Arrays

• What an array actually is, and why it exists (fixed-size, single data type, ordered).
• The index war: Python starts counting at 0, CIE pseudocode starts counting at 1.
• How to declare, access, and modify elements in a 1D array — in both pseudocode and Python.
• Using len() / a length variable to find how many elements are in an array.
• Iterating through a 1D array with a FOR...NEXT loop.
• 2D arrays — thinking of them as a grid with rows and columns, and how to navigate row-first, then column.
• Declaring, accessing, and modifying elements in a 2D array — pseudocode uses [row, col], Python uses [row][col].
• Nested FOR...NEXT loops to iterate through every cell of a 2D array.

Imagine you need to store the test scores of 5 students. You create five separate variables — score1, score2, score3, score4, score5 — but that's clunky, and it falls apart completely if you have 500 students instead of 5. You'd need 500 variable names, and looping through them would be impossible because you can't easily say "give me variable number 347" in a loop.

An array solves this. Think of it like a strip of numbered pigeonholes bolted to a wall. The whole strip has one name (say, scores), and each pigeonhole has a position number. To get a value out, you don't need a new variable name — you just say "give me the value in pigeonhole 3."

Formally, an array is defined by three properties, and all three matter for exam answers:

• Ordered — every element has a fixed position (an index), so the array always keeps the same sequence unless you explicitly change it.
• Static / fixed-size — once you declare an array to hold, say, 5 elements, it always has exactly 5 slots. You can't stretch it to hold a 6th value without creating a new array. (This is different from a Python , which can grow — but for IGCSE purposes, arrays are treated as fixed-size.)
• Single data type — an array of integers can only hold integers. You can't mix a string into an array of numbers.

Visualising a 1D array

Here's the array ["B","E","A","D","S"] drawn out — notice this uses zero-indexing (the Python/most-languages convention):

Array length is 5 · First element is at index 0 · The 4th element ("D") sits at index 3