Append

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

Append originates in the programming language Lisp. The append procedure takes zero or more (linked) lists as arguments, and returns the concatenation of these lists.

(append '(1 2 3) '(a b) '() '(6)) ;Output: (1 2 3 a b 6) 

Since the append procedure must completely copy all of its arguments except the last, both its time and space complexity are O(n) for a list of ${\displaystyle n}$ elements. It may thus be a source of inefficiency if used injudiciously in code.

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

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

(define append   (lambda (ls1 ls2)     (if (null? ls1)       ls2       (cons (car ls1) (append (cdr ls1) ls2))))) 

Append can also be implemented using fold-right:

(define append    (lambda (a b)       (fold-right cons b a))) 

Following Lisp, other high-level programming languages which feature linked lists as primitive data structures have adopted an append. To append lists, as an operator, Haskell uses ++, OCaml uses @.

Other languages use the + or ++ symbols to nondestructively concatenate a string, list, or array.

The logic programming language Prolog features a built-in append predicate, which can be implemented as follows:

append([],Ys,Ys). append([X|Xs],Ys,[X|Zs]) :-     append(Xs,Ys,Zs). 

This predicate can be used for appending, but also for picking lists apart. Calling

 ?- append(L,R,[1,2,3]). 

yields the solutions:

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

In 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] 

In Haskell, this right-fold has the same effect as the Scheme implementation above:

append :: [a] -> [a] -> [a] append xs ys = foldr (:) ys xs 

This is essentially a reimplementation of Haskell's ++ operator.

In Perl, the push function is equivalent to the append method, and can be used in the following way.

my @list; push @list, 1; push @list, 2, 3; 

The end result is a list containing [1, 2, 3]

The unshift function appends to the front of a list, rather than the end

my @list; unshift @list, 1; unshift @list, 2, 3; 

The end result is a list containing [2, 3, 1]

When opening a file, use the ">>" mode to append rather than over write.

open(my $fh, '>>', "/some/file.txt"); print $fh "Some new text\n"; close $fh; 

Note that when opening and closing file handles, one should always check the return value.

In Python, use the list method extend or the infix operators + and += to append lists.

>>> l = [1, 2] >>> l.extend([3, 4, 5]) >>> l [1, 2, 3, 4, 5] >>> l + [6, 7] [1, 2, 3, 4, 5, 6, 7] 

Do not confuse with the list method append, which adds a single element to a list:

>>> l = [1, 2] >>> l.append(3) >>> l [1, 2, 3] 

In Bash the append redirect is the usage of ">>" for adding a stream to something, like in the following series of shell commands:

echo Hello world! >text; echo Goodbye world! >>text; cat text 

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! 

• Hughes, John (1989). "Why functional programming matters" (PDF). Computer Journal. 32 (2): 98–107. doi:10.1093/comjnl/32.2.98. Archived from the original (PDF) on 2007-04-13.
• Steele, Guy L. Jr. (1990). "Common Lisp: The Language" (2nd ed.): 418. {{cite journal}}: Cite journal requires |journal= (help) Description of append.