Giới thiệu về Tập dữ liệu

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.