Emptystack

Emptystack is a term used to describe a stack data structure that contains zero elements. In data structures, a stack provides last-in-first-out access via push, pop, and peek operations. The emptystack state is defined by a size or count of zero; most implementations offer a way to query whether the stack is empty.

Implementation approaches vary. In an array-based stack, a contiguous block of memory stores elements and a top index tracks the current top element. The stack is empty when the top index indicates no elements (often top equals -1 or size equals 0). In a linked-list stack, a head pointer refers to the top node, and emptiness corresponds to a null or nil head. Some languages provide built-in stack types whose initial state is emptystack.

Underflow is a common concern with emptystack. Popping or peeking from an empty stack typically triggers an underflow error or exception. Correct usage relies on prechecking emptiness (isEmpty) before removal or retrieval, or using exception handling, sentinel values, or optional results.

Performance characteristics are generally straightforward: push and pop operations are constant time (O(1)) in well-implemented stacks, with memory usage proportional to the current number of elements. Fixed-capacity stacks may reach emptystack only when elements are removed; dynamic resizing can mitigate capacity-related concerns at the cost of possible reallocation.

Common applications of emptystack concepts include maintaining function call frames in programming languages, evaluating expressions in compilers and calculators, backtracking algorithms, and iterative implementations of recursive processes. The term is also used informally as a variable or identifier in code to denote an initially empty stack.