firstout

Firstout is a term used in computer science to denote the operation of removing and returning the first element from a data sequence or container. It is commonly associated with FIFO (first-in, first-out) data structures such as queues and streams, where the element that has been in the structure the longest is the first to be removed. In contrast to last-out or LIFO approaches, where the most recently added element is removed first (such as stacks), firstout emphasizes predictable, chronological processing.

In practice, firstout can refer to the dequeue operation in a queue: retrieving and deleting the element at the front. Depending on implementation, this operation can run in constant time, typically O(1), when the underlying structure is a linked list or a circular buffer. Some languages expose explicit firstout operations or methods named removeFirst, poll, or popleft, all serving the same semantic.

Common use cases for firstout processing include task scheduling, output pipelines, and message queues in asynchronous systems, where earlier items should be processed before later ones to preserve ordering.

Variants and related concepts include first-in, first-out (FIFO) discipline, and first-out queues with bounded capacity. Some systems distinguish between strict and relaxed firstout depending on whether the data must be returned in exact original order in the presence of concurrent producers and consumers.

This article uses firstout in a general sense to describe the principle of consuming the earliest element available, rather than a specific language feature or library name. See also FIFO, queue, dequeue, and popleft.