• Week 2– Objects and Classes– Properties and behaviours– VB Objects/Controls– VB.NET IDE – Built a simple splash screen

• Week 3– TOE Charts– Designing User Interface– In-built functions: Val(), Format()– Classes Convert and Random– Variables/Data Types– Literals and Constants– Option Explicit and Implicit– Input/Output

• Week 4– Selection– If statements– Case statements

• Week 5Repetition (looping)– The For Next Loop– The Do While Loop– The Repeat Until Loop

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TEST 1 Overview

• Test will be 50 mins long• Counts for 10% of total mark• Question types

– Short answer written responses– Writing pseudo code – Develop TOE chart + User Interface– VB.NET coding questions

• Wishing you all the success you deserve!• Reminder TEST 1 starts at 10am next week!