Append Online Tutorials

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.

Implementation edit

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.

Prolog edit

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

append(,Ys,Ys).
append(,Ys,) :-
    append(Xs,Ys,Zs).

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

 ?- append(L,R,).

yields the solutions:

L = , R =  ;
L = , R =  ;
L = , R =  ;
L = , R = 

Miranda edit

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   = reduce cons  
    = (reduce cons ) (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 edit

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.

Perl edit

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 edit

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 edit

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!