Append

In computer programming,   is the operation for concatenating linked lists or arrays in some high-level programming languages.

Lisp
originates in the Lisp programming language. The  procedure takes zero or more (linked) lists as arguments, and returns the concatenation of these lists. Since the  procedure must completely copy all of its arguments except the last, both its time and space complexity are O(n) for a list of $$n$$ elements. It may thus be a source of inefficiency if used injudiciously in code.

The  procedure (called   in Scheme) performs the same function as , but destructively: it alters the cdr of each argument (save the last), pointing it to the next list.

Implementation
can easily be defined recursively in terms of. The following is a simple implementation in Scheme, for two arguments only:

Append can also be implemented using fold-right:

Other languages
Following Lisp, other high-level languages which feature linked lists as primitive data structures have adopted an. Haskell uses the  operator to append lists. OCaml uses the  operator to append lists.

Other languages use the  or   symbols for nondestructive string/list/array concatenation.

Prolog
The logic programming language Prolog features a built-in  predicate, which can be implemented as follows: This predicate can be used for appending, but also for picking lists apart. Calling yields the solutions:

L = [], R = [1, 2, 3] ; L = [1], R = [2, 3] ; L = [1, 2], R = [3] ; L = [1, 2, 3], R = []

Miranda
This right-fold, from Hughes (1989:5-6), has the same semantics (by example) as the Scheme implementation above, for two arguments.

append a b = reduce cons b a

Where reduce is Miranda's name for fold, and cons constructs a list from two values or lists.

For example,

append [1,2] [3,4] = reduce cons [3,4] [1,2] = (reduce cons [3,4]) (cons 1 (cons 2 nil)) = cons 1 (cons 2 [3,4]))        (replacing cons by cons and nil by [3,4])     = [1,2,3,4]

Haskell
This right-fold has the same effect as the Scheme implementation above: This is essentially a reimplementation of Haskell's  operator.

Perl
In Perl, the push function is equivalent to the append method, and can be used in the following way. The end result is a list containing [1, 2, 3]

The unshift function appends to the front of a list, rather than the end The end result is a list containing [2, 3, 1]

When opening a file, use the ">>" mode to append rather than over write. Note that when opening and closing file handles, one should always check the return value.

Python
In Python, use the list method "extend" or the infix operators + and += to append lists. After executing this code, l is a list containing [1, 2, 3, 4, 5], while the output generated is the list [1, 2, 3, 4, 5, 6, 7].

Do not confuse with the list method "append", which adds a single element to a list: Here, the result is a list containing [1, 2, 3].

Bash
In Bash the append redirect is the usage of ">>" for adding a stream to something, like in the following series of shell commands: The stream "Goodbye world!" is added to the text file written in the first command. The ";" implies the execution of the given commands in order not simultaneously. So, the final content of the text file is:

Hello world! Goodbye world!