# A-level Computing 2009/AQA/Problem Solving, Programming, Data Representation and Practical Exercise/Fundamentals of Programming/Selection

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:
The Else part is optional, you can just ignore the commoner! (and dump the Else)
You might also want to test multiple things in the If statement. For example:
* Exercise: IF statements
* Write a single IF statement for the following:
* Ask a user for their eye colour, if they say green call them a "Goblin", else they must be a different type of monster:
* Code Output
* What eyes have thee?
* Green
* Thou art a Goblin?
* Alternatively:
* Code Output
* What eyes have thee?
* Blue
* Pray tell, be thou another form of beast?
* Answer:
* dim eyes as string
* console.writeline("What eyes have thee?")
* eyes = console.readline
* If eyes = "Green" Then
* console.writeline("Thou art a Goblin?")
* Else
* console.writeline("Pray tell, be thou another form of beast?")
* End If
* Try the code by inputting "green". It doesn't work! We need to adjust the IF statement:
* If eyes = "Green" or eyes = "green" Then the Or part makes sure it picks up upper and lower case letters
* alternatively we could use UCase we'll find out more about this later
* If UCase(eyes) = "GREEN" Then 'UCase converts the entire input into capitals
* Code Output
* How old are you:
* 11.8
* You're probably at secondary school
* Code Output
* How old are you:
* 9
* You're not at secondary school
* Code Output
* How old are you:
* 19
* You're not at secondary school
* Using one IF statement write code to handle the above. HINT: you might need more than one clause in the IF ... THEN section.
* Answer:
* dim age as single
* console.writeline("How old are you:")
* age = console.readline
* If age >= 11 And age < 17 Then
* console.writeline("You're probably at secondary school")
* Else
* console.writeline("You're not at secondary school")
* End If
* Now for some very simple AI:
* Code Output
* How do you feel today: Happy or Sad?
* Sad
* Have you had some exercise: Yes or No?
* No
* Go for a walk, you might feel better
* In all other situations the program should say: "Sorry I don't know how to help". Using one IF statement write code to handle the above:
* Answer:
* dim feel as string
* dim exercise as string
* console.writeline("How do you feel today: Happy or Sad?")
* feel = console.readline
* console.writeline("Have you had some exercise: Yes or No?")
* exercise = console.readline
* If feel = "Sad" AND exercise = "No" Then
* console.writeline("Go for a walk, you might feel better")
* Else
* console.writeline("Sorry I don't know how to help")
* 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:
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.
Try the examples above with the following data, both solutions should provide the same answer:
* Exercise: Nested IF statements
* Write nests of IF statements for the following:
* A car can be hired when a person is over 21 and not intoxicated.
* Code Output
* How old are you?
* 22
* Good, that's old enough. Have you been drinking?
* Yes
* Come back tomorrow
* It should also handle:
* Code Output
* console.writeline("How old are you?")
* 20
* console.writeline("You're too young I'm afraid. Come back in a few years")
* Answer:
* dim age as integer
* dim drinking as string
* console.writeline("How old are you?")
* age = console.readline
* if age >= 21 then
* console.writeline("Good, that's old enough. Have you been drinking?")
* drinking = console.readline
* if drinking = "Yes" then
* console.writeline("Come back tomorrow")
* else
* console.writeline("Your carriage awaits")
* end if
* else
* console.writeline("You're too young I'm afraid. Come back in a few years")
* end if
* Create a login screen to do the following:
* Code Output
* Enter username
* Jonny5
* RECOGNISED! Enter password:
* Alive
* Please enter Jonny5
* If they get the username wrong it should immediately kick them out and not ask for the password. If they get the password wrong it should kick them out.
* Answer:
* dim name as string
* dim password as string
* console.writeline("Enter username:")
* name = console.readline
* if name = "Jonny5" then
* console.writeline("RECOGNISED! Enter password:")
* password = console.readline
* if password = "Alive" then
* console.writeline("Please enter " & name)
* else
* console.writeline("INCORRECT! Get out!")
* end if
* else
* console.writeline("Username not recognised. Get out!")
* end if
* 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.
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
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.
Answer:
Dim animal As string
animal = console.readline
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.
NoteFrequency
A220.00
B246.94
C261.63
D293.66
E329.63
F349.23
G392.00
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
Answer:
dim note as char
console.writeline("please insert a musical note:")
While True
note = Console.ReadKey.KeyChar
'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 :