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Data Structures

Array

Read by index: O(1), instant · Add or remove at the end: O(1) · Search for a value: O(n)

The idea, in plain English

An array is like an egg carton. It has a row of numbered slots, and each slot holds one thing. The slots stay in order. Each slot has a number, called its index. The first index is 0.

How it works

  1. 1Put items in a row of slots, in order.
  2. 2Reach any item right away by its slot number. This number is called the index, and it starts at 0.
  3. 3Add an item to the end, remove one from the end, or loop through every slot.

When you'd use it

You'll use arrays almost everywhere. Use one whenever you have a list of things in a specific order, like a to-do list, search results, or the players in a game.

Common beginner mistakes

  • You may forget indexes start at 0. So the first item is arr[0], not arr[1].
  • Don't go past the end of the array, like arr[arr.length]. There's nothing there.

Try it — edit and run

Click the code to edit · press ⌘/Ctrl+↵ to run

Editable code. Tab and Shift+Tab indent. Press Escape, then Tab, to move focus out of the editor.

Expected output — hit Run to try it
Items: apple, banana, cherry, date
Count: 4
First: apple
Removed: date

See it in motion

Watch the arrayArray · indexed slotsRead by index: O(1), instant · Add or remove at the end: O(1) · Search for a value: O(n)

Ready. Press play — read by index is instant, but inserting or deleting shifts the rest.

3
Length
0
Reads (O(1))
0
Shifts
0/6
Step
Accessed by indexShifting (the cost)New value placedIdle slot

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