dottedpair

A dotted pair, in the Lisp family of languages, is a simple two-element data structure formed from a cons cell. It consists of a car (the first element) and a cdr (the second element) and is written in dot notation as (car . cdr). The dot signals that the cdr is not a list.

In Lisp, a proper list is built from cons cells whose cdr so far forms another list or nil. For example, (a b c) is equivalent to (a . (b . (c . nil))). A dotted pair such as (a . b) has a non-list cdr, and prints with the dot: (a . b). If the cdr is nil, the pair is a single element list like (a) which is treated as (a . nil) and prints without the dot.

Printing conventions vary by context. Lists print without dots, while dotted pairs print with the explicit dot, reflecting the underlying pair structure. The functions cons, car, and cdr operate on these constructs; for instance, (cons 'a 'b) yields (a . b), while (cons 'a '(b c)) yields (a b c).

Uses and significance: Dotted pairs are the fundamental building blocks of lists and more general pair-based data structures. They are used to represent association lists (key . value), to build trees, graphs, or other structures where a simple two-element linkage is needed. The concept is central to Lisp, Scheme, and related languages, where every list is ultimately a chain of such pairs ending in nil, and dotted notation remains a common way to express non-list cdrs.