Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
, append is the name of a procedure for concatenating (linked
Linked list
In computer science, a linked list is a data structure consisting of a group of nodes which together represent a sequence. Under the simplest form, each node is composed of a datum and a reference to the next node in the sequence; more complex variants add additional links...
In computer science, an array type is a data type that is meant to describe a collection of elements , each selected by one or more indices that can be computed at run time by the program. Such a collection is usually called an array variable, array value, or simply array...
A high-level programming language is a programming language with strong abstraction from the details of the computer. In comparison to low-level programming languages, it may use natural language elements, be easier to use, or be from the specification of the program, making the process of...
Lisp is a family of computer programming languages with a long history and a distinctive, fully parenthesized syntax. Originally specified in 1958, Lisp is the second-oldest high-level programming language in widespread use today; only Fortran is older...
. The append procedure takes zero or more (linked) list
Linked list
In computer science, a linked list is a data structure consisting of a group of nodes which together represent a sequence. Under the simplest form, each node is composed of a datum and a reference to the next node in the sequence; more complex variants add additional links...
s 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
Computational complexity theory
Computational complexity theory is a branch of the theory of computation in theoretical computer science and mathematics that focuses on classifying computational problems according to their inherent difficulty, and relating those classes to each other...
In mathematics, big O notation is used to describe the limiting behavior of a function when the argument tends towards a particular value or infinity, usually in terms of simpler functions. It is a member of a larger family of notations that is called Landau notation, Bachmann-Landau notation, or...
for a list of 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
In-place algorithm
In computer science, an in-place algorithm is an algorithm which transforms input using a data structure with a small, constant amount of extra storage space. The input is usually overwritten by the output as the algorithm executes...
car and cdr are primitive operations on cons cells introduced in the Lisp programming language. A cons cell is composed of two pointers; the car operation extracts the first pointer, and the cdr operation extracts the second.Thus, the expression evaluates to x, and evaluates to...
of each argument (save the last), pointing it to the next list.
Recursion is the process of repeating items in a self-similar way. For instance, when the surfaces of two mirrors are exactly parallel with each other the nested images that occur are a form of infinite recursion. The term has a variety of meanings specific to a variety of disciplines ranging from...
In computer programming, cons is a fundamental function in most dialects of the Lisp programming language. cons constructs memory objects which hold two values or pointers to values. These objects are referred to as cells, conses, non-atomic s-expressions , or pairs...
. 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)))
Other languages
Following Lisp, other high-level languages which feature linked list
Linked list
In computer science, a linked list is a data structure consisting of a group of nodes which together represent a sequence. Under the simplest form, each node is composed of a datum and a reference to the next node in the sequence; more complex variants add additional links...
In computer science, a data structure is a particular way of storing and organizing data in a computer so that it can be used efficiently.Different kinds of data structures are suited to different kinds of applications, and some are highly specialized to specific tasks...
Haskell is a standardized, general-purpose purely functional programming language, with non-strict semantics and strong static typing. It is named after logician Haskell Curry. In Haskell, "a function is a first-class citizen" of the programming language. As a functional programming language, the...
uses the ++ operator to append lists. OCaml uses the @ operator to append lists.
Other languages use the + or ++ symbols for nondestructive string
String (computer science)
In formal languages, which are used in mathematical logic and theoretical computer science, a string is a finite sequence of symbols that are chosen from a set or alphabet....
/list/array concatenation.
Prolog
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 functional programming, fold – also known variously as reduce, accumulate, compress, or inject – are a family of higher-order functions that analyze a recursive data structure and recombine through use of a given combining operation the results of recursively processing its...
, from Hughes (1989:5-6), has the same semantics (by example) as the Scheme implementation above, for two arguments.
In functional programming, fold – also known variously as reduce, accumulate, compress, or inject – are a family of higher-order functions that analyze a recursive data structure and recombine through use of a given combining operation the results of recursively processing its...
In computer programming, cons is a fundamental function in most dialects of the Lisp programming language. cons constructs memory objects which hold two values or pointers to values. These objects are referred to as cells, conses, non-atomic s-expressions , or pairs...
In functional programming, fold – also known variously as reduce, accumulate, compress, or inject – are a family of higher-order functions that analyze a recursive data structure and recombine through use of a given combining operation the results of recursively processing its...
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.
Perl
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.
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
, the list append method can be used in the following way.
list = [1, 2]
list.append(3)
The end 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:
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:
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...
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.
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elements. It may thus be a source of inefficiency if used injudiciously in code.