WACE Computer Science cheat sheet

Data types: integer (whole number), real/float (fractional), Boolean (true/false), character (single symbol), string (sequence of characters); each type fixes the operations and storage allowed.

Data structures: array (fixed-size, same type, index access O(1)); record (named fields of mixed types); list (dynamic size); stack (LIFO: push/pop at one end); queue (FIFO: enqueue at rear, dequeue at front).

Boolean logic: AND is true only if both inputs true; OR is true if at least one input true; NOT inverts; XOR is true when inputs differ. De Morgan: NOT(A AND B) = (NOT A) OR (NOT B); NOT(A OR B) = (NOT A) AND (NOT B).

Algorithm efficiency: linear search is O(n); binary search is O(log n) but requires a sorted list; bubble and insertion sort are O(n^2); merge sort is O(n log n).

Number systems: binary base 2, octal base 8, hexadecimal base 16. To convert binary to hex, group bits in fours from the right; each hex digit maps to 4 bits.

Two's complement: to negate a binary integer, invert all bits and add 1. An n-bit two's complement range is -2^(n-1) to 2^(n-1)-1.

Program constructs: sequence (statements in order), selection (IF/ELSE, CASE), iteration (count-controlled FOR, condition-controlled WHILE/REPEAT). A loop must have a terminating condition to avoid infinite execution.

Abstraction hides detail behind an interface; encapsulation bundles data with the methods that operate on it; modularity splits a program into independently testable units.

Error types: syntax errors break language rules and stop compilation/interpretation; logic errors run but give wrong output; runtime errors (e.g. division by zero, out-of-range index) occur during execution.

CPU cycle: fetch (retrieve instruction from memory at the program counter), decode, execute, then store result; the program counter holds the address of the next instruction.

Memory hierarchy by speed and cost: CPU registers fastest, then cache (L1/L2/L3), then RAM, then secondary storage (SSD/HDD); volatile memory (RAM, cache) loses contents on power loss, non-volatile storage retains data.

OSI reference model has 7 layers (physical, data link, network, transport, session, presentation, application); TCP/IP model condenses these to 4 (network access, internet, transport, application).

Protocol roles: IP handles addressing and routing of packets; TCP provides reliable, ordered, connection-oriented delivery with acknowledgements; UDP is connectionless and faster but unreliable; HTTP/HTTPS transfer web content; DNS resolves domain names to IP addresses.

Network topologies: star uses a central switch (single point of failure but easy to isolate faults); bus shares one backbone; mesh gives redundant paths at higher cabling cost. LAN is local, WAN spans large distances.

Security controls: authentication verifies identity (passwords, biometrics, MFA); encryption converts plaintext to ciphertext using a key; symmetric encryption uses one shared key, asymmetric uses a public/private key pair; hashing is one-way and used for integrity checks.

Threats: malware (virus attaches to files, worm self-replicates across networks, trojan disguises as legitimate software), phishing (deceptive messages harvesting credentials), and denial-of-service (flooding a service to exhaust resources).

Legal and ethical frameworks: copyright protects original expression; the Australian Privacy Principles govern collection and handling of personal information; software licences (proprietary, open source, freeware) define permitted use and modification.

Emerging impacts: automation shifts employment demand toward design and maintenance roles; machine learning systems depend on training data quality and can inherit bias present in that data.