Lecture Outline
IMS1906 Programming in VB.NET
Week 5 – Lecture 1 and 2
Repetition (looping)
• The For Next Loop
• The Do While Loop
• The Repeat Until Loop
The repetition structure (Part I and II)
© Angela Carbone
Monash University
School of Information Management and Systems
Application Development
www.monash.edu.au
2
www.monash.edu.au
Control Structures
Repetition Structures/Loops
• 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
• 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
www.monash.edu.au
www.monash.edu.au
3
4
Repetition structures have …
Repetition Structures
• 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
Counted
Loops
(For)
Pre-test
Loops
(Do-while)
– tested in the condition
Post-test
Loops
(Repeat -Until)
www.monash.edu.au
www.monash.edu.au
5
6
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
DOWHILE - Algorithm
number = 0
DOWHILE number <10
display number
number = number + 1
ENDDO
DOWHILE a condition evaluates to TRUE
statement block
ENDDO
For example:
number = 0
DOWHILE number <10
display number
number = number + 1
ENDDO
www.monash.edu.au
www.monash.edu.au
7
8
DO WHILE Loops
DOWHILE - Problem Example
• 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
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 displayed
on the screen.
Celsius = (Fahrenheit – 32) x 5/9
– otherwise endless loop (“infinite” loop)
www.monash.edu.au
www.monash.edu.au
9
10
Defining Diagram
INPUT
fahrenheit
DOWHILE - Solution Algorithm
PROCESS
Read fahrenheit
OUTPUT
celsius
Calculate celsius
Go Back
Display celsius
Sometimes called an IPO chart (for Input, Processing, Output)
Initialise the test variable
Set temperature_count to zero
Test the condition:
DOWHILE temperature_count < 15
If condition is true, go here
Prompt for f_temp
If condition is false, go here
Get f_temp
Compute c_temp = (f_temp - 32)* 5/9
Display c_temp
Increment temperature_count
Change the test
variable
ENDDO
Display ‘All temperatures processed’ to the
screen
END
www.monash.edu.au
www.monash.edu.au
11
12
DOWHILE - Desk Check
DO WHILE – Desk Checking
Input data:
test value 1: f_temp = 32,
Statement
Temp_count
Initialize
0
f_temp
DO WHILE
c_temp = (f_temp - 32) * 5/9 = 0
32
Compute
test value 2: f_temp = 50
Add
Expected outcome: c_temp = 10
Temp_count
14
f_temp
c_temp
DO WHILE
www.monash.edu.au
13
14
DOWHILE
condition
• Forgetting to give initial values to loop
counters
• Forgetting to change the condition
variables inside the loop
10
Display
Add
Common Traps with Loops
50
Compute
TRUE …
www.monash.edu.au
TRUE
Read
Display
1
DO WHILE
DO WHILE – Desk Checking …
… Add
0
Display
c_temp = (f_temp - 32) * 5/9 = 10
DOWHILE
condition
TRUE
Read
Expected outcome: c_temp = 0
Statement
c_temp
Display
15
DO WHILE
FALSE
Display
END.
www.monash.edu.au
www.monash.edu.au
15
16
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)
REPEAT UNTIL - Algorithm
number = 0
REPEAT
display number
number = number + 1
UNTIL number = 10
REPEAT
statement block
UNTIL a condition evaluates TRUE
For example:
number = 0
REPEAT
Display number
Increment number by 1
UNTIL number = 10
www.monash.edu.au
www.monash.edu.au
17
18
REPEAT UNTIL – VB.NET Syntax
Counted loops
•
• A Repeat Until in an algorithm becomes a
DO .. LOOP UNTIL in VB.NET code:
•
•
Do
…
Statements
…
•
•
Loop Until condition
www.monash.edu.au
www.monash.edu.au
19
20
DO … END DO - Algorithm
DO counter = m to n
Counted Loops - Problem Example
[ step x ]
Fahrenheit - Celsius conversion (Robertson, p.52)
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 displayed on the screen.
statement block
END DO
For example:
DO i = 1 to 10
display i
END DO
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 10
display i
END DO
Another example:
DO i = 1 to 11 step 2
display i
END DO
www.monash.edu.au
www.monash.edu.au
21
22
Counted Loops - Solution Algorithm
FOR NEXT – VB.NET Syntax
Fahrenheit_Celsius_conversion
DO temperature_count = 1 to 15
Prompt operator for f_temp
Get f_temp
Compute c_temp = (f_temp - 32)* 5/9
Display c_temp
END DO
Display ‘All temperatures processed’ to the
screen
END
For counter_var = m to n
statement block
Next counter_var
For counter_var = m to n step s
statement block
Next counter_var
www.monash.edu.au
www.monash.edu.au
23
24
Hints for Programming with Loops
Repetition – Summary
• Minimise the steps inside any loop
• Pre-test loops:
Do While …
• Post-test loops:
• Counted loops:
Repeat … Until
For … Next
–
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
You have now learnt all the elements of the
Structure Theorem.
• Enter the loop from only one location
• Leave the loop only by satisfying the exit
condition
– Structured programming requires this
www.monash.edu.au
www.monash.edu.au
25
26
Questions/Reading
•
•
TEST 1 (Next week) Overview
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
•
•
Week 1
–
–
–
–
•
•
Objects and Classes
Properties and behaviours
VB Objects/Controls
VB.NET IDE
Built a simple splash screen
Week 3
–
–
–
–
–
–
–
–
Algorithm design
Structure Theorem
Steps in writing a program
Desk checking an algorithm
Week 2
–
–
–
–
–
•
Week 4
–
–
–
•
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
Selection
If statements
Case statements
Week 5
Repetition (looping)
– The For Next Loop
– The Do While Loop
– The Repeat Until Loop
www.monash.edu.au
www.monash.edu.au
27
28
TEST 1 Overview
• Test will be 50 mins long
• Counts for 10% of total mark
• Question types
–
–
–
–
•
Test will cover topics weeks
1-5
Topics include:
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!
www.monash.edu.au
29