Programming and algorithms - Computer Science IB Study Notes

Programming is the process of designing, writing, testing, and maintaining source code to create computer programs that solve specific problems. Algorithms are step-by-step procedures or formulas for solving problems—they form the logical foundation of all programs.

Key Algorithmic Concepts:

Sequence refers to instructions executed in order, one after another. Selection (conditional statements) allows different paths based on conditions using IF-THEN-ELSE structures. Iteration (loops) repeats instructions—either a fixed number of times (count-controlled with FOR loops) or until a condition is met (condition-controlled with WHILE/REPEAT-UNTIL loops).

Variables are named storage locations holding data values that can change during program execution. Constants hold fixed values that never change. Data types include INTEGER (whole numbers), REAL/FLOAT (decimal numbers), STRING (text), BOOLEAN (TRUE/FALSE), and CHAR (single character).

Operators perform operations: arithmetic (+, -, *, /, MOD, DIV), comparison (=, ≠, <, >, ≤, ≥), and logical (AND, OR, NOT).

Pseudocode is a structured, English-like notation for describing algorithms without strict syntax rules. Cambridge uses specific conventions: DECLARE for variables, INPUT/OUTPUT for I/O, IF-THEN-ELSE-ENDIF for selection, and FOR/WHILE/REPEAT-UNTIL loops.

Memory Aid (SISAL): Sequence, Iteration, Selection form ALgorithms—the three fundamental programming constructs that can solve any computable problem (this is the structured programming theorem).

Essential Formula: Algorithm Efficiency = f(n) where n = input size. Linear algorithms O(n) scale proportionally; quadratic O(n²) algorithms slow dramatically with larger inputs.

Think of an algorithm like a recipe: precise, ordered instructions anyone can follow to achieve the same result. Just as recipes have ingredients (variables/constants), preparation steps (sequence), conditional instructions ("if mixture is too thick, add milk"), and repetitive actions ("stir until smooth"), programs mirror this structure.

Real-World Algorithm: Traffic Light System

Traffic lights demonstrate all three programming constructs. The sequence cycles through states (red→amber→green). Selection occurs when sensors detect vehicles ("IF car waiting AND cross-street clear THEN change light"). Iteration continuously loops through the cycle until the system powers down.

Netflix Recommendation Algorithm

When you browse Netflix, complex algorithms analyze your viewing history (stored in variables). The system uses selection to categorize you: "IF user watches >5 sci-fi shows THEN increase sci-fi recommendations." It employs iteration to examine thousands of titles, calculating match percentages. Arrays store your watch history, while Boolean flags track "liked/disliked."

E-commerce Shopping Cart

Adding items uses sequence (select product → add to cart → update total). Variables track quantity and price. Selection applies discounts: "IF total > $50 THEN apply 10% discount." Iteration processes each cart item to calculate the final bill. The MOD operator might determine: "IF itemCount MOD 2 = 0 THEN free_shipping = TRUE" (free shipping on even quantities).

GPS Navigation

Your GPS uses Dijkstra's algorithm—a sophisticated example of iteration and selection working together. It repeatedly (iteration) examines possible routes, selecting (selection) the shortest path based on conditions like traffic, distance, and road type. Variables store current location, destination, and estimated time.