1.1Number systems

A computer is built from billions of tiny switches called transistors. Each switch is either off or on, so the natural way to store anything is as a pattern of two states: 0 and 1. That is why every kind of data (numbers, letters, pictures, music, program instructions) is stored as binary.

Binary to denary

Each bit has a place value that doubles as you move left. Add up the place values where the bit is 1.

128 + 16 + 8 + 4 = 156. To go the other way, subtract place values from the largest downwards: 156 − 128 = 28, then 28 − 16 = 12, 12 − 8 = 4, 4 − 4 = 0, which gives 10011100.

Binary and hexadecimal

One hex digit represents exactly four bits (a nibble), so conversion is a matter of splitting into groups of four. Hex letters: A = 10, B = 11, C = 12, D = 13, E = 14, F = 15.

1001 1100 → 9 and C → 9C. In the other direction, 2B → 0010 and 1011 → 00101011 = 43.

Binary addition and overflow

Rules for each column, working right to left: 0 + 0 = 0; 0 + 1 = 1; 1 + 1 = 0 carry 1; 1 + 1 + 1 = 1 carry 1.

  10110110   (182)
+ 01101011   (107)
----------
 100100001   (289)  ← 9 bits

Register holds 8 bits, so the left-hand 1 is lost:  00100001 (33)  → OVERFLOW ERROR

Overflow happens when the result is too big for the number of bits available. In 8 bits the largest value is 255, so anything above 255 overflows and the answer stored is wrong.

Logical shifts

A logical shift moves every bit left or right by a number of places. Bits that fall off the end are lost, and the gaps are filled with 0.

• Left shift by 1 multiplies by 2. 00001101 (13) shifted left 2 places gives 00110100 (52).
• Right shift by 1 divides by 2 (whole number). 10110100 (180) shifted right 3 places gives 00010110 (22). The three bits 100 that fell off were lost, which is why 180 ÷ 8 = 22.5 became 22.
• Shifting can cause loss of precision or overflow if a 1 falls off the left end.

Two's complement (negative numbers)

The left-most bit has the value −128 instead of +128. Numbers with a leading 1 are negative. The range of 8-bit two's complement is −128 to +127.

1. Write the positive number in binary: 45 = 00101101.
2. Invert every bit: 11010010.
3. Add 1: 11010011.

Check: −128 + 64 + 16 + 2 + 1 = −45.

1.2Text, sound and images

Text

Every character is given a number in a character set, and that number is stored in binary. Letters follow in order, so once you know 'A' = 65 you know 'B' = 66; lower-case 'a' = 97; the digit character '0' = 48.

Sound

Real sound is analogue (a continuous wave). To store it, the wave is measured at regular intervals; this is called sampling. Each measurement is stored as a binary number.

Sample rate

Number of samples taken each second, in hertz (Hz). CD audio uses 44,100 Hz.

Sample resolution

Number of bits used for each sample (bit depth). CD audio uses 16 bits.

Raising either value makes the stored sound a closer match to the original wave, but the file becomes larger.

Images

An image is a grid of tiny squares called pixels. Each pixel stores a binary number that represents its colour.

Resolution

Width × height in pixels. More pixels means more detail and a larger file.

Colour depth

Bits per pixel. 1 bit = 2 colours, 8 bits = 256 colours, 24 bits = about 16.7 million colours.

Metadata is data about the file, such as the image width, height and colour depth, or the sound file's sample rate and length. The computer needs it to rebuild the file correctly.

1.3Data storage and compression

File size formulas

Image size (bits) = width × height × colour depth
Sound size (bits) = sample rate × sample resolution × length in seconds
Bytes = bits ÷ 8       KiB = bytes ÷ 1024       MiB = KiB ÷ 1024

Compression

Compression reduces file size, so files use less storage, take less bandwidth and transfer faster.

Run-length encoding: store each value once with a count of how many times it repeats. AAAABBBCC becomes 4A3B2C (9 characters shrink to 6). It works best on data with long runs, such as simple graphics.

2.1Types and methods of transmission

Packets

Data sent over a network is split into small units called packets. Each packet has three parts:

Header

Destination IP address, sender's IP address, packet number (so they can be put back in order), and the number of packets.

Payload

The actual piece of data being carried.

Trailer

Marks the end of the packet and usually holds an error-checking value.

Packet switching

1. The message is broken into packets, each numbered.
2. Each packet is sent separately. Routers read the header and choose the best route at that moment, so packets may take different routes.
3. Packets can arrive out of order. The receiving device uses the packet numbers to reassemble them.
4. If a packet is missing or damaged, the receiver asks for it to be sent again.

Advantages: no single route is tied up, so the network is used efficiently; if one route fails, packets take another; it is harder to intercept a whole message. Disadvantages: packets may arrive late or out of order, and reassembly takes time (which can hurt live audio or video).

Serial and parallel

Direction of transmission

Universal Serial Bus (USB)

USB sends data serially. Advantages: one standard connector on most devices; plug-and-play (the computer detects the device); can supply power; backwards compatible with older versions; USB-C plugs in either way round. Drawbacks: limited cable length; a lower maximum speed than some specialised connections; older versions are slower.

2.2Methods of error detection

Errors happen during transmission because of interference, electrical noise or a weak signal. Data can be lost, gained (extra bits) or changed (a 0 becomes a 1). Detection methods find out that something has gone wrong; they do not fix it.

Parity check

The sender and receiver agree on even or odd parity. One extra bit (the parity bit) is added so the total number of 1s is even (or odd). The receiver counts the 1s; if the count is wrong, an error occurred.

Example (even parity): 1011001 has four 1s, which is already even, so the parity bit is 0. 1011011 has five 1s, so the parity bit is 1.

Parity block (parity byte) check: the data is arranged in rows and columns and a parity bit is added for every row and every column. A single wrong bit is located where the failing row and column meet.

Checksum

A value is calculated from the data using an agreed algorithm and sent with the data. The receiver repeats the calculation. If the results differ, the data is corrupt and is sent again.

Echo check

The receiver sends a copy of the data it received back to the sender, who compares it with the original. If they differ, the data is sent again. It is not efficient because it doubles the traffic, and an error could have happened on the return trip instead.

Check digit

An extra digit at the end of a number (such as an ISBN or barcode) is calculated from the other digits. When the number is typed or scanned, the digit is recalculated. It catches data-entry mistakes such as a wrong digit or two digits swapped.

Automatic repeat query (ARQ)

1. The receiver checks each packet for errors.
2. If it is correct, a positive acknowledgement is sent. If not, a negative acknowledgement asks for it again.
3. The sender starts a timeout. If no acknowledgement arrives in time, it resends the packet.
4. This repeats until the packet is acknowledged, or a set number of attempts is reached.

2.3Encryption

Encryption scrambles data (plaintext) into unreadable ciphertext using an algorithm and a key. It does not stop data being intercepted; it makes intercepted data useless without the key.

To send a secret message to someone with asymmetric encryption, you encrypt with their public key. Only their private key can decrypt it.

3.1Computer architecture

The central processing unit (CPU) processes data and instructions. The Von Neumann architecture stores both program instructions and data in the same main memory (RAM) and fetches them one after another.

Arithmetic logic unit (ALU)

Carries out calculations and logical comparisons.

Control unit (CU)

Fetches and decodes instructions and sends control signals to coordinate the rest of the computer.

Program counter (PC)

Holds the address of the next instruction.

Memory address register (MAR)

Holds the address of the memory location to be read from or written to.

Memory data register (MDR)

Holds data or the instruction just fetched from memory, or about to be written to it.

Current instruction register (CIR)

Holds the instruction being decoded and executed.

Accumulator (ACC)

Holds the results of ALU calculations.

Buses

The fetch–decode–execute cycle

1. The address in the PC is copied to the MAR.
2. The PC is increased by 1, ready for the next instruction.
3. The instruction at that address is fetched from RAM along the data bus into the MDR.
4. The instruction is copied from the MDR to the CIR.
5. The CU decodes the instruction.
6. The instruction is executed (the ALU does any calculation, with results held in the ACC). The cycle then starts again.

What affects CPU performance

The instruction set is the list of machine-code commands a given CPU can carry out. An embedded system is a computer built into a device to do one dedicated job, such as a washing machine controller, a microwave, traffic lights or a car engine unit. A general-purpose computer, by contrast, can run many different programs.

3.2Input and output devices

An input device sends data into the computer; an output device sends data or results out.

Sensors

A sensor measures a physical quantity and turns it into a signal the computer can use. Know the common types and one use for each:

3.3Data storage

Primary storage is directly accessed by the CPU (RAM, ROM). Secondary storage is non-volatile and keeps files long-term; the CPU cannot access it directly.

Virtual memory

If RAM fills up, the operating system moves sections of data that are not currently needed (pages) out to a reserved area of secondary storage, and brings them back when needed. This lets the computer run more programs than RAM alone can hold, but secondary storage is much slower than RAM, so too much paging makes the computer sluggish.

Cloud storage

Data is stored on remote servers and accessed over the internet. Benefits: reachable from any device, automatic backup, easy to add space. Drawbacks: needs an internet connection, ongoing subscription cost, you rely on the provider's security and reliability.

3.4Network hardware

Network interface card (NIC)

Allows a device to connect to a network, wired or wireless. It stores the device's MAC address.

Router

Connects networks together (for example your home LAN to the internet). It reads each packet's destination IP address and forwards it, and it gives IP addresses to devices on the local network.

IPv6 exists because the world is running out of IPv4 addresses. An IP address can be static (never changes, used for servers) or dynamic (a new one is given each time a device connects, which is the norm at home).

4.1Types of software and interrupts

What an operating system does

• User interface: a GUI (icons, windows) or a command-line interface.
• Memory management: decides which programs use which part of RAM and controls virtual memory.
• Process management and multitasking: shares CPU time so several programs appear to run at once.
• File management: creates, names, moves, copies and deletes files and folders.
• Peripheral management: controls input and output devices, using device drivers.
• Security: user accounts, passwords and access rights.
• Providing a platform on which application software can run.

Utility software and drivers

Utilities are small programs that maintain the computer: antivirus, backup, disk formatting, defragmentation (only useful on hard disks, not SSDs), file compression and disk clean-up. A device driver tells the OS how to communicate with one particular piece of hardware such as a printer.

Interrupts

An interrupt is a signal sent to the processor asking it to stop what it is doing because something needs attention.

1. An interrupt signal is generated.
2. At the end of each fetch–decode–execute cycle the CPU checks for interrupts.
3. If the interrupt has a higher priority than the current task, the CPU saves the current state (register contents).
4. The interrupt service routine (ISR) for that interrupt is run.
5. The saved state is restored and the interrupted task carries on.

4.2Languages, translators and IDEs

An assembler translates assembly language into machine code.

Integrated development environment (IDE)

An IDE bundles the tools a programmer needs into one program:

• Code editor with syntax highlighting, auto-completion, auto-indentation and pretty-printing.
• Error diagnostics to point out mistakes and a debugger with breakpoints, single-stepping and variable watches.
• A built-in translator (compiler or interpreter) and a run-time environment to test the program.

5.1The internet and the World Wide Web

URL structure

https://www.example.com/school/index.html
└─┬─┘   └──────┬──────┘└────┬────┘└───┬────┘
protocol   domain name       path    file name

The web browser reads the HTML and displays the page. The web server stores the pages and sends them when requested.

What happens when you type a URL

1. The browser asks a DNS server for the IP address of the domain name.
2. If that server does not know it, it asks other DNS servers until the IP address is found and returned.
3. The browser sends a request to the web server at that IP address.
4. The web server sends back the page (as HTML), and the browser renders it.

HTTP and HTTPS

HTTP is the protocol for sending web pages, but the data is not encrypted. HTTPS adds SSL/TLS: the browser checks the server's digital certificate, they agree a session key using asymmetric encryption, and all data after that is encrypted. A padlock icon shows HTTPS is in use.

Cookies

A cookie is a small text file stored by the browser when you visit a site.

• Session cookies are held in memory and deleted when the browser closes (for example a shopping basket or keeping you logged in during a visit).
• Persistent cookies stay on the device until they expire or are deleted (remembering login details, language and preferences, or tracking browsing to target adverts).

5.2Digital currency

Digital currency exists only in electronic form. Cryptocurrencies are decentralised: there is no bank in control.

A blockchain is the digital ledger that records every transaction:

• Transactions are grouped into blocks. Each block stores its data, a timestamp, its own hash and the hash of the previous block, so the blocks form a chain.
• If someone changes a block, its hash changes and no longer matches the next block. The chain is broken and the tampering is obvious.
• Many computers hold a copy of the ledger and agree on new blocks together, so there is no single point to attack.

Benefits include fast transfers and no bank fees. Drawbacks include price swings, high energy use for some systems, no way to recover lost keys, and use in scams.

5.3Cyber security

Malware

Protection methods

Firewall

Checks incoming and outgoing traffic against a set of rules and blocks anything unauthorised.

Anti-malware

Scans for, quarantines and removes malicious software; keep it updated.

Authentication

Passwords, biometrics (fingerprint, face, iris, voice) and two-factor authentication (two different proofs, such as a password plus a code sent to a phone).

Proxy server

Sits between users and the internet. It can filter websites, cache pages and hide users' IP addresses.

Access levels

Users only see the data they need for their role.

Software updates

Fix known security holes; turn on automatic updates.

6.1Automated systems

An automated system uses sensors, a microprocessor and actuators to run a process with little or no human help. It works as a feedback loop:

1. Sensors measure a physical value (for example temperature).
2. If the signal is analogue, an analogue-to-digital converter (ADC) turns it into digital data.
3. The microprocessor compares the reading with a stored target value.
4. If action is needed, it sends a signal (through a digital-to-analogue converter, DAC, if required) to an actuator such as a heater or motor.
5. The sensor keeps measuring, so the result of the action feeds back in. The loop repeats.

Other examples: automatic street lighting, central heating, automatic doors, air-conditioning, self-driving vehicles, farm drones and factory production lines.

6.2Robotics and artificial intelligence

Robotics

A robot is a machine with a mechanical structure, electrical parts (sensors, motors and a controller) and a program that lets it carry out tasks. Uses: welding and painting in factories, exploring places people cannot go, surgery, vacuum cleaning, warehouse picking and farming.

Benefits: precise, tireless, safe in hazardous places. Drawbacks: expensive, can replace workers, cannot adapt to situations they were not programmed for.

Artificial intelligence (AI)

AI is a computer system that carries out tasks normally needing human intelligence, such as learning, reasoning, problem solving and recognising speech or images.

Machine learning is a part of AI in which a system improves at a task by learning from data instead of being told every rule. It is used in recommendations, spam filters and voice assistants.

Expert systems

An expert system imitates a human specialist to give advice or a diagnosis. It has four main parts:

Knowledge base

A large database of facts about the subject.

Rule base

IF…THEN rules that link facts to conclusions.

Inference engine

The reasoning part. It applies the rules to the knowledge base and the user's answers to reach a conclusion.

User interface

Lets the user answer questions and read the results, often with an explanation of how the conclusion was reached.

Uses: medical diagnosis, finding faults in car engines, mineral prospecting, financial advice, chess.

1Program development life cycle (SDLC)

Programs are built in stages, so that mistakes are found early when they are cheap to fix.

2A program: input, process, output

Every program takes input, applies a process and produces output. Storage (files and databases) keeps data between runs. Designs are drawn as flowcharts or structure diagrams.

Flowchart symbols

Structure diagram

A structure diagram breaks a system into sub-systems, from the overall task at the top down to small jobs at the bottom (top-down design). It shows what the parts are, not the order they run in.

• Library system
  • Members
    • Register member
    • Update details
  • Books
    • Add book
    • Search for book
  • Loans
    • Issue book
    • Return book
    • Calculate fine

3Data types

4Operators

5Variables and constants

A variable is a named memory location whose value can change while the program runs (a score, a counter, a total). A constant is a named value that does not change (VAT rate, pi, maximum class size). Use meaningful names, and use constants so a value only needs changing in one place.

DECLARE Score : INTEGER
CONSTANT VATRate ← 0.2
Score ← 0
Score ← Score + 10

VAT_RATE = 0.2        # constant by convention (capitals)
score = 0
score = score + 10

6Selection

Selection lets the program choose which statements to run, depending on a condition.

INPUT Mark
IF Mark >= 70 THEN
   OUTPUT "Distinction"
ELSE
   IF Mark >= 40 THEN
      OUTPUT "Pass"
   ELSE
      OUTPUT "Fail"
   ENDIF
ENDIF

mark = int(input("Mark: "))
if mark >= 70:
    print("Distinction")
elif mark >= 40:
    print("Pass")
else:
    print("Fail")

When one variable is compared with many fixed values, use CASE OF (Python 3.10+: match):

CASE OF Choice
   1 : OUTPUT "Start game"
   2 : OUTPUT "Options"
   OTHERWISE : OUTPUT "Invalid choice"
ENDCASE

7Iteration (loops)

FOR Count ← 1 TO 5
   OUTPUT Count
NEXT Count

REPEAT
   INPUT Password
UNTIL Password = "cs0478"

WHILE Lives > 0 DO
   PlayRound
ENDWHILE

for count in range(1, 6):    # 1 to 5
    print(count)

while True:                  # REPEAT-UNTIL pattern
    password = input("Password: ")
    if password == "cs0478":
        break

while lives > 0:
    play_round()

8Maximum, minimum, counting and totalling

These four standard algorithms appear in almost every Paper 2 question. Start the total and count at 0, and start the max and min at the first value, not at 0.

Total ← 0
Count ← 0
Highest ← Marks[1]
Lowest ← Marks[1]
FOR Index ← 1 TO 5
   Total ← Total + Marks[Index]
   Count ← Count + 1
   IF Marks[Index] > Highest THEN
      Highest ← Marks[Index]
   ENDIF
   IF Marks[Index] < Lowest THEN
      Lowest ← Marks[Index]
   ENDIF
NEXT Index
Average ← Total / Count

marks = [72, 85, 60, 91, 55]
total = 0
count = 0
highest = marks[0]
lowest = marks[0]
for m in marks:
    total = total + m
    count = count + 1
    if m > highest:
        highest = m
    if m < lowest:
        lowest = m
average = total / count

Result for this data: total 363, count 5, highest 91, lowest 55, average 72.6.

9Lists and one-dimensional arrays

An array stores several items of the same data type under one name. Each item is reached by its index. A Python list written [item1, item2, …] is used for arrays. Two-dimensional arrays (rows and columns) are covered in Chapter 8.

DECLARE Names : ARRAY[1:3] OF STRING
Names[1] ← "Aung"
Names[2] ← "Su"
Names[3] ← "Zin"
FOR Index ← 1 TO 3
   OUTPUT Names[Index]
NEXT Index

names = ["Aung", "Su", "Zin"]
print(names[0])          # Aung
names[1] = "Mya"         # change an item
names.append("Kyaw")     # add to the end
for i in range(len(names)):
    print(names[i])

10Linear search and bubble sort

Linear search

Check each item in turn from the start until the target is found or the end is reached. It works on unsorted data, but is slow on long lists because in the worst case every item is checked.

Found ← FALSE
Index ← 1
WHILE Index <= 5 AND Found = FALSE
   IF Data[Index] = Target THEN
      Found ← TRUE
   ELSE
      Index ← Index + 1
   ENDIF
ENDWHILE
IF Found THEN
   OUTPUT "Found at ", Index
ELSE
   OUTPUT "Not found"
ENDIF

found = False
index = 0
while index < len(data) and not found:
    if data[index] == target:
        found = True
    else:
        index = index + 1
if found:
    print("Found at", index)
else:
    print("Not found")

Bubble sort

Compare each pair of neighbouring items and swap them if they are in the wrong order. After each pass the largest unsorted value has "bubbled" to the end. Keep going until a full pass makes no swaps.

n = len(data)
for i in range(n - 1):
    swapped = False
    for j in range(n - 1 - i):
        if data[j] > data[j + 1]:
            temp = data[j]
            data[j] = data[j + 1]
            data[j + 1] = temp
            swapped = True
    if not swapped:
        break

11Validation and verification

Validation is an automatic check that data is reasonable (sensible and in the right form). It cannot prove the data is correct: a date of birth of 01/01/2000 passes even if the real date is 05/03/2000. Verification checks that the data entered is the same as the source data or was not changed in transfer.

Verification methods: double entry (type it twice and the computer compares, as with a new password) and a visual check (a person compares the screen with the original document).

mark = int(input("Mark (0-100): "))
while mark < 0 or mark > 100:
    print("Invalid: enter a number from 0 to 100")
    mark = int(input("Mark (0-100): "))

Test data

Test data is chosen to show the program works for every kind of input. Example: a program that accepts a whole-number mark from 0 to 100.

12Trace tables

A trace table is a "dry run" on paper: one column for each variable (and output), one row for each change, so you can follow the algorithm step by step. It is used to find logic errors and to work out what an algorithm does. Fill in a value only in the row where it changes, unless the question says otherwise.

Total ← 0
Count ← 0
FOR i ← 1 TO 4
   INPUT Num
   IF Num MOD 2 = 0 THEN
      Total ← Total + Num
      Count ← Count + 1
   ENDIF
NEXT i
OUTPUT Total, Count

Inputs in order: 5, 8, 3, 6. The algorithm totals and counts the even numbers.

8.1Input, output and casting

input() always returns a string. Convert it (cast it) before doing arithmetic.

INPUT Name
INPUT Age
OUTPUT "Hello ", Name
OUTPUT "Next year you will be ", Age + 1

name = input("Name: ")
age = int(input("Age: "))    # int() casts text
print("Hello", name)
print("Next year you will be", age + 1)

Whole-number division and remainder: 17 // 5 is 3 and 17 % 5 is 2 (pseudocode DIV and MOD). Use round(x, 2) to round to 2 decimal places.

8.2Strings and upper / lower case

Strings cannot be changed in place; the methods above return a new string. Some versions of the pseudocode guide write TO_UPPER and TO_LOWER instead of UCASE and LCASE.

Why change case? To compare fairly. A user might type "YES", "yes" or "Yes"; converting the input first means one test covers them all.

answer = input("Play again? (yes/no): ").lower()
if answer == "yes":
    print("Restarting")

word = input("Word: ").lower()
vowels = 0
for ch in word:
    if ch == "a" or ch == "e" or ch == "i" or ch == "o" or ch == "u":
        vowels = vowels + 1
print("Vowels:", vowels)

8.3One-dimensional arrays

Use a loop to fill and process an array. A loop with an index is the standard way to read, total or search every item.

DECLARE Marks : ARRAY[1:5] OF INTEGER
FOR Index ← 1 TO 5
   OUTPUT "Enter mark ", Index
   INPUT Marks[Index]
NEXT Index

marks = [0, 0, 0, 0, 0]          # 5 items, indexes 0 to 4
for i in range(5):
    marks[i] = int(input("Enter mark " + str(i + 1) + ": "))

Useful list tools: len(marks) (number of items), marks.append(x) (add to the end), marks[-1] (last item).

8.4Two-dimensional arrays

A 2D array is a table with rows and columns, and needs two indexes: Grid[row][column] in Python and Grid[row, column] in pseudocode. Use a nested loop: the outer loop moves down the rows and the inner loop moves across the columns.

DECLARE Grid : ARRAY[1:3, 1:3] OF INTEGER
FOR Row ← 1 TO 3
   FOR Col ← 1 TO 3
      Grid[Row, Col] ← 0
   NEXT Col
NEXT Row
Grid[2, 3] ← 7

grid = [[0, 0, 0],
        [0, 0, 0],
        [0, 0, 0]]
grid[1][2] = 7          # row 2, column 3 (counting from 0)

total = 0
for r in range(3):
    for c in range(3):
        total = total + grid[r][c]
print("Total:", total)   # 7

8.5File handling

Variables lose their values when a program ends. A text file keeps data between runs. The three modes are read, write (creates the file, or overwrites the old contents) and append (adds to the end). Always close the file when finished.

OPENFILE "scores.txt" FOR WRITE
WRITEFILE "scores.txt", "Aung,85"
WRITEFILE "scores.txt", "Su,91"
CLOSEFILE "scores.txt"

file = open("scores.txt", "w")
file.write("Aung,85\n")
file.write("Su,91\n")
file.close()

DECLARE Line : STRING
OPENFILE "scores.txt" FOR READ
WHILE NOT EOF("scores.txt") DO
   READFILE "scores.txt", Line
   OUTPUT Line
ENDWHILE
CLOSEFILE "scores.txt"

file = open("scores.txt", "r")
for line in file:            # loops until the end
    print(line.strip())      # strip() removes \n
file.close()

file = open("scores.txt", "a")
file.write("Zin,67\n")
file.close()

names = []
scores = []
file = open("scores.txt", "r")
for line in file:
    parts = line.strip().split(",")     # "Aung,85" → ["Aung", "85"]
    names.append(parts[0])
    scores.append(int(parts[1]))
file.close()

8.6Procedures and functions

A procedure carries out a task. A function carries out a task and returns a value. Both can take parameters. Variables created inside are local (exist only in that subprogram); a global variable is available everywhere. Subprograms make code shorter, easier to test and reusable.

PROCEDURE Greet(Name : STRING)
   OUTPUT "Hello ", Name
ENDPROCEDURE

FUNCTION Area(L : REAL, W : REAL) RETURNS REAL
   RETURN L * W
ENDFUNCTION

CALL Greet("Aung")
Size ← Area(4, 2.5)

def greet(name):
    print("Hello", name)

def area(l, w):
    return l * w

greet("Aung")
size = area(4, 2.5)

8.7Linear search and bubble sort: complete programs

Chapter 7 explained the ideas. These are full versions with input, so you can run them.

names = ["Aung", "Su", "Zin", "Kyaw", "Mya"]
target = input("Name to find: ")
found = False
for i in range(len(names)):
    if names[i].lower() == target.lower():
        print("Found at position", i + 1)
        found = True
if not found:
    print("Not found")

marks = [72, 85, 60, 91, 55]
n = len(marks)
for i in range(n - 1):
    swapped = False
    for j in range(n - 1 - i):
        if marks[j] > marks[j + 1]:
            temp = marks[j]
            marks[j] = marks[j + 1]
            marks[j + 1] = temp
            swapped = True
    if not swapped:
        break
print(marks)       # [55, 60, 72, 85, 91]

To sort into descending order, change > to < in the comparison.

9.1Database concepts

A database is an organised collection of data. In a table, each field is a column (one piece of data about every item), each record is a row (everything about one item), and the table holds many records. The syllabus uses single-table databases only.

Primary key

A field whose value is unique for every record, so it identifies exactly one record. It cannot be empty or duplicated. Examples: StudentID, ISBN.

Data types

Text (alphanumeric), Character, Integer, Real, Boolean (Yes/No) and Date/Time. Choose the one that matches the data; a phone number is text.

Validation

Range, length, type, presence and format checks stop bad data entering a field.

The sample table used below is called Students. StudentID is the primary key.

9.2Structured Query Language (SQL)

SELECT   field1, field2       -- which columns to show (* means all fields)
FROM     table                -- which table
WHERE    condition            -- which records (optional)
ORDER BY field ASC|DESC;      -- sort order (optional; ASC is the default)

Worked queries on the Students table

SELECT FirstName, LastName
FROM Students
WHERE Grade >= 80
ORDER BY LastName ASC;

SELECT StudentID, FirstName, Grade
FROM Students
WHERE Class = "10A" AND Grade > 60;

SELECT *
FROM Students
ORDER BY Grade DESC;

Shows all fields, with the highest grade first: S002 (91), S006 (88), S001 (82), S004 (78), S003 (67), S005 (55).

SELECT COUNT(StudentID)
FROM Students
WHERE Class = "10A";           -- result: 3

SELECT SUM(Grade)
FROM Students
WHERE Class = "10B";           -- result: 233 (67 + 78 + 88)

10.1Logic gates

A logic gate takes one or more binary inputs (1 = true / on, 0 = false / off) and gives one output. NOT has one input; every other gate you need has two. Try each one:

Truth tables

10.2Logic circuits and truth tables

Circuits join gates so that the output of one is the input of another. Work from the inputs towards the output, and add one column for every gate. With n inputs a truth table has 2ⁿ rows (two inputs: 4 rows; three inputs: 8 rows). List the input rows in binary counting order.

From a statement to an expression

"An alarm (X) sounds if the door is open (A) and the system is armed (B), or the panic button (C) is pressed." Becomes X = (A AND B) OR C. Words map straight to gates: and → AND, or → OR, not → NOT.