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Fundamentals of Programming: Selection

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PAPER 1 - ⇑ Fundamentals of programming ⇑

← Input and output 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

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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


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

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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: 

 if name = "Queen" and age >= 18:
    print("Hello your Majesty, I can serve you beer")

we will get an error message similar to this:

   Code Output

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.NET Python 
 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

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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.

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.

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

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
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