Peek (data type operation)

In computer science, peek is an operation on certain abstract data types, specifically sequential collections such as stacks and queues, which returns the value of the top ("front") of the collection without removing the element from the collection. It thus returns the same value as operations such as "pop" or "dequeue", but does not modify the data.

The name "peek" is similar to the basic "push" and "pop" operations on a stack, but the name for this operation varies depending on data type and language. Peek is generally considered an inessential operation, compared with the more basic operations of adding and removing data, and as such is not included in the basic definition of these data types. However, since it is a useful operation and generally easily implemented, it is frequently included in practice, and in some definitions peek is included as basic, with pop (or analog) defined in terms of peek; see abstract definition.

Sequential types for which peek is often implemented include:

Single-ended types, such as stack, generally only admit a single peek, at the end that is modified. Double-ended types, such as deques, admit two peeks, one at each end.

Names for peek vary. "Peek" or "top" are common for stacks, while for queues "front" is common. Operations on deques have varied names, often "front" and "back" or "first" and "last". The name "peak" is also occasionally found, in the sense of "top, summit", though this also occurs as a spelling error for the verb "peek".

Intuitively, peek returns the same value as pop, but does not change the data. Behavior when the collection is empty varies – most often this yields an underflow error, identically to a pop on an empty collection, but some implementations provide a function which instead simply returns (without error), essentially implementing if isempty then return, else peek.

This behavior can be axiomatized in various ways. For example, a common VDM (Vienna Development Method) description of a stack defines top (peek) and remove as atomic, where top returns the top value (without modifying the stack), and remove modifies the stack (without returning a value). In this case pop is defined in terms of top and remove.

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