Append
In computer programming, append
is the operation for concatenating linked lists or arrays in some high-level programming languages.
Lisp[]
Append
originates in the Lisp programming language. 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 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.
Implementation[]
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)))
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.
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 = []
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:
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[]
In Python, use the list method "extend" or the infix operators + and += to append lists.
l = [1, 2]
l.extend([3, 4, 5])
print l + [6, 7]
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:
l = [1, 2]
l.append(3)
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:
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!
References[]
- Hughes, John. 1989. Why functional programming matters. Computer Journal 32, 2, 98-107. https://web.archive.org/web/20070413005952/http://www.math.chalmers.se/~rjmh/Papers/whyfp.pdf
- Steele, Guy L. Jr. Common Lisp: The Language, Second Edition. 1990. pg. 418, description of
append
.
- Functional programming
- Lisp (programming language)
- Programming constructs
- DOS on IBM PC compatibles