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Append

In computer programming, append is an operation that concatenates two or more lists or arrays by adding the elements of one to the end of another, often used in high-level languages for manipulating dynamic data structures. Originating in the Lisp programming language in the late 1950s, it enables efficient handling of sequences without modifying the original structures in functional paradigms. Append is fundamental in algorithms, file operations, and database management, with implementations varying across languages like Prolog, Haskell, and Python.

Definition and Purpose

Core Operation

The append operation serves as a core construct in for concatenating two or more s—such as lists or arrays—into a single new that preserves the relative order of from each input. Specifically, it combines the sequences end-to-end, placing all from the first before those from the second, and extending this pattern for any additional sequences involved. This results in a unified structure where the original ordering within each remains intact, making append a key tool for composition without altering the inputs. The semantics of the append operation can be illustrated through simple , which emphasizes creating a new by sequentially incorporating s from the inputs: 
[function](/page/Function) append(seq1, seq2):
    result = empty_[sequence](/page/Sequence)()
    for each [element](/page/Element) in seq1:
        result.add([element](/page/Element))
    for each [element](/page/Element) in seq2:
        result.add([element](/page/Element))
    return result
In this formulation, the operation iterates over the elements of the first followed by the second, ensuring the output reflects their concatenated order; extensions to more than two s follow the same iterative principle. Unlike related operations such as insertion—which places elements at arbitrary positions within a —or prepending—which attaches elements to the beginning, append exclusively adds to the end, thereby maintaining a of the structure. This end-focused behavior distinguishes it in manipulation, where it facilitates tasks like building accumulators through successive or merging disparate data streams into cohesive outputs. Linked lists commonly employ append for efficient tail-to-head linking in such scenarios.

Applications in Data Structures

In linked lists, the append operation for concatenating two lists typically involves traversing the first list to locate its node and then linking that 's next pointer to the head of the second list, achieving time where n is the length of the first list, without requiring additional space beyond the existing . For , the append behavior differs significantly between fixed-size and dynamic variants. In fixed-size , appending an beyond the allocated capacity necessitates creating a new of larger , copying all existing into it, and then adding the new , resulting in time for each such operation where n is the current . Dynamic , however, mitigate this through resizing strategies like doubling the capacity when full, which involves copying to the new but yields an amortized O(1) time per append across a sequence of operations, as the total cost for m appends remains O(m). String append operations also vary based on mutability. For immutable strings, concatenation creates a new string object containing the combined contents, copying both original strings, which can lead to O(n²) time for repeated appends in a due to successive allocations. Mutable string buffers, such as those implemented via resizable arrays, allow in-place addition by extending the as needed, similar to dynamic arrays, enabling efficient O(1) amortized appends without full recopying on each operation. Append operations find practical use in algorithms for building result structures, such as in the merge step of where elements from two sorted sublists are sequentially appended to a new based on comparisons, ensuring the final sorted output in O(n) time for the merge. In systems, append facilitates extending files or buffers by adding new records to the end, often using mutable structures to maintain sequential integrity and support atomic writes for reliability.

Historical Development

Introduction in Lisp

The append function was developed in 1958 by John McCarthy as one of the foundational list-processing primitives in the Lisp programming language, designed to enable efficient manipulation of symbolic expressions central to early artificial intelligence research. Lisp, short for LISt Processor, emerged from McCarthy's work at MIT to formalize recursive functions over symbolic data structures, where lists served as the primary representation for atoms and expressions. As part of this system, append provided a means to concatenate lists, supporting the construction of complex symbolic forms without relying on low-level machine instructions. In , the syntax for append is (append list1 list2 ... ), which takes one or more lists as arguments and returns a new list formed by copying the elements from all but the last input list and then attaching the last list, sharing its structure to avoid unnecessary duplication. This behavior ensures the operation is non-destructive, preserving the original lists while producing a fresh result, which aligns with Lisp's emphasis on functional composition. The recursive definition underlying append—append[x; y] = [null[x] → y; T → cons[car[x]; append[cdr[x]; y]]]—reflects the language's theoretical roots in and recursive function theory. The design rationale for append centered on facilitating symbolic computation and recursive algorithms in a functional style, allowing programmers to build and transform lists for tasks like and without mutating data in place. This non-mutating approach was crucial for and reasoning about programs in contexts, where predictability in list operations prevented unintended side effects. Early from the 1.5 system, released in 1962, illustrates append's usage in programs, such as merging lists in the Wang algorithm for in , where it combined clause representations to explore logical inferences. Its influence later extended to other programming languages, adapting list concepts beyond 's symbolic paradigm.

Adoption Across Paradigms

The append operation, originating in as a functional list concatenation tool, transitioned to paradigms in the 1970s, profoundly shaping 's approach to list manipulation. , pioneered by Alain Colmerauer and his team at the University of in , drew inspiration from Lisp's list structures to define predicates like append/3, which declaratively concatenates lists through unification and . This predicate, often exemplified in early Prolog systems for tasks such as member checks or reversal, enabled relational definitions without explicit , marking a shift from Lisp's applicative-order evaluation to logic-based resolution. By the mid-1970s, append became a cornerstone of Prolog's list-processing capabilities, influencing implementations in systems like DEC-10 Prolog and facilitating applications in and theorem proving. In the 1980s, append concepts were integrated into subsequent pure functional languages, emphasizing immutability and non-strict evaluation to maintain . Miranda, developed by starting in 1985, introduced the infix operator ++ for , treating lists as polymorphic, lazy data structures that avoid side effects during composition. This design choice aligned with Miranda's goal of higher-order , where ++ recursively builds new lists without modifying originals, supporting equational reasoning in programs. , formalized in 1990 by a committee including Turner, adopted the same ++ operator in its , standardizing append as a core function for pure expressions and enabling optimizations like fusion in list comprehensions. These integrations reinforced append's role in functional purity, distinguishing it from mutable variants by prioritizing over in-place updates. During the 1990s, append adapted to imperative paradigms in languages like and , where it manifested as methods for extending mutable arrays to support dynamic, side-effecting growth. 's list.append() method, introduced in version 0.9.0 in 1991 by , adds elements in-place to resizable arrays using amortized constant-time over-allocation, catering to procedural scripting needs. 's push function, present since the initial Perl 1.0 release in 1987 and refined in Perl 5 (1994), similarly appends values to the end of arrays, returning the new size and enabling stack-like operations in text processing tasks. These adaptations prioritized efficiency for mutable structures, contrasting functional purity by allowing direct modification, though they retained conceptual ties to list extension for practical utility in . Key milestones in append's standardization included its formalization in the ANSI specification (INCITS 226-1994), which defined append as a non-destructive copying all but the last list argument, ensuring consistent behavior across dialects for portable code. This event solidified append's semantics in object-oriented extensions like CLOS. Additionally, append influenced handling in scripting environments, such as version 3.1 (2005), where the += facilitates appending to indexed s (building on support introduced in 2.0 in ), borrowing list-processing idioms for shell automation despite Bash's imperative roots.

Efficiency and Implementation Theory

Complexity Analysis

The append operation, which concatenates two lists of lengths m and n (with m denoting the prefix list), exhibits varying time and space complexities depending on the underlying data structure. In immutable singly linked lists, typical of functional programming paradigms, the time complexity is O(m), as the operation recursively constructs a new list by copying the prefix list (via cons operations on each element) and sharing the suffix list, without modifying the originals. This construction dominates the cost, formalized as T(\text{append}(l_1, l_2)) = \Theta(|l_1|) + c, where c is a constant and |l_1| = m is the length of the prefix list. In mutable singly linked lists, common in , append also has O(m) time complexity, requiring traversal of the prefix list to locate its end before updating the tail pointer to the suffix list's head in time. For array-based representations, generally incurs O(m + n) due to the need to copy both arrays into a new contiguous block, ensuring no in-place modification disrupts the original structures. Space complexity for append varies significantly with mutability: in immutable structures, such as those prevalent in functional languages, it is O(m) to allocate a fresh list that copies the prefix while sharing the suffix, introducing overhead from non-destructive updates. Conversely, destructive append in mutable contexts achieves O(1) auxiliary space by modifying the prefix in place, though this assumes no resizing is needed. Key factors influencing performance include immutability, which necessitates prefix reconstruction and elevates both time and space costs in persistent settings, and amortization techniques in dynamic arrays. For single-element appends to dynamic arrays (e.g., via capacity doubling), the amortized is O(1), as infrequent O(k) resizes (for size k) are offset by many O(1) operations, with total cost over m appends bounded by O(m). These considerations highlight append's efficiency trade-offs across behaviors.

Functional vs Imperative Variants

In paradigms, the append operation constructs and returns a new by combining the input sequences without altering the originals, thereby maintaining immutability and enabling structure sharing where possible, such as through cons-cell construction that reuses the of the second . This approach ensures that the original s remain unchanged, supporting pure functions that depend solely on their arguments. In contrast, imperative variants of append typically modify an existing mutable structure in place, such as extending an or directly, which avoids creating additional copies and leverages side effects for direct state updates. This style aligns with procedural , where operations like resizing or pointer manipulation occur to integrate the new elements efficiently into the target structure. The primary trade-offs between these variants revolve around purity versus performance: functional append preserves , allowing expressions to be substituted with their values without affecting program behavior, but incurs higher space overhead from constructing new structures. Imperative append offers superior runtime efficiency and reduced memory usage through in-place modifications, yet introduces risks such as , where unintended changes propagate through shared references, complicating reasoning about program state. Hybrid approaches in functional languages mitigate some inefficiencies via , where list concatenation defers computation until elements are accessed, potentially sharing unevaluated thunks between structures to delay full materialization. This technique, as seen in Haskell's list operations, balances immutability with on-demand efficiency without resorting to explicit mutation.

Specific Language Implementations

In , the append function concatenates any number of lists (or other objects) into a new list by copying the structure of all arguments except the last, which is shared without modification. The syntax is (append &rest lists), where each lists except the last must be a proper list, and the last can be any object; it returns a new list unless all preceding lists are empty, in which case it returns the last argument. For instance, (append '(1 2) '(3 4) '()) yields (1 2 3 4), demonstrating how empty lists are handled without altering the originals. This non-destructive behavior ensures the input lists remain unchanged, making append suitable for functional-style programming where immutability is preferred. A key nuance of append is its shallow copying: only the cons cells of the prefix lists are duplicated, while substructures (such as nested lists) and the final list are shared, which can lead to unintended if the shared portions are later modified. In contrast, nconc provides a destructive variant that modifies the input lists by linking the end of each (except the last) to the start of the next via rplacd, recycling existing structure for efficiency but risking side effects. For example, (let ((x '(1 2))) (nconc x '(3 4)) x) results in (1 2 3 4) and permanently alters x. also includes revappend and its destructive counterpart nreconc for cases requiring reversal during concatenation: revappend non-destructively reverses the first list before appending the second, as in (revappend '(1 2 3) '(a b)) => (3 2 1 a b), equivalent to (nconc (reverse '(1 2 3)) '(a b)). Similarly, nreconc reverses the first list in place before destructive append. A frequent pitfall arises from using append in loops to accumulate results, as each call copies the growing list, leading to quadratic time complexity O(n²) for n elements due to repeated traversals and allocations. Instead, programmers are advised to build lists by consing elements to the front (O(1) per operation) and reversing the result at the end, or to use loop macros with accumulation keywords like collect that handle efficient construction internally. This approach avoids unnecessary garbage collection and maintains linear performance.

Prolog

In , the predicate is defined as a append(?List1, ?List2, ?List3), which succeeds if List3 unifies with the of List1 and List2. This syntax treats the arguments as logical variables that can be instantiated in any direction, reflecting the declarative nature of where the predicate expresses a relationship rather than a strict computational direction. The behavior of append is inherently bidirectional and non-deterministic, allowing it to perform both and of lists through unification. For instance, when all arguments are , it verifies if List3 is the result of appending List1 and List2; however, if List3 is and the others are variables, such as append(X, Y, [1,2,3]), it generates all possible partitions, yielding solutions like X = [], Y = [1,2,3]; X = [1], Y = [2,3]; and so on until exhaustion. This relational flexibility enables append to support search and generation tasks unique to , where multiple solutions can be explored via . Append serves as a foundational primitive for recursive list processing and is integral to higher-level constructs like Definite Clause Grammars (DCGs) for parsing. In DCGs, which expand to predicates using difference lists for efficient stream handling, append underpins the conceptual model of accumulating parsed tokens without repeated full concatenations during recursion. For example, membership can be proven relationally using append: member(H, List) :- append(_, [H|_], List), which succeeds if H appears in List, by decomposing List into a prefix followed by a tail starting with H, allowing non-deterministic search for the position of H.

Haskell

In Haskell, list concatenation is primarily achieved using the (++) operator, defined in the Prelude as (++) :: [a] -> [a] -> [a], which appends the second list to the end of the first. For example, [1,2] ++ [3,4] evaluates to [1,2,3,4]. This operator is infix and right-associative with precedence 5 (infixr 5), meaning expressions like xs ++ ys ++ zs are parsed as xs ++ (ys ++ zs). The behavior of (++) leverages Haskell's lazy evaluation strategy, where computation is deferred until the results are needed, allowing the operator to handle potentially infinite lists efficiently. If the first list is infinite, such as [0..] ++ [10,20], the result is equivalent to the infinite first list, and the second list's elements are never accessible, as the first list cannot be fully consumed. For example, take 10 ([0..] ++ [10,20]) yields [0,1,2,3,4,5,6,7,8,9], ignoring the second list. However, if the second list is infinite and the first is finite, the concatenation produces the finite prefix followed by the infinite tail, which may not terminate without additional constraints like take, as laziness requires forcing evaluation incrementally. This purity ensures that append operations produce immutable results without side effects, aligning with Haskell's functional paradigm. Efficiency considerations arise in chained concatenations due to the right-associativity of (++), which prevents that would occur with left-associative . A left-associated chain like ((xs ++ ys) ++ zs) traverses and reconstructs the growing prefix repeatedly, leading to O(n²) time for n elements, whereas right-association builds nested structures efficiently in linear time overall when combined with functions like foldr (++) []. This design mitigates deep buildup in long chains, though Haskell's generally avoids traditional overflows by suspending computations as thunks. Lists in also form a Monoid instance, where the mappend function from Data.Monoid (or GHC.Base) serves as an alias for (++), enabling generic concatenation via operations: mappend [1,2] [3,4] yields [1,2,3,4]. This integration supports abstracting append in libraries like Data.List for polymorphic code, such as in folds or operations, while the empty list [] acts as the identity.

In , the append operation for s is primarily handled by the built-in list type's append method, which adds a single element to the end of the in amortized O(1) time. For example, given a mylist = [1, 2], executing mylist.append(3) results in mylist becoming [1, 2, 3]. To append multiple elements from an iterable, the extend method is used, which adds each item individually to the end; for instance, mylist.extend([4, 5]) modifies mylist to [1, 2, 3, 4, 5]. These methods modify the in place and return None, distinguishing them from operations that create new lists like slicing or . Lists are commonly built using append within loops for dynamic accumulation, such as processing data sequentially: 
python
data = []
for item in some_iterable:
    data.append(process(item))
This approach is efficient for most use cases due to the amortized constant-time append. List comprehensions offer a more concise alternative for static constructions without explicit append calls, like [process(item) for item in some_iterable], which creates and populates a new list directly. Strings in are immutable sequences, lacking a direct append ; instead, creates new strings using the + or the join . For simple cases, 'a' + 'b' yields 'ab', but repeated in loops leads to O(n²) runtime due to repeated object creation and copying. To avoid this inefficiency, accumulate strings in a list and use ''.join(list_of_strings) for O(n) performance; for example: 
python
chunks = []
for part in parts:
    chunks.append(part)
result = ''.join(chunks)
This idiom is recommended for building strings from multiple fragments. For file handling, appending content uses the built-in open function with mode 'a', which opens a for writing and adds data to the end without overwriting existing content; if the file does not exist, it is created. For instance: 
python
with open('file.txt', 'a') as f:
    f.write('appended text\n')
This ensures all writes occur at the file's end, ignoring the current file pointer position on most systems, and supports text mode by default for Unicode handling. The context manager (with statement) automatically closes the file, preventing resource leaks.

Other Languages

In Perl, arrays are extended by appending elements to the end using the built-in push function, which efficiently adds one or more values and returns the new length of the array. For strings, concatenation with assignment is achieved via the .= operator, which appends the right operand to the left string variable. Bash, as a shell scripting language, supports array appending through the += operator in compound assignments, allowing elements to be added to the end of an indexed or associative array. Strings in Bash are similarly concatenated and assigned using the += operator, which appends the right-hand side value to the existing string. The functional language provides list concatenation via the infix ++ , which non-destructively combines two lists of the same type into a new list, akin to Haskell's implementation. In modern languages like , arrays are merged using the concat() method, which returns a new array without modifying the originals, or the ES6 spread (...) for more flexible shallow copying and appending. Rust's implements vector appending through the extend method on Vec<T>, which consumes an and appends its elements, leveraging the Extend for compatibility across collection types. Post-2010 trends in statically typed languages emphasize generic mechanisms for append operations, such as and interfaces, to enable polymorphic extensions across data structures while maintaining , as exemplified by Rust's adoption of the Extend and similar features in languages like .