How does a game turn 135 seconds into 2:15 on its timer?

Variables, constants, input and output

• A variable is a named memory location whose value can change while the program runs.
• A constant is a named value that cannot change while the program runs. Naming it means one place to update it, and clearer code.
• Assignment puts a value into a variable; the first assignment is initialisation. Use meaningful identifiers like lives, not x.

CONSTANT MAX_LIVES ← 3
lives ← MAX_LIVES
name ← USERINPUT
OUTPUT 'Good luck, ', name
lives ← lives - 1
OUTPUT lives

Data types

• A data type tells the computer what kind of data a variable holds and which operations make sense.

• Some languages call a real a float; AQA exams use the names in the table.

Arithmetic operators

• +, - and * work as in maths. Real division / can give a fractional answer.
• Integer division gives only the whole-number part. The remainder (modulus) is what is left over.

total ← 200
minutes ← total DIV 60    # 3
seconds ← total MOD 60    # 20

Casting

• Casting converts a value from one data type to another, e.g. the string '15' to the integer 15.
• Keyboard input usually arrives as a string, so cast it before doing arithmetic.
• Cast numbers to strings to join them into a message.
• AQA calls these string conversion operations: STRING_TO_INT, STRING_TO_REAL, INT_TO_STRING, REAL_TO_STRING.

ageText ← USERINPUT
age ← STRING_TO_INT(ageText)
OUTPUT age + 1

How can a few lines of code print all 100 times-table answers?

Sequence and selection

• Sequence: statements run one after another, in order.
• Selection: a condition that is true or false decides which block of code runs.
• Compare with <, >, ≤, ≥, equal to = and not equal to ≠.
• Join conditions with AND, OR, NOT.

mark ← 72
IF mark ≥ 70 THEN
  OUTPUT 'Distinction'
ELSE IF mark ≥ 40 THEN
  OUTPUT 'Pass'
ELSE
  OUTPUT 'Fail'
ENDIF

Count-controlled loops

• Iteration repeats a block of code.
• A count-controlled loop (definite iteration) repeats a fixed number of times. A counter variable goes up by 1 each time unless a step is given.

FOR count ← 1 TO 5
  OUTPUT count * 3
ENDFOR

All output 3, 6, 9, 12, 15.

Condition-controlled loops

• A condition-controlled loop (indefinite iteration) repeats until a condition changes, so you need not know the number of passes.
• A WHILE loop tests the condition at the start: it may run zero times.
• REPEAT … UNTIL (or DO … WHILE) tests at the end, so it always runs at least once.

total ← 1
WHILE total < 50
  total ← total * 2
ENDWHILE
OUTPUT total

All output 64.

Nesting

• Nesting puts one construct inside another: an IF inside a loop, a loop inside an IF, or a loop inside a loop.
• With nested loops, the inner loop runs in full for every pass of the outer loop.
• Total passes of the inner block = outer passes × inner passes.

FOR row ← 1 TO 3
  FOR col ← 1 TO 3
    OUTPUT row * col
  ENDFOR
ENDFOR

Output: 1, 2, 3, 2, 4, 6, 3, 6, 9 (9 values).

How can one name hold all 9 squares of a noughts and crosses board?

Arrays

• An array is a data structure: it stores many values under one identifier. Each value is an element.
• All elements have the same data type.
• An index picks out one element. The first element has index 0.
• AQA exam questions index from 0 unless they say otherwise.

scores ← [12, 7, 9, 15, 4]
OUTPUT scores[1]      # 7
scores[3] ← 20        # 15 becomes 20

Looping through an array

• Use a variable as the index in a count-controlled loop to visit every element in turn.
• The counter runs from 0 to length − 1.
• You can also loop over every item directly: FOR t IN temps.

temps ← [18, 21, 17, 22, 19]
total ← 0
FOR i ← 0 TO LEN(temps) - 1
  total ← total + temps[i]
ENDFOR
OUTPUT total

All output 97.

2D arrays

• A two-dimensional (2D) array is like a table: one identifier, with rows and columns.
• Each element needs two indexes: board[row][col] (row first here).
• A noughts and crosses board is a 3 × 3 2D array of characters.

board ← [[' ', ' ', ' '],
         [' ', ' ', ' '],
         [' ', ' ', ' ']]
board[1][1] ← 'X'    # centre square
board[2][0] ← 'O'    # bottom-left square

Records

• A record groups related data about one item under one name.
• Each item of data in a record is a field. Fields can have different data types.
• Compare: an array's elements all share one data type, but a record mixes types.

Records in code

• Declare the record's fields and types once, create a record from it, then reach a field with a dot: myPet.age.

RECORD Pet
  name : String
  species : String
  age : Integer
  weight : Real
ENDRECORD

myPet ← Pet('Biscuit', 'dog', 4, 12.5)
myPet.age ← 5
OUTPUT myPet.name     # Biscuit

Two subroutines both use a variable called total. Why no clash?

Subroutines

• A subroutine is a named, out-of-line block of code. It runs when you call it by its name, then control returns to the line after the call.
• A procedure carries out a task. A function carries out a task and returns a value.
• In AQA pseudo-code, a subroutine containing RETURN is a function; one without is a procedure.

SUBROUTINE welcome()
  OUTPUT 'Welcome to the quiz'
ENDSUBROUTINE

welcome()
welcome()

Parameters and return values

• A parameter passes data into a subroutine when it is called. Values are matched to parameters in order.
• A function sends its result back with return. The returned value replaces the call, so store it or use it straight away.
• AQA uses the word parameter both for the names in the definition and for the values passed in.

SUBROUTINE area(width, height)
  result ← width * height
  RETURN result
ENDSUBROUTINE

roomArea ← area(4, 5)
OUTPUT roomArea    # 20

Local and global variables

• A local variable is declared inside a subroutine: it only exists while that subroutine runs and is only usable inside it.
• (Context) A global variable is declared in the main program and can be used anywhere.
• Good practice: nothing outside can change a local variable, and its name can be reused elsewhere.

SUBROUTINE addBonus(score)
  newScore ← score + 5    # newScore is local
  RETURN newScore
ENDSUBROUTINE

OUTPUT addBonus(10)    # 15
OUTPUT newScore        # error: newScore does not exist here

Why use subroutines?

• Reuse: write code once and call it many times, so there is less to write and check.
• Decomposition: each subroutine solves one part and can be written and tested on its own.
• Maintenance: fix or improve the code in one place.
• The structured approach: a modular program whose subroutines have clear, well-documented interfaces (parameters, return values, local variables).

How does an app show only the last 4 digits of your card number?

Strings have positions

• A string is a sequence of characters. Each character has a position, its index.
• The first character is at index 0, so the last is at length − 1.
• The length of a string is how many characters it holds, spaces included.
• A card number is stored as a string, not an integer: it is never used in arithmetic.

card ← '4929123456789012'
OUTPUT LEN(card)
# outputs 16

Taking a substring

• A substring is part of a string, picked out by position.
• SUBSTRING(start, end, string) uses a start and an end index; both are included.
• Using the length makes the code work for any card length.

card ← '4929123456789012'
last4 ← SUBSTRING(LEN(card) - 4, LEN(card) - 1, card)
OUTPUT '****' + last4

More string tools

• Concatenation joins strings with +: 'computer' + 'science' gives 'computerscience'.
• POSITION('computer', 'p') gives 3, the index of the first 'p'.
• CHAR_TO_CODE('a') gives 97; CODE_TO_CHAR(97) gives 'a'.
• STRING_TO_INT('16') gives 16; INT_TO_STRING, STRING_TO_REAL and REAL_TO_STRING convert the other ways.

Random numbers

• Games, quizzes and simulations need unpredictable values, such as a dice roll.
• The range you give includes both ends: (1, 6) can give 1, 2, 3, 4, 5 or 6.

dice ← RANDOM_INT(1, 6)
OUTPUT dice

You typed 41, not 14, for your age. Why did the form accept it?

What validation does

• Validation is an automatic check that input is sensible and follows rules before the program uses it.
• Invalid data is rejected, and the user is told why and asked again.
• It stops bad data causing crashes or wrong results later.
• Validation cannot tell whether data is correct: a mistyped value that still follows the rules gets through.

Types of validation check

Writing a validation routine

• Ask for input, test it, and keep asking until it passes.
• A loop does this: the user cannot move on with invalid data.
• This WHILE loop is a range check for 1 to 10.

score ← STRING_TO_INT(USERINPUT)
WHILE score < 1 OR score > 10
  OUTPUT 'Must be 1 to 10'
  score ← STRING_TO_INT(USERINPUT)
ENDWHILE

Authentication

• Authentication confirms the identity of a user before they get access, e.g. with a username and password.
• The program compares what is typed with the stored details.

username ← USERINPUT
password ← USERINPUT
IF username = 'sam01' AND password = 'Tr33house' THEN
  OUTPUT 'Access granted'
ELSE
  OUTPUT 'Access denied'
ENDIF

It passed every test. So why did it fail for an 18-year-old?

Why test?

• Testing checks that a program works as intended and meets its requirements.
• It finds errors so they can be fixed before users meet them.
• A program can run without crashing and still give wrong results.
• Passing a few easy tests proves little: choose test data that could expose errors.

Normal, edge and wrong data

For a program that accepts whole numbers from 1 to 10:

• Normal (typical) data: sensible values that should be accepted, e.g. 5.
• Boundary (extreme) data: values either side of each limit: 0, 1, 10 and 11.
• Erroneous data: data that should be rejected, e.g. ten or 15.

Types of error

• A syntax error breaks the rules of the language, e.g. a missing bracket. The program cannot be translated or run.
• A logic error lets the program run, but it gives the wrong or unexpected result, e.g. > where ≥ was needed.
• Testing with good test data is how you find logic errors.

Big ideas

• A variable is a named memory location whose value can change; a constant keeps one value while the program runs.
• Choose the data type that fits the data: integer, real, Boolean, character or string, and cast between types when needed (input text to an integer).
• Integer division gives the whole-number part; the remainder (modulus) gives what is left over.
• Sequence runs statements in order; selection uses a true/false condition to choose a branch.
• Count-controlled loops repeat a fixed number of times; condition-controlled loops repeat until a condition changes, tested at the start or the end.
• In nested loops the inner loop runs fully for every pass of the outer loop.
• An array holds many values of one data type under one identifier; an index picks out an element (the first is index 0).
• A 2D array is a table of rows and columns, so each element needs two indexes.
• A record groups fields of different data types about one item.
• A subroutine is a named block of code called by its name; a function returns a value and a procedure does not.
• Parameters pass data in, in order; return passes a result back to the call.
• Local variables exist only inside their subroutine while it runs, so the same name can be reused safely.
• A string is a sequence of characters, each with an index; the length is how many characters it has. Indexes start at 0.
• A substring picks out part of a string by position.
• Concatenation joins strings with +.
• Random number functions give a value in a range, including both ends.
• Validation automatically checks input is sensible, e.g. range, length and presence checks, and rejects data that breaks the rules; it cannot check data is correct.
• Authentication confirms who a user is, e.g. by matching a username and password.
• Choose test data of every kind: normal, at the edges of the range, and data that should be rejected (normal, boundary, erroneous).
• Syntax errors stop a program running; logic errors give wrong results.