CO1403 Visual Basic Programming

1
CO1403 Visual Basic Programming

• Week 6

2
Introduction

• This week we will look at,
• Conditional IF statements
• LO3, Syllabus Item 3 (See Matrix)

3
Decisions

• It is a really useful tool to be able to make
  decisions
• As humans, we do it every single day
• VB also gives us the ability to make decisions
  within our program code
• For example, you may only want to display the
  message Congratulations if someone has entered
  the correct answer to one of your questions.

4
Decisions

• To make decisions within our programs, we have to
  make use of an IF statement.
• An IF statement only executes some lines of code
  if and only if a condition is satisfied.
• IF condition THEN statements ELSE statements

5
Decisions

• IF condition THEN statement ELSE statements
• Is made up of,
• IF is a VB keyword which tells VB that we are
  making some kind of decision
• condition is a boolean expression that must
  evaluate to true or false
• THEN is a VB keyword
• statement is a line of code that you can
  potentially write these are executed if the
  condition evaluates to true
• ELSE statement is a line of code that you can
  write that is executed if the condition evaluates
  to false

6
Decisions

• Consider pseudo-code forms of this IF statement.
• IF John is giving a VB Lecture THEN Students
  are supposed to be in LH14 ELSE Students could
  be anywhere
• IF You understand what John is talking about
  THEN You can write IF statements ELSE You need
  to listen more.

7
Decisions

• The previous example of an IF statement works
  only when there is ONE line of code after the
  THEN and after the ELSE parts of the IF
  statement.
• There is also a form of IF statement that allows
  you to execute more than one line of code
• IF condition THEN
• statements
• ELSE
• statements
• END IF

8
Decisions

• This form of IF statement can word EXACTLY the
  same as the previous example (i.e. with only one
  line of code)
• But also gives us the advantage that we can have
  more than one line of code
• But it also makes your programming code more
  readable, and I would prefer it if you always
  used this style of IF statement
• (It is untidy to attempt to fit too much on a
  signle line)

9
Decisions

• We may also want to have more than one condition
  that we want to test.
• For example, how could we test to see if a
  username was John or Chris and to output
  different results based on the username?
• This introduces us to the ELSEIF statement.

10
Decisions

• IF condition THEN
• statements
• ELSEIF condition THEN
• statements
• ELSE
• statements
• END IF

11
Decisions

• Using the ELSEIF keyword allows us to add more
  than one condition to test for, and makes the IF
  statement very flexible.

12
Decisions

• Show VB examples on OHP

13
What makes a condition?

• As far as programming is concerned, a condition
  is something that evaluates to either true or
  false.
• In order to be able to decide something, a
  computer must have either a definite yes or a
  definite no answer.
• We use booleans to help us define our conditions.
  True / False is the same as Yes or No.
• When writing a condition in VB, you must write it
  in a way that has a yes or no answer (true or
  false)

14
What makes a condition?

• Sometimes, it is useful to be able to combine
  more than one condition. For example,
• IF The username entered was John OR Chris THEN
  Output to screen you are course leaders ELSE
  Output to screen that you are not course
  leaders.
• To achieve this we must look at some boolean
  operators.

15
Boolean operators

• Boolean operators work on True/False values.
  There are a limited number of boolean operators
• All Boolean operators make sense, and you use
  them in the real world (probably without
  realising it)

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Boolean Operators

• AND is a boolean operator. We use it everyday
  in life If the sandwich has cheese AND ham,
  then it is a cheese and ham sandwich.
• AND works on two boolean conditions, in the case
  of the example above, the two boolean expressions
  are
• 1.The sandwich has cheese
• 2.The sandwich has ham
• If they both evaluate to true, then we definitely
  have a cheese and ham sandwich
• Both conditions need to be true for the whole
  condition to evaluate to true

17
Boolean Operators

• OR is a boolean operator. We use it every day
  in real life. For example, If it is 9am OR 10am
  on a Monday morning, THEN I have John in CAT
  building
• OR works on two boolean conditions,
• Is it 9AM?
• OR is it 10AM?
• If either condition is true, then you should be
  in the CAT building.
• Only one condition needs to be true for the whole
  condition to be true

18
Boolean Operators

• NOT is a boolean operator. We also use this
  everyday in life. For example, John is the best
  lecturer aboutNOT!
• The NOT operator works on ONE condition only, and
  totally negates the result.
• If a condition evaluates to true, using the NOT
  operator will make the whole condition evaluate
  to false
• If a condition evaluates to false, using the NOT
  operator will make the whole condition evaluate
  to true

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Boolean Operators

• You can use a boolean operator in your VB code
  easily,
• IF aCondition OR anotherCondition THEN
• statement
• END IF
• IF NOT aCondition THEN
• statement
• END IF

20
Boolean Operators

• VB evaluates operators from left to right. This
  may cause potential confusion with expected
  results. For example,
• IF John AND Chris OR Kate are present
• Which is it? What happens if John is present,
  but not Chris or Kate?
• To avoid confusion such as this, VB implements an
  operator precedence
• This means that some operators will be evaluated
  before others

21
Boolean Operators

• Order of operator evaluation
• 1.() Parenthesis
• 2.NOT
• 3.AND
• 4.OR
• 5.XOR
• 6. (Equivalence)
• Which means an AND expression will evaluate to
  true before an OR expression will evaluate.

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Boolean Operators

• You dont have to, but I always recommend you
  make full use of parenthesis
• This is the only way that you are 100 certain of
  how things are being evaluated
• I.e.
• IF (John AND Chris) OR Kate
• IF John AND (Chris OR Kate)
• You can be 100 certain of the outcomeit is
  predictable!!!