B065: PROGRAMMINGOPERATORS AND SELECTION 1

Starter Have a go at the All Logic Signs activity.

COMP1 Programming 8 Selection All Logic Signs.xls

Objectives• Understand what is meant by selection in programming.

• Explore selection programming commands: IF

• Understand the use of logical operators.

Operators Recap• Answer these questions with a TRUE or FALSE

• 6 > 3• 72 < 75• 115 >= 115• 92 > 42+2• 12 = 6+6• 32 <= 16*2

> MORE than

< LESS than

<= LESS than or EQUAL to

>= MORE than or EQUL to

Logical Operators in Programming

Evaluation Symbol Example

Equal to = Number = 1

Less Than < Age < 21

Less Than or Equal To <= Age <=17

Greater Than > Debt > 25

Greater Than or Equal To >= Debt > = 2000

Not Equal To <> Number <> 1

AND AND Number = 1 AND Age < 12

OR OR Number = 1 OR Number = 2

NOT NOT NOT Age < 17

IF Logic Recap

Which goes where to make a logical sentence? Let’s do one at a time. CLICK on a question mark to reveal.

What you believe in…

Question What to do if TRUE What to do if FALSE

IF 12 > 25 Clap twice Clap once

IF you are at Crypt Stand up, then sit down Spin your head around

IF 30 < 42 Wink twice Shake your hands

IF 12 > 6*1 Put your hands up Click twice

IF you are in ICT Stand up, turn around, sit back down

Yawn

IF 4*2 >= 12 Roar like a lion Moo like a cow

You will be given a logical question. If you think it is TRUE, do what is says under TRUE. If you think it is FALSE, do the FALSE action.

A Separate Example• Let’s look at a spreadsheet version (From GCSE) which you

should be familiar with.

• It nicely sets the scene for selection in programming.

• IF Example

IF…THEN…END IF• Sometimes in programming, you may only want to execute

lines of code if certain conditions are met.

• For example, you would not send out a bill to a customer if they did not owe anything.

• The way to write an IF statement is simple:

If <condition> Then <statements>End If

ExamplesIf score = 100 Then Console.WriteLine("Full Marks!")End If

If pay > tax Then pay = pay - taxEnd If If mark < 40 Then result = "Fail" console.writeline(result)End If

The semantics are simple: if the condition is True, the following statement(s) will be executed; otherwise they will be omitted.

If, Then, and End If are all keywords - the code editor will capitalise them and colour them blue when you finish typing any of them, as it does with all keywords.

What can you use?• The statements can be any legal VB statements - and that includes If … Then … End

If statements now! So we can quite easily have a nested structure, like the following example:

If age > 65 Then If theDay = "Thursday" Then 'Senior Citizens get a 10% discount! cost = 0.9 * cost End IfEnd If

• Note that each If requires a corresponding End If. See how we INDENT to make the code easier to understand.

IF…THEN…ELSE…END IF• What if you want some code to be executed if the condition IS

NOT met?

If mark < 40 Then result = "Fail" Else result = "Pass"End IfConsole.WriteLine(result)

• You can use ELSE. What happens in the code above?

Multiple Conditions? ELSEIF• What if you have multiple conditions (or rules) which you

want to check?

Summary• IF…END IF Basic Checks.

• IF…ELSE…END IF Basic Checks with a catch situation.

• IF…ELSEIF…ELSE…END IF Multiple conditions.

• You can also use logical operators ( >, >=, <, <=, AND, OR, NOT) in your selection statements.

TASK• Complete the questions in your guide.

• Attempt Tasks 10, 11 and 12.

• We will go through each task throughout the lesson.

• Complete for homework.

Objectives Review• Understand what is meant by selection in programming.

• Explore selection programming commands: IF

• Understand the use of logical operators.

Required Reading• Each week you will be given required reading.

• If you fail to do this, you will 100% find the lessons which follow it EXTREMELY difficult.

• Before next lesson you should have read:

• Read Chapter 4• Homework – Finish exercises 1-10 Chapter 4

Plenary• If the day is Monday and a person is 18 or over (but younger

than 21), they qualify for a discount on drinks over £2. They receive 35p off the price of the drink.

• What would be the IF block which would express this in code?