Programming Fundamentals › Collections

Indexing

Accessing elements by position, usually starting at zero.

Also known as: array index, zero-based indexing, subscript

Indexing is accessing an element by its position in an ordered collection. In most languages, positions start at 0.

letters = ["a", "b", "c", "d"]
letters[0]       # "a"   first
letters[3]       # "d"   last
letters[-1]      # "d"   Python counts back from the end
letters[4]       # IndexError: past the end

letters[0];          // "a"
letters[4];          // undefined: no error, just nothing
letters.at(-1);      // "d"

Why zero?

The index is an offset from the start: element 0 is at the start, with no offset. A list of length n has valid indexes 0 to n − 1, which is the source of many off-by-one errors.

Works on more than arrays

Strings ("hello"[1] is "e"), tuples and dictionaries (indexed by key instead of position).

Common problems

• Out of range. Check the length first, or use a safe accessor. Empty lists have no [0].
• Negative indexes work in Python, and mean “from the end”. In JavaScript, a[-1] is undefined, so use .at(-1).
• Wrong type. An index must be an integer: items["1"] isn’t items[1].
• Changing the list while indexing shifts positions.
• One-based languages exist (Lua, R, MATLAB, SQL’s SUBSTRING). Check yours.

To get a range of elements, use slicing. To go through all of them, prefer looping over items instead of indexes (iteration).