# Fundamentals of Programming: Selection

 ← Constant Definitions Selection Iteration →

An important part of programming is the use of selection, that is the ability to do something if certain criteria is met. This may be as simple as increasing your health bar in a computer game if you eat a chicken drumstick or inserting the cooling rods into the nuclear reactor if the temperature exceeds a certain value.

### IF Statement

The most common selection statement is the IF statement, the idea is that you compare a value to some criteria, IF the value and criteria match then you proceed in a certain way, otherwise you do something else. For example:

If It is the queen Then
Salute her
Else
Treat them like a commoner
End
If name = "Queen" Then
Else
console.writeline("Get to the back of the queue!")
End If

The Else part is optional, you can just ignore the commoner! (and dump the Else)

If name = "Queen" Then
End If

You might also want to test multiple things in the If statement. For example:

If name = "Queen" And age >= 18 Then
console.writeline("Hello your Majesty, I can serve you beer")
Else
console.writeline("Get out of my bar!")
End If

#### Nested Ifs

Sometimes when we are trying to write complex code we will need to use a combination of IFs. In the example above we might want to still treat an under-age queen with respect, an under-age commoner with contempt, serve an 18+ queen with respect, and serve an 18+ commoner with common manners. In fact it seems as if we need 4 different IF statements. We could solve it like this:

If name = "Queen" And age >= 18 Then
console.writeline("Hello your Majesty, may one serve you beer?")
End If
If name = "Queen" And age < 18 Then
End If
If name <> "Queen" And age >= 18 Then '<> means not equal (so does !=)
console.writeline("Hello mate, can I serve you beer?")
End If
If name <> "Queen" And age < 18 Then
console.writeline("Get out of my pub, you are too young to buy beer")
End If

This seems awfully cumbersome and we will now look a more elegant way of solving this, using Nested IF's. First of all, nested means placing one thing inside another, so we are going to place an IF inside another.

If name = "Queen" Then
If age < 18 Then
Else
console.writeline("Hello your Majesty, may one serve you beer?")
End If
Else
If age >= 18 Then
console.writeline("Hello mate, can I serve you beer?")
Else
console.writeline("Get out of my pub, you are too young to buy beer")
End If
End If

Try the examples above with the following data, both solutions should provide the same answer:

name = "Queen" age = 18
name = "Quentin" age = 28
name = "Queen" age = 17
name = "Aashia" age = 15

 Extension: Single line IF statements As you should be aware by now a lot of programming is doing things as quickly as possible. You might be fed up with writing long if statements, having to keep hitting that enter key to make new lines. There is a faster way to code: single line IF statements. If(input >= 18,console.writeline("drink beer"),console.writeline("drink cola")) This is a much shorter way of writing: If input >= 18 then console.writeline("drink beer") Else console.writeline("drink cola") End if But be careful, code like this can often be harder to read and therefore debug. Once it has been through the interpreter / compiler it almost certainly won't be running any faster either, it's just there for you to save a little space. For the exam keep to the longer version.

### Case Statement

The other type is the Case statement, this can be summarised by several if statements where the value is compared to several criteria and the action of first criteria matched is performed, otherwise a default action may be performed.

Case Enter Restaurant and pick up menu
If Egg and Chips available Then
Order Egg and Chips
End If
If Pie and Chips available Then
Order Pie and Chips
End If
If Curry and Chips available Then
Order Curry and Chips
End If
If Pizza and Chips available Then
Order Pizza and Chips
End If
Default
Leave hungry
End

However, most programming languages will give you a shortened way of implementing a case statement without the need to write all of these if statements. For example in VB.NET we use the select case

Dim number As Integer = 5
Select Case number
Case 1, 2, 3
Console.WriteLine("Between 1, 2, 3 inclusive")
Case 4 to 8
Console.WriteLine("Between 4 and up to 8")
'This is the only case that is true
Case 9
Console.WriteLine("Equal to 9")
Case 10
Console.WriteLine("Equal to 10")
Case Else
Console.WriteLine("Not between 1 and up to 10")
End Select
Code Output

Between 4 and up to 8

Exercise: Case statements
Create a program where someone types in the name of an animal and it outputs the sound the animal makes. The animals it should handle are:
• Pig - Oink
• Cow - Moo
• Bear - Grr
• Sheep - Baa
• Tiger - Grr
• everything else - Meow

Try and complete this task by only using 5 case statements.

Dim animal As string
Select Case animal
Case "Pig"
Console.WriteLine("Oink")
Case "Cow"
Console.WriteLine("Moo")
Case "Bear", "Tiger"
Console.WriteLine("Grr")
Case "Sheep"
Console.WriteLine("Baa")
Case Else
Console.WriteLine("Meow")
End Select
You are now going to use a case statement to create an electronic piano.
Note Frequency
A 220.00
B 246.94
C 261.63
D 293.66
E 329.63
F 349.23
G 392.00

Create a case statement in the code below, that will play the notes written above. The while true loop means that the code will never end and will continue for ever. For bonus points try and get the code to accept both upper and lower case inputs

'instruction statement here
While True
'input and case statements here
While End

Remember to make sound you use:

console.beep(2000,500) 'beep at 2000Hz for 0.5 seconds

dim note as char
While True
'note =  Console.Readline() 'also works, but it less fun
Select Case UCase(note)
Case "A"
Console.Beep(220,50)
Case "B"
Console.Beep(247,50)
Case "C"
Console.Beep(262,50)
Case "D"
Console.Beep(294,50)
Case "E"
Console.Beep(330,50)
Case "F"
Console.Beep(349,50)
Case "G"
Console.Beep(392,50)
End Select
End While