In computer programming, append is the name of a procedure for concatenating (linked) lists or arrays in some high-level programming languages.


Append originates in the Lisp programming language. The append procedure takes two 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 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)))))

Other languages

Following Lisp, other high-level languages which feature linked lists as primitive data structures have adopted an append 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.


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 = []


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]


This right-fold has the same effect as the Scheme implementation above:

append :: [a] -> [a] -> [a] append xs ys = foldr (:) ys xsThis is essentially a reimplementation of Haskell's ++ operator.

DOS command

append is a DOS command that allows programs to open data files in specified directories as if they were in the current directory. It appends the directories to the search path list.


* Hughes, John. 1989. Why functional programming matters. Computer Journal 32, 2, 98-107.
* Steele, Guy L. Jr. "Common Lisp: The Language, Second Edition". 1990. pg. 418, description of append.

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Look at other dictionaries:

  • append — ap‧pend [əˈpend] verb [transitive] to attach or add something to a piece of writing: • I have appended a letter which you sent to us last year. append something to something • The director has the right to append comments to the final report. * * …   Financial and business terms

  • Append — Ap*pend ([a^]p*p[e^]nd ), v. t. [imp. & p. p. {Appended}; p. pr. & vb. n. {Appending}.] [L. appendere or F. appendre: cf. OE. appenden, apenden, to belong, OF. apendre, F. appendre, fr. L. append[=e]re, v. i., to hang to, append[e^]re, v. t., to… …   The Collaborative International Dictionary of English

  • append — I verb add, addere, adiungere, affix, annex, attach, augment, conjoin, connect, extend, fasten include, insert, join, subjoin, supplement II index affix, annex (add) …   Law dictionary

  • append — (v.) late 14c., to belong to as a possession or right, from O.Fr. apendre (13c.) belong, be dependent (on); attach (oneself) to; hang, hang up, and directly from L. appendere to cause to hang (from something), weigh, from ad to (see AD (Cf. ad )) …   Etymology dictionary

  • append — *add, subjoin, annex, superadd Analogous words: affix, attach, *fasten Contrasted words: *detach, disengage: curtail (see SHORTEN) …   New Dictionary of Synonyms

  • append — [v] add, join adjoin, affix, annex, attach, conjoin, fasten, fix, hang, subjoin, supplement, tack on*, tag on*; concepts 85,113,160 Ant. disjoin, subtract, take away …   New thesaurus

  • append — ► VERB ▪ add to the end of a document or piece of writing. ORIGIN Latin appendere hang on …   English terms dictionary

  • append — [ə pend′] vt. [ME appenden < OFr apendre < L appendere < ad , to + pendere, hang: see SPIN] to attach or affix; add as a supplement or appendix …   English World dictionary

  • append — v. (D; tr.) to append to (to append a translation to a document) * * * [ə pend] (D; tr.) to append to (to append a translation to a document) …   Combinatory dictionary

  • append — 01. The secretary [appended] a note at the end of the memo, asking people to verify reception. 02. Many people [append] a little quotation or joke to their e mail signature. 03. The director s name is [appended] to all official documents. 04.… …   Grammatical examples in English

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