# A-level Computing/AQA/Paper 1/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:
| VB.NET | Python |
| If name = "Queen" Then console.writeline("Hello your Majesty") Else console.writeline("Get to the back of the queue!") End If | if name == "Queen": print ("Hello your Majesty") else: print ("Get to the back of the queue!") |
The Else part is optional, you can just ignore the commoner! (and dump the Else)
| VB.NET | Python |
| If name = "Queen" Then console.writeline("Hello your Majesty") End If | if name == "Queen": print ("Hello your Majesty") |
You might also want to test multiple things in the If statement. For example:
* VB.NET
* 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
* Python
* if name == "Queen" and age >= 18:
* print ("Hello your Majesty, I can serve you beer")
* else:
* print ("Get out of my bar!")

## Relational operators

We often want to write IF statements that do some kind of comparison or test. We just did exactly that in the example above with age >= 18 which tests if the value of the age variable is greater or equal to 18.
Most of these operators you will recognise from learning maths but some are slightly different in computing. The following operators are straightforward:
| Operator | Meaning |
| > | Greater than |
| < | Less than |
| >= | Greater than or equal to |
| <= | Less than or equal to |
The most important operator that is different in computing is one that you have already used many, many times probably without even noticing, which is the = operator. In most programming languages the = operator is used for assignment, for example if we want to assign the value "bunny" to a variable called animal we write animal = "bunny" which is the same in both VB.NET and Python. These two languages are different when it comes to equals which we saw in the example above testing if the value of the name variable was equal to "Queen". In VB.NET the equals operator is just = whereas Python uses == instead. This can lead to a very common programming mistake when writing Python - if we try to write an IF statement which uses = by mistake:
we will get an error message similar to this:
Traceback (most recent call last):  File "python", line 4  if name = "Queen" and age >= 18 then:  ^ SyntaxError: invalid syntax
Finally we need an operator for not equals. In VB.NET we use <> whereas in Python we use !=. Here's an example in the form of a really quite rubbish game:
| VB.NET | Python |
| Dim secret_word, your_guess As String secret_word = "unlikely" console.WriteLine("Try to guess the secret word:") your_guess = console.ReadLine If your_guess <> secret_word Then console.WriteLine("You lose") End If | secret_word = "unlikely" print("Try to guess the secret word:") your_guess = input if your_guess != secret_word: print("You lose") |
* 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
* </syntaxhighlight>
* 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
* Example: Multiple Ifs versus 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:
* VB.NET
* 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
* console.writeline("I'm sorry your Majesty, you are too young to buy beer")
* 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
* Python 3
* if name == "Queen" and age >= 18:
* print ("Hello your Majesty, may one serve you beer?")
* elif name == "Queen" and age <18:
* print ("I'm sorry your Majesty, you are too young to buy beer")
* elif name != "Queen" and age >= 18:
* print ("Hello mate, can I serve you beer?")
* elif name != "Queen" and age <18:
* print ("Get out of my pub, you are too young to buy beer")
* 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.
* VB.NET
* If name = "Queen" Then
* If age < 18 Then
* console.writeline("I'm sorry your Majesty, you are too young to buy beer")
* 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
* Python 3
* if name == "Queen":
* if age < 18:
* print ("I'm sorry your Majesty, you are too young to buy beer")
* else:
* print ("Hello your Majesty, may one serve you beer?")
* else:
* if age >= 18:
* print ("Hello mate, can I serve you beer?")
* else:
* print ("Get out of my pub, you are too young to buy beer")
* Try the examples above with the following data, both solutions should provide the same answer:
* 1. The name is Queen and the age is 18
* 2. The name is Quentin and the age is 28
* 3. The name is Queen and the age is 17
* 4. The name is Aashia and the age is 15
* 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.
VB.NET
If(input >= 18,console.writeline("drink beer"),console.writeline("drink cola"))
Python
print ("drink beer") if int(input) >= 18 else print ("drink cola")
This is a much shorter way of writing:
VB.NETPython
If input >= 18 then
console.writeline("drink beer")
Else
console.writeline("drink cola")
End if
if int(input) >= 18:
print ("drink beer")
else:
print ("drink cola")
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 :