Array

An ordered, indexed collection of elements.

Backend Engineer · Junior Frontend Engineer · Junior Data Engineer · Junior

Also known as: arrays, array data structure

An array is an ordered collection of elements, each reached by its position (index). Elements sit next to each other in memory, so jumping to any position is fast.

const scores = [90, 72, 85];
scores[0];            // 90
scores.length;        // 3
scores.push(60);      // add at the end
scores.map(s => s + 5);

scores = [90, 72, 85]       # Python's list plays this role

What’s cheap and what isn’t

Operation	Cost
Read or write `a[i]`	constant time
Add at the end	usually constant time
Insert or remove in the middle or at the start	slow: elements must shift (linear time)
Search for a value	linear time, unless sorted (big O)

If you search by value often, a set or dictionary is faster (hash tables).

Details

Indexes start at 0, and the last one is length - 1. Going past the end is an error or undefined (indexing, off-by-one errors).
Arrays are mutable in most languages, and variables share them (mutable vs immutable).
Copying an array with b = a doesn’t copy it. Both names refer to one array (shallow vs deep copy).
Fixed vs dynamic. In C and Java, a plain array has a fixed size. JavaScript and Python arrays grow automatically.
Mixed types are allowed in JavaScript and Python, though mixing is usually a bad idea.

Use arrays for ordered sequences. Use a dictionary when you look things up by key.