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 maxLives = 3
lives = maxLives
name = input("Enter your name")
print("Good luck, " + name)
lives = lives - 1
print(lives)

Data types

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

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.

• OCR calls DIV the quotient and MOD modulo.

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.
• OCR: int(), float() or real(), str(), bool(). The change is temporary: the original variable keeps its type.

age = int(input("Enter your age"))
print(age + 1)
print("Next year: " + str(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
    print("Distinction")
elseif mark >= 40 then
    print("Pass")
else
    print("Fail")
endif

CASE statements

• A switch (CASE SELECT) statement picks one branch by comparing one variable with a list of values.
• Cases are checked in order; the first match runs, then the statement ends.
• default runs if no case matches.
• It is neater than a long chain of IFs when you test one variable for exact values.

day = input("Enter a day")
switch day:
    case "Sat":
        print("Weekend")
    case "Sun":
        print("Weekend")
    default:
        print("Weekday")
endswitch

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.

for count = 1 to 5
    print(count * 3)
next count

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.
• do … until tests at the end, so it always runs at least once.

total = 1
while total < 50
    total = total * 2
endwhile
print(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
        print(row * col)
    next col
next row

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, and the array has a fixed length (static).
• An index picks out one element. The first element has index 0.

array scores = [12, 7, 9, 15, 4]
print(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.

array temps = [18, 21, 17, 22, 19]
total = 0
for i = 0 to 4
    total = total + temps[i]
next i
print(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.
• A 2D array can emulate a database table: each row is a record and each column a field.

array board[3,3]
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.

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 sub programs.

procedure welcome()
    print("Welcome to the quiz")
endprocedure

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)
    result = width * height
    return result
endfunction

roomArea = area(4, 5)
print(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.
• 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.
• Declare a global with the global keyword. Constants can also be local or global.

global bonus = 5
function addBonus(score)
    newScore = score + bonus    // newScore is local
    return newScore
endfunction

print(addBonus(10))    // 15
print(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.
• Arrays can be passed into, and returned from, sub programs.

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"
print(card.length)
// prints 16

Taking a substring

• A substring is part of a string, picked out by position.
• string.substring(x, i) starts at index x and takes i characters; .left(i) and .right(i) take characters from either end.
• Using the length makes the code work for any card length.

card = "4929123456789012"
last4 = card.substring(card.length - 4, 4)
print("****" + last4)

More string tools

• Concatenation joins strings with +.
• .upper and .lower change case: "Hi".upper gives "HI".
• ASC("A") gives 65; CHR(97) gives "a".

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(1, 6)
print(dice)

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.
• open() opens a file; .readLine() reads the next line; .writeLine() adds a line to the end; .endOfFile() is True after the last line; newFile() creates a file.

scores = open("scores.txt")
scores.writeLine("Zara,42")
scores.close()
scores = open("scores.txt")
while NOT scores.endOfFile()
  print(scores.readLine())
endwhile
scores.close()

How can one line of SQL find every overdue book in a library?

SELECT and FROM

• SQL (Structured Query Language) is used to search for data in a database.
• SELECT lists the fields you want to see, separated by commas.
• FROM names the table to search.
• This query shows the Title and DaysLate of every record in Loan:

SELECT Title, DaysLate
FROM Loan

WHERE picks the records

• WHERE gives a condition. Only records that meet it are returned.
• Compare with = < > <= >= and <> (not equal).
• Put text values in quotes: Genre = 'Fantasy'. Numbers need no quotes.

SELECT Title, DaysLate
FROM Loan
WHERE Genre = 'Fantasy'

Result: Coraline 0, Skellig 7, Matilda 5.

Writing a query

• Read the question for the fields to show (SELECT), the table (FROM) and the rule (WHERE).
• Fields in WHERE do not have to appear in SELECT.

SELECT Title, Genre
FROM Loan
WHERE DaysLate > 4

Returns Skellig Fantasy and Matilda Fantasy.

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 (if or switch).
• 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.
• Concatenation joins strings with +.
• Files keep data after a program ends: open, read or write, close.
• Random number functions give a value in a range, including both ends.
• SELECT fields FROM table WHERE condition: the fields to show, the table to search and the records to keep.