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.

CONST INTEGER MAX_LIVES
SET MAX_LIVES TO 3
SET lives TO MAX_LIVES
RECEIVE name FROM (STRING) KEYBOARD
SEND 'Good luck, ' & name TO DISPLAY
SET lives TO lives - 1
SEND lives TO DISPLAY

Data types

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

• Command-set keywords: INTEGER, REAL, BOOLEAN, CHARACTER; a STRING is treated as a data structure.

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.

SET total TO 200
SET minutes TO total DIV 60    # 3
SET seconds TO 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.
• The command set calls this type coercion: integer + real gives a real (3 + 8.25 = 11.25), and RECEIVE … FROM (INTEGER) reads input as an integer.

RECEIVE age FROM (INTEGER) KEYBOARD
SEND age + 1 TO DISPLAY
SET total TO 3 + 8.25    # coerced to REAL: 11.25

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.

SET mark TO 72
IF mark >= 70 THEN
   SEND 'Distinction' TO DISPLAY
ELSE
   IF mark >= 40 THEN
      SEND 'Pass' TO DISPLAY
   ELSE
      SEND 'Fail' TO DISPLAY
   END IF
END IF

Count-controlled loops

• Iteration repeats a block of code.
• A count-controlled loop repeats a fixed number of times. A counter variable goes up by 1 each time unless a step is given.
• REPEAT 5 TIMES … END REPEAT is another count-controlled loop.

FOR count FROM 1 TO 5 DO
   SEND count * 3 TO DISPLAY
END FOR

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

Condition-controlled loops

• A condition-controlled loop 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 tests at the end, so it always runs at least once.

SET total TO 1
WHILE total < 50 DO
   SET total TO total * 2
END WHILE
SEND total TO DISPLAY

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 FROM 1 TO 3 DO
   FOR col FROM 1 TO 3 DO
      SEND row * col TO DISPLAY
   END FOR
END FOR

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.

SET scores TO [12, 7, 9, 15, 4]
SEND scores[1] TO DISPLAY    # 7
SET scores[3] TO 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 EACH t FROM temps DO.

SET temps TO [18, 21, 17, 22, 19]
SET total TO 0
FOR i FROM 0 TO LENGTH(temps) - 1 DO
   SET total TO total + temps[i]
END FOR
SEND total TO DISPLAY

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 is a 3 x 3 array of characters
SET board[1, 1] TO 'X'    # centre square
SET board[2, 0] TO '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.

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.
• Your board calls subroutines subprograms.

PROCEDURE welcome ()
BEGIN PROCEDURE
   SEND 'Welcome to the quiz' TO DISPLAY
END PROCEDURE

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.

FUNCTION area (width, height)
BEGIN FUNCTION
   SET result TO width * height
   RETURN result
END FUNCTION

SET roomArea TO area (4, 5)
SEND roomArea TO DISPLAY    # 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.
• 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.

FUNCTION addBonus (score)
BEGIN FUNCTION
   SET newScore TO score + bonus    # newScore is local
   RETURN newScore
END FUNCTION

SET bonus TO 5                      # bonus is global
SEND addBonus (10) TO DISPLAY       # 15
SEND newScore TO DISPLAY            # 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.
• Library routines are ready-made subroutines, e.g. LENGTH(), RANDOM(n).

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.

SET Card TO '4929123456789012'
SEND LENGTH(Card) TO DISPLAY
# displays 16

Taking a substring

• A substring is part of a string, picked out by position.
• Read characters one at a time by index, string[index], and join them with &.
• Using the length makes the code work for any card length.

SET Card TO '4929123456789012'
SET Last4 TO ''
FOR Index FROM LENGTH(Card) - 4 TO LENGTH(Card) - 1 DO
  SET Last4 TO Last4 & Card[Index]
END FOR
SEND '****' & Last4 TO DISPLAY

More string tools

• & joins (appends) strings; a non-string joined with & becomes a string: 'Fred' & 18 gives 'Fred18'.

Random numbers

• Games, quizzes and simulations need unpredictable values, such as a dice roll.
• RANDOM(n) gives a number from 0 to n, so add 1 to start at 1.

SET Dice TO RANDOM(5) + 1
SEND Dice TO DISPLAY

Reading and writing files

• Variables are lost when a program ends. A file in secondary storage keeps data, such as high scores, for next time.
• Open the file, read or write it, then close it so the data is saved and the file is freed.
• READ reads the next record from a file; WRITE writes a record to it.

WRITE Scores.txt 'Zara', 42
READ Scores.txt Record
SEND Record TO DISPLAY

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.

RECEIVE Score FROM (INTEGER) KEYBOARD
WHILE Score < 1 OR Score > 10 DO
  SEND 'Must be 1 to 10' TO DISPLAY
  RECEIVE Score FROM (INTEGER) KEYBOARD
END WHILE

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 data: sensible values that should be accepted, e.g. 5.
• Boundary data: values at the edges of the range, 1 and 10, and just outside them, 0 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.
• A runtime error happens while the program runs and makes it crash, e.g. dividing by zero or using an index past the end of an array.
• Testing with good test data is how you find logic errors.
• Read the error message: it gives the error type and line number. Fix that line, then run again.

Test plans

• A test plan lists each test before it is run: what it tests, the test data, its type and the expected result.
• After running, record the actual result. If they differ, fix the code and test again.

• Test 2 failed, so the range check needs fixing.

Code that is easy to maintain

• Other people (and you, later) must be able to read and change code. This is maintainability.
• Descriptive names: totalScore, not t.
• Comments explain what each part of the code does.
• Indentation shows which lines belong inside loops, selection and subprograms.
• Subprograms break code into named, reusable parts.

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; global variables can be used anywhere.
• 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.
• Files keep data after a program ends: open, read or write, close.
• 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.
• 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; runtime errors crash it while it runs.
• Meaningful names, comments, indentation and subprograms make code easy to maintain.