Arrays and Lists

Explainer

Think of a class register: 30 students, one numbered seat each, one piece of data per seat. An array, a List in Python, is exactly that: one row of numbered slots, each holding one value.

This is the single most common source of lost marks on this statement: not the concept of an array, but which number points to the first element. If you’re working across boards, the two conventions genuinely conflict:

There is no universal rule. It depends entirely on which syllabus (and which language) the question is written in. Always check which one you’re in before writing a loop bound.

Explainer

Two-dimensional arrays and lists

One week of temperatures is a single row. Now track four weeks, not one: that’s a table, one row per week, one column per day. A 2D array (Java) or 2D list (Python) stores exactly that shape, and every element needs two indices to reach it, not one.

# 4 weeks, 7 days each: a list of lists, one inner list per week
weekly_temps = [[0] * 7 for week in range(4)]

# Outer loop for weeks (rows), inner loop for days (columns)
for week in range(4):
    for day in range(7):
        weekly_temps[week][day] = int(input("Enter a temperature: "))

# Week 2 (index 1), Thursday (index 3): row first, then column
print(weekly_temps[1][3])

// 4 rows (weeks), 7 columns (days), both dimensions fixed upfront
int[][] weeklyTemps = new int[4][7];

Scanner input = new Scanner(System.in);
for (int week = 0; week < 4; week++) {
    for (int day = 0; day < 7; day++) {
        weeklyTemps[week][day] = input.nextInt();
    }
}

// Week 2 (index 1), Thursday (index 3): row first, then column
System.out.println(weeklyTemps[1][3]);

Two nested loops, always. The outer loop moves down the rows (weeks), the inner loop moves across the columns (days) within whichever row the outer loop is currently on.

A month of temperatures, indexed by row and column

That double loop has now filled all 28 cells. Here is the same weekly_temps table read back, row (week) first, column (day) second.

Explainer

Dynamic lists: add, remove, traverse

The week of temperatures above was sized in advance, 7 slots, fixed. But readings from a sensor arrive throughout the day, with no way to know in advance how many there’ll be, including a faulty spike that needs deleting once it’s spotted. That calls for a structure that can genuinely grow and shrink, not just be pre-sized: this is B2.2.1’s dynamic data structure, in practice.

# Starts empty: we don't know in advance how many readings today will bring
readings = []

readings.append(18)   # add: a new reading comes in
readings.append(21)
readings.append(97)   # a faulty sensor spike

readings.remove(97)   # remove: deletes the VALUE 97, wherever it is

# traverse: visit every reading that's left
for reading in readings:
    print(reading)

// Java's arrays can't grow, so a dynamic collection needs ArrayList instead
ArrayList<Integer> readings = new ArrayList<Integer>();

readings.add(18);     // add: a new reading comes in
readings.add(21);
readings.add(97);     // a faulty sensor spike

readings.remove(Integer.valueOf(97));   // remove BY VALUE, see note below

// traverse: visit every reading that's left
for (int reading : readings) {
    System.out.println(reading);
}

Python's list was always capable of this, append() and remove() work on any list, growing or shrinking it as needed. Java's plain array from the section above cannot: once declared with a size, it never changes, which is exactly why a genuinely dynamic collection in Java needs ArrayList instead.