Introduction to Data Sets

Vocabulary

English	Meaning
data structure/ˈdeɪtə ˈstrʌktʃə/	A way of organising and storing data to support particular operations.
array/əˈreɪ/	An indexed collection of elements arranged in one or more dimensions.

Many values, one name

Often you need to store many values — a whole class's scores, a list of names.
A single variable holds one value; a data structure 数据结构 holds a collection.
One name (scores) plus an index or position reaches each value.
Collections let one loop process all the values at once.

Arrays and lists

An array 数组 is a fixed-size collection of values, all the same type.
An ArrayList is a resizable list — it can grow and shrink as you add or remove.
Both store many values under one name and let you loop over them.
Unit 4 builds algorithms on both.

A filing cabinet: collections store many values under one name so algorithms can process them

Why loops love collections

A collection plus a loop is the pattern behind almost every data task.
"Add up all the scores," "find the biggest," "count the passes" — all one loop.
The same accumulator/max/count patterns from Unit 2 apply directly.
Without collections, you'd need a separate variable for every value.

From 1-D to 2-D

A one-dimensional collection is a simple line of values.
A 2-D array stores a grid (rows and columns) — like a spreadsheet or board.
Nested loops (Unit 2) traverse a 2-D grid.
Unit 4 covers arrays, ArrayLists, and 2-D arrays in turn.

Watch out

A collection stores many values, but you still reach each one by its position. For an array, that's an index (scores[i]); for an ArrayList, a get(i). Choose the right structure: a fixed-size array when the count is known, an ArrayList when it grows or shrinks. Looping is what makes a collection powerful.

Example

Storing test scores:

One value: int score = 85; — only one student.
A collection: int[] scores = {85, 90, 78}; — a whole class in one name.
A loop can then total, average, or search all of them.

Key idea

A data structure stores a collection of values under one name, reached by index or position, so a loop can process them all. An array is fixed-size and same-type; an ArrayList is resizable. Unit 4 builds algorithms on arrays, ArrayLists, and 2-D arrays.

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