Reverse a string

Reverse a string
You are encouraged to solve this task according to the task description, using any language you may know.

Take a string and reverse it. For example, "asdf" becomes "fdsa".

For extra credit, preserve Unicode combining characters. For example, "as⃝df̅" becomes "f̅ds⃝a", not "̅fd⃝sa".

0815

This program reverses each line of its input.

}:r:     Start reader loop.
  !~>&   Push a character to the "stack".
  <:a:=- Stop reading on newline.
^:r:
@>       Rotate the newline to the end and enqueue a sentinel 0.
{~       Dequeue and rotate the first character into place.
}:p:
  ${~    Print the current character until it's 0.
^:p:
#:r:     Read again.

Output:

echo -e "foo\nbar" | 0815 rev.0
oof
rab

ACL2

(reverse "hello")

ACL2 does not support unicode.

ActionScript

function reverseString(string:String):String
{
	var reversed:String = new String();
	for(var i:int = string.length -1; i >= 0; i--)
		reversed += string.charAt(i);
	return reversed;
}
 
function reverseStringCQAlternative(string:String):String
{
	return string.split('').reverse().join('');
}

Ada

with Ada.Text_IO; use Ada.Text_IO;
 
procedure Reverse_String is
   function Reverse_It (Item : String) return String is
      Result : String (Item'Range);
   begin
      for I in Item'range loop
         Result (Result'Last - I + Item'First) := Item (I);
      end loop;
      return Result;
   end Reverse_It;
begin
   Put_Line (Reverse_It (Get_Line));
end Reverse_String;

Agda2

Using the Agda standard library, version 0.6.

module reverse_string where
 
open import Data.String
open import Data.List
 
reverse_string : String → String
reverse_string s = fromList (reverse (toList s))

Aime

text
reverse(text s)
{
    data b;
    integer i;
 
    i = length(s);
    while (i) {
        i -= 1;
        b_insert(b, -1, character(s, i));
    }
 
    return b_string(b);
}
 
integer
main(void)
{
    o_text(reverse("Hello, World!"));
    o_byte('\n');
 
    return 0;
}

ALGOL 68

Works with: ALGOL 68 version Standard - no extensions to language used

Works with: ALGOL 68G version Any - tested with release mk15-0.8b.fc9.i386

Works with: ELLA ALGOL 68 version Any (with appropriate job cards) - tested with release 1.8.8d.fc9.i386

PROC reverse = (REF STRING s)VOID:
  FOR i TO UPB s OVER 2 DO
    CHAR c = s[i];
    s[i] := s[UPB s - i + 1];
    s[UPB s - i + 1] := c
  OD;
 
main:
(
  STRING text := "Was it a cat I saw";
  reverse(text);
  print((text, new line))
)

Output:

was I tac a ti saW

APL

      ⌽'asdf'
fdsa

AppleScript

get reverse_string("as⃝df̅")
 
on reverse_string(str)
	set old_delim to (get AppleScript's text item delimiters)
	set AppleScript's text item delimiters to ""
 
	set temp to (reverse of text items of str)
	set temp to (text items of temp) as Unicode text
 
	set AppleScript's text item delimiters to old_delim
	return temp
end reverse_string

AutoHotkey

"Normal" version:

MsgBox % reverse("asdf")
 
reverse(string)
{
  Loop, Parse, string
    reversed := A_LoopField . reversed
  Return reversed
}

A ''much'' faster version:

Reverse(String){ ; credit to Rseding91
   If (A_IsUnicode){
      SLen := StrLen(String) * 2
      VarSetCapacity(RString,SLen)
 
      Loop,Parse,String
         NumPut(Asc(A_LoopField),RString,SLen-(A_Index * 2),"UShort")
   } Else {
      SLen := StrLen(String)
      VarSetCapacity(RString,SLen)
 
      Loop,Parse,String
         NumPut(Asc(A_LoopField),RString,SLen-A_Index,"UChar")
   }
 
   VarSetCapacity(RString,-1)
 
   Return RString
}

AutoIt

#AutoIt Version: 3.2.10.0
$mystring="asdf"
$reverse_string = ""
$string_length = StringLen($mystring)
 
For $i = 1 to $string_length
   $last_n_chrs = StringRight($mystring, $i)
   $nth_chr = StringTrimRight($last_n_chrs, $i-1)
   $reverse_string= $reverse_string & $nth_chr 
Next
 
MsgBox(0, "Reversed string is:", $reverse_string)

AWK

function reverse(s)
{
  p = ""
  for(i=length(s); i > 0; i--) { p = p substr(s, i, 1) }
  return p
}
 
BEGIN {
  print reverse("edoCattesoR")
}

Recursive

function reverse(s   ,l)
{
  l = length(s)
  return l < 2 ? s:( substr(s,l,1) reverse(substr(s,1,l-1)) )
}
 
BEGIN {
  print reverse("edoCattesoR")
}

Babel

This example will handle UTF-8 encoded Unicode but doesn't handle combining characters.

strrev: { str2ar ar2ls reverse ls2lf ar2str }

• str2ar - this operator converts a UTF-8 encoded string to an array of Unicode codepoints
• ar2ls - this operator converts the array to a linked-list
• reverse - this operator reverses a linked-list
• ls2lf - this operator undoes the effect of ar2ls
• ar2str - this operator undoes the effect of str2ar

BASIC

Works with: QuickBasic version 4.5

FUNCTION reverse$(a$)
   b$ = ""
   FOR i = 1 TO LEN(a$)
      b$ = MID$(a$, i, 1) + b$
   NEXT i
   reverse$ = b$
END FUNCTION

Batch File

@echo off
setlocal enabledelayedexpansion
call :reverse %1 res
echo %res%
goto :eof
 
:reverse
set str=%~1
set cnt=0
:loop
if "%str%" equ "" (
	goto :eof
	)
set chr=!str:~0,1!
set str=%str:~1%
set %2=%chr%!%2! 
goto loop

BBC BASIC

      PRINT FNreverse("The five boxing wizards jump quickly")
      END
 
      DEF FNreverse(A$)
      LOCAL B$, C%
      FOR C% = LEN(A$) TO 1 STEP -1
        B$ += MID$(A$,C%,1)
      NEXT
      = B$

Befunge

To see this in action, it's best to use an interpreter that animates the process.

v  The string to reverse. The row to copy to.
   |                      | The actual copying happens here.
   |                      | |    Increment column to write to.
   |                      | |    |  Store column #.
   v                      v v    v  v
> "reverse me" 3 10p >10g 4 p 10g1+ 10pv
               ^ ^   |:                <
First column --| |   @  ^
to write to.     |   ^  Get the address
   All calls to 10   |  to copy the next
 involve saving or   |  character to.
       reading the   End when stack is empty or
   column to write   explicit zero is reached.
               to.

Bracmat

  ( reverse
  = L x
    .     :?L
        & @( !arg
           :   ?
               ( %?x
               & utf$!x
               & !x !L:?L
               & ~`
               )
               ?
           )
      | str$!L
  )
& out$reverse$Ελληνικά

Output:

άκινηλλΕ

Brainf***

[-]>,+[->,+]<[.<]

The former wont stop taking input bytes unless a special compiler was made to stop at ENTER. The following checks for 10 ascii (line feed) and stops taking input at that point

,----- ----- [+++++ +++++ > , ----- -----]	If a newline is hit counter will be zero and input loop ends
<[.<]	run all chars backwards and print them
 
just because it looks good we print CRLF
+++++ +++++ +++ . --- .

Brat

p "olleh".reverse  #Prints "hello"

Burlesque

 
"Hello, world!"<-
 

C

#include <stdio.h>
#include <stdlib.h>
#include <locale.h>
#include <wchar.h>
 
const char *sa = "abcdef";
const char *su = "as⃝df̅"; /* Should be in your native locale encoding. Mine is UTF-8 */
 
int is_comb(wchar_t c)
{
	if (c >= 0x300 && c <= 0x36f) return 1;
	if (c >= 0x1dc0 && c <= 0x1dff) return 1;
	if (c >= 0x20d0 && c <= 0x20ff) return 1;
	if (c >= 0xfe20 && c <= 0xfe2f) return 1;
	return 0;
}
 
wchar_t* mb_to_wchar(const char *s)
{
	wchar_t *u;
	size_t len = mbstowcs(0, s, 0) + 1;
	if (!len) return 0;
 
	u = malloc(sizeof(wchar_t) * len);
	mbstowcs(u, s, len);
	return u;
}
 
wchar_t* ws_reverse(const wchar_t* u)
{
	size_t len, i, j;
	wchar_t *out;
	for (len = 0; u[len]; len++);
	out = malloc(sizeof(wchar_t) * (len + 1));
	out[len] = 0;
	j = 0;
	while (len) {
		for (i = len - 1; i && is_comb(u[i]); i--);
		wcsncpy(out + j, u + i, len - i);
		j += len - i;
		len = i;
	}
	return out;
}
 
char *mb_reverse(const char *in)
{
	size_t len;
	char *out;
	wchar_t *u = mb_to_wchar(in);
	wchar_t *r = ws_reverse(u);
	len = wcstombs(0, r, 0) + 1;
	out = malloc(len);
	wcstombs(out, r, len);
	free(u);
	free(r);
	return out;
}
 
int main(void)
{
	setlocale(LC_CTYPE, "");
 
	printf("%s => %s\n", sa, mb_reverse(sa));
	printf("%s => %s\n", su, mb_reverse(su));
	return 0;
}

Output:

abcdef => fedcba
as⃝df̅ => f̅ds⃝a

Library: GLib

#include <glib.h>
gchar *srev (const gchar *s) {
    if (g_utf8_validate(s,-1,NULL)) {
        return g_utf8_strreverse (s,-1);
}   }
// main
int main (void) {
    const gchar *t="asdf";
    const gchar *u="as⃝df̅";
    printf ("%s\n",srev(t));
    printf ("%s\n",srev(u));
    return 0;
}

C++

#include <iostream>
#include <string>
#include <algorithm>
 
int main()
{
  std::string s;
  std::getline(std::cin, s);
  std::reverse(s.begin(), s.end()); // modifies s
  std::cout << s << std::endl;
  return 0;
}

C#

C# does not have a built-in Reverse method for strings, which are immutable. One way to implement this is to convert the string to an array of characters, reverse that, and return a new string from the reversed array:

private static string ReverseString(string input)
{
	char[] inputChars = input.ToCharArray();
	Array.Reverse(inputChars);
	return new string(inputChars);
}

As of C# 4.0 you can call the Reverse method on a string, however it returns an Enumerable of chars. It appears that this only helps by letting you do the above in a different order (reverse the string, convert that to an array of chars, and return a new string from the reversed array):

return new string(input.Reverse().ToArray());

Caché ObjectScript

USER>Write $Reverse("Hello, World")
dlroW ,olleH

Clojure

Basic reverse

For normal strings, the reverse function can be used to do the bulk of the work. However, it returns a character sequence, which has to be converted back to a string.

(defn str-reverse [s] (apply str (reverse s)))

Reverse words in a string

(apply str (interpose " " (reverse (.split "the quick brown fox" " "))))

Supporting combining characters

Handling combining characters present a trickier task. We need to protect the relative ordering of the combining character and the character to its left. Thus, before reversing, the characters need to be grouped.

(defn combining? [c]
  (let [type (Character/getType c)]
    ;; currently hardcoded to the types taken from the sample string
    (or (= type 6) (= type 7))))
 
(defn group
  "Group normal characters with their combining characters"
  [chars]
  (cond (empty? chars) chars
	(empty? (next chars)) (list chars)
	:else
	(let [dres (group (next chars))]
	  (cond (combining? (second chars)) (cons (cons (first chars)
							(first dres))
						  (rest dres))
		:else (cons (list (first chars)) dres)))))
 
(defn str-reverse
  "Unicode-safe string reverse"
  [s]
  (apply str (apply concat (reverse (group s)))))

Output:

user=> s
"as⃝df̅"
user=> (str-reverse s)
"f̅ds⃝a"[
user=> (str-reverse (str-reverse s))
"as⃝df̅"
user=>

COBOL

FUNCTION REVERSE-STRING('QWERTY')

CoffeeScript

"qwerty".split("").reverse().join ""

ColdFusion

You can reverse anything that can be written to the document in hashmarks (i.e. strings, numbers, now( ), etc.).

<cfset myString  = "asdf" />
<cfset myString  = reverse( myString ) />

Common Lisp

(reverse my-string)

D

import std.stdio, std.range, std.conv;
 
void main() {
    string s1 = "hello"; // UTF-8
    string s1r = text(retro("hello"));
    assert(s1r == "olleh");
 
    wstring s2 = "hello"w; // UTF-16
    wstring s2r = wtext(retro("hello"));
    assert(s2r == "olleh");
 
    dstring s3 = "hello"d; // UTF-32
    dstring s3r = dtext(retro("hello"));
    assert(s3r == "olleh");
}

Dart

Dart apparently has no String or List reverse method

String reverse(String s) {
  StringBuffer sb=new StringBuffer();
  for(int i=s.length-1;i>=0;i--) {
    sb.add(s[i]);
  }
  return sb.toString();
}
 
main() {
  print(reverse('a string.'));
}

Delphi

function ReverseString(const InString: string): string;
var
  i: integer;
begin
  for i := Length(InString) downto 1 do
    Result := Result + InString[i];
end;

You could also use this RTL function Introduced in Delphi 6:

StrUtils.ReverseString

DWScript

See Delphi.

E

pragma.enable("accumulator")
def reverse(string) {
  return accum "" for i in (0..!(string.size())).descending() { _ + string[i] }
}

Emacs Lisp

(concat (reverse (append "Hello World" nil)))

Output:

"dlroW olleH"

Eiffel

class
    APPLICATION
create
    make
feature
    make
            -- Demonstrate string reversal.
        do
            my_string := "Hello World!"
            my_string.mirror
            print (my_string)
        end
    my_string: STRING
            -- Used for reversal
end

Output:

!dlroW olleH

EGL

function reverse( str string ) returns( string )
	result string;
	for ( i int from StrLib.characterLen( str ) to 1 decrement by 1 )
		result ::= str[i:i];
	end
	return( result );
end

Erlang

1> lists:reverse("reverse!").
"!esrever"

Erlang also supports binary strings, which uses its binary format. There is no standard function to reverse a binary sequence, but the following one does the job well enough. It works by changing the endianness (from little to big or the opposite) of the whole sequence, effectively reversing the string.

reverse(Bin) ->
    Size = size(Bin)*8,
    <<T:Size/integer-little>> = Bin,
    <<T:Size/integer-big>>.

Output:

1> test:reverse(<<"hello">>).
<<"olleh">>

Euler Math Toolbox

 
>function strrev (s) := chartostr(fliplr(strtochar(s)))
>strrev("This is a test!")
 !tset a si sihT
 

Euphoria

 
include std/sequence.e
printf(1, "%s\n", {reverse("abcdef") })
 

FBSL

A slow way

FUNCTION StrRev1(BYVAL $p1)
	DIM $b = ""
	REPEAT LEN(p1)
		b = b & RIGHT(p1,1)
		p1 = LEFT(p1,LEN(p1)-1)
	END REPEAT
	RETURN b
END FUNCTION
 

A much faster (twice at least) way

FUNCTION StrRev2(BYVAL $p1)
	DIM $b = "", %i
	FOR i = LEN(p1) DOWNTO 1
		b = b & MID(p1,i,1)
	NEXT
	RETURN b
END FUNCTION

An even faster way using PEEK, POKE, double-calls and quantity-in-hand

FUNCTION StrRev3( $s )
	FOR DIM x = 1 TO LEN(s) \ 2
		PEEK(@s + LEN - x, $1)
		POKE(@s + LEN - x, s{x})(@s + x - 1, PEEK)
	NEXT
	RETURN s
END FUNCTION
 

An even faster way using the DynC (Dynamic C) mode

DynC StringRev($theString) As String
   void rev(char *str)
   {
		int len = strlen(str);
		char *HEAD = str;
		char *TAIL = str + len - 1;
		char temp;
		int i;
		for ( i = 0; i <= len / 2; i++, HEAD++, TAIL--) {
			temp = *HEAD;
			*HEAD = *TAIL;
			*TAIL = temp;
		}
   }
   char *main(char* theString)
   {
      rev(theString);
      return theString;
   }
End DynC

Using DynASM, the Dynamic Assembler mode.

DYNASM RevStr(BYVAL s AS STRING) AS STRING
   // get length of string
   // divide by two
   // setup pointers to head and tail
   // iterate from 1 to (length \ 2)
   //   swap head with tail
   //   increment head pointer
   //   decrement tail pointer
 
   ENTER 0, 0 // = PUSH EBP: MOV EBP, ESP
   PUSH EBX // by Windows convention EBX, EDI, ESI must be saved before modification
 
   MOV EAX, s // get string pointer
   MOV ECX, EAX // duplicate it
 
   .WHILE BYTE PTR [ECX] <> 0
 
	INC ECX // propagate to tail 
 
   .WEND
 
   MOV EDX, ECX // duplicate tail pointer
   DEC EDX // set it to last byte before trailing zero
 
   SUB ECX, EAX // get length in ECX in 1 CPU cycle
   SHR ECX, 1 // get length \ 2 in 1 CPU cycle; that's the beauty of power-of-two division
 
   .WHILE ECX > 0
 
      MOV BL, [EDX] // no need to XOR; just overwrite BL and BH contents
      MOV BH, [EAX] // DynAsm deduces data size from destination register sizes
 
      MOV [EDX], BH // ditto, source register sizes
      MOV [EAX], BL
 
      INC EAX // propagate pointers
      DEC EDX
 
      DEC ECX // decrement counter
 
   .WEND
 
   // point to start of string again
   MOV EAX, s // MOV = 1 CPU cycle, PUSH + POP = 2 CPU cycles
 
   POP EBX // by Windows convention ESI, EDI, EBX must be restored if modified
   LEAVE // = POP EBP
   RET
END DYNASM
 

F#

let ReverseString (s:string) = new string(Array.rev (s.ToCharArray()))

Factor

A string is a sequence and there is a default reverse implementation for those.

"hello" reverse

string-reverse preserves graphemes:

"as⃝df̅" string-reverse "f̅ds⃝a" = .

Fancy

"hello world!" reverse

Forth

: exchange ( a1 a2 -- )
  2dup c@ swap c@ rot c! swap c! ;
: reverse ( c-addr u -- )
  1- bounds begin 2dup > while
    2dup exchange
    -1 /string
  repeat 2drop ;
 
s" testing" 2dup reverse type   \ gnitset

Fortran

Works with: Fortran version 90 and later

PROGRAM Example
 
  CHARACTER(80) :: str = "This is a string"
  CHARACTER :: temp
  INTEGER :: i, length
 
  WRITE (*,*) str
  length = LEN_TRIM(str) ! Ignores trailing blanks. Use LEN(str) to reverse those as well
  DO i = 1, length/2
     temp = str(i:i)
     str(i:i) = str(length+1-i:length+1-i)
     str(length+1-i:length+1-i) = temp
  END DO
  WRITE(*,*) str
 
END PROGRAM Example

Output:

This is a string
gnirts a si sihT

Another implementation that uses a recursive not-in-place algorithm:

program reverse_string
 
  implicit none
  character (*), parameter :: string = 'no devil lived on'
 
  write (*, '(a)') string
  write (*, '(a)') reverse (string)
 
contains
 
  recursive function reverse (string) result (res)
 
    implicit none
    character (*), intent (in) :: string
    character (len (string)) :: res
 
    if (len (string) == 0) then
      res = ''
    else
      res = string (len (string) :) // reverse (string (: len (string) - 1))
    end if
 
  end function reverse
 
end program reverse_string

Output:

no devil lived on
no devil lived on

Frink

The built-in reverse function reverses a string or the elements of a list.

println[reverse["abcdef"]]

GAP

Reversed("abcdef");
# "fedcba"

Gema

Reverse each line in the input stream. Using built in function:

\L<U>=@reverse{$1}

Not using built in function (recursively apply substring to same rule):

\L<U1><U>=@{$2}$1

Go

Functions below assume UTF-8 encoding. (The task mentions Unicode but does not specify an encoding.) Strings in Go are not restricted to be UTF-8, but Go has good support for it and works with UTF-8 most natually. As shown below, certain string conversions work in UTF-8 and the range clause over a string works in UTF-8. Go also has a Unicode package in the standard library that makes easy work of recognizing combining characters for this task.

package main
 
import (
    "fmt"
    "unicode"
    "unicode/utf8"
)
 
// no encoding
func reverseBytes(s string) string {
    r := make([]byte, len(s))
    for i := 0; i < len(s); i++ {
        r[i] = s[len(s)-1-i]
    }
    return string(r)
}
 
// reverseCodePoints interprets its argument as UTF-8 and ignores bytes
// that do not form valid UTF-8.  return value is UTF-8.
func reverseCodePoints(s string) string {
    r := make([]rune, len(s))
    start := len(s)
    for _, c := range s {
        // quietly skip invalid UTF-8
        if c != utf8.RuneError {
            start--
            r[start] = c
        }
    }
    return string(r[start:])
}
 
// reversePreservingCombiningCharacters interprets its argument as UTF-8
// and ignores bytes that do not form valid UTF-8.  return value is UTF-8.
func reversePreservingCombiningCharacters(s string) string {
    if s == "" {
        return ""
    }
    p := []rune(s)
    r := make([]rune, len(p))
    start := len(r)
    for i := 0; i < len(p); {
        // quietly skip invalid UTF-8
        if p[i] == utf8.RuneError {
            i++
            continue
        }
        j := i + 1
        for j < len(p) && (unicode.Is(unicode.Mn, p[j]) ||
            unicode.Is(unicode.Me, p[j]) || unicode.Is(unicode.Mc, p[j])) {
            j++
        }
        for k := j - 1; k >= i; k-- {
            start--
            r[start] = p[k]
        }
        i = j
    }
    return (string(r[start:]))
}
 
func main() {
    test("asdf")
    test("as⃝df̅")
}
 
func test(s string) {
    fmt.Println("\noriginal:      ", []byte(s), s)
    r := reverseBytes(s)
    fmt.Println("reversed bytes:", []byte(r), r)
    fmt.Println("original code points:", []rune(s), s)
    r = reverseCodePoints(s)
    fmt.Println("reversed code points:", []rune(r), r)
    r = reversePreservingCombiningCharacters(s)
    fmt.Println("combining characters:", []rune(r), r)
}

Output:

original:       [97 115 100 102] asdf
reversed bytes: [102 100 115 97] fdsa
original code points: [97 115 100 102] asdf
reversed code points: [102 100 115 97] fdsa
combining characters: [102 100 115 97] fdsa

original:       [97 115 226 131 157 100 102 204 133] as⃝df̅
reversed bytes: [133 204 102 100 157 131 226 115 97] ��fd���sa
original code points: [97 115 8413 100 102 773] as⃝df̅
reversed code points: [773 102 100 8413 115 97] ̅fd⃝sa
combining characters: [102 773 100 115 8413 97] f̅ds⃝a

Groovy

Solution:

println "Able was I, 'ere I saw Elba.".reverse()

Output:

.ablE was I ere' ,I saw elbA

Haskell

reverse = foldl (flip (:)) []

This function as defined in the Haskell Prelude.

Supporting combining characters

import Data.Char (isMark)
import Data.List (groupBy)
myReverse = concat . reverse . groupBy (const isMark)

groupBy (const isMark) is an unusual way of splitting a string into its combined characters

HicEst

CHARACTER string = "Hello World", tmp
 
L = LEN( string )
DO i = 1, L/2
  tmp = string(i)
  string(i) = string(L-i+1)
  string(L-i+1) = tmp
ENDDO
 
WRITE(Messagebox, Name) string 

Icon and Unicon

procedure main(arglist)
s := \arglist[1] | "asdf"
write(s," <-> ", reverse(s))    # reverse is built-in
end

Io

"asdf" reverse

J

Reverse (|.) reverses a list of items (of any shape or type).

   |.'asdf'
fdsa

Extra credit: First, a function to determine whether a Unicode character is a combining character:

   ranges=.16b02ff 16b036f, 16b1dbf 16b1dff, 16b20cf 16b20ff, 16bfe1f 16bfe2f
   iscombining=. 2 | ranges&I.

Then we need to box groups of letters and combining characters, reverse, and unbox. The boxing function can be carried out easily with dyad cut, which uses the indices of the ones on the right as the starting points for groups of characters. For clarity, its inverse will be defined as raze, which simply runs together the items inside boxes of its argument.

   split=. (<;.1~ -.@iscombining) :. ;

After this, the solution is just to reverse under the split transformation. This also takes place under J code to convert from Unicode to integers.

   |.&.split&.(3 u: 7&u:) 'as⃝df̅'
f̅ds⃝a

Java

public static String reverseString(String s) {
    return new StringBuffer(s).reverse().toString();
}

Works with: Java version 1.5+

public static String reverseString(String s) {
    return new StringBuilder(s).reverse().toString();
}

JavaScript

var a = "cat".split("");
a.reverse();
print(a.join(""));   // tac

Julia

julia> reverse("hey")
"yeh"

LabVIEW

This image is a VI Snippet, an executable image of LabVIEW code. The LabVIEW version is shown on the top-right hand corner. You can download it, then drag-and-drop it onto the LabVIEW block diagram from a file browser, and it will appear as runnable, editable code.

Lang5

: flip "" split reverse "" join ;
"qwer asdf" flip .

Liberty BASIC

input$ ="abcdefgABCDEFG012345"
print input$
print ReversedStr$( input$)
 
end
 
function ReversedStr$(in$)
    for i =len(in$) to 1 step -1
    ReversedStr$ =ReversedStr$ +mid$( in$, i, 1)
    next i
end function

Logo

REVERSE works on both words and lists.

print reverse "cat   ; tac

Lua

Built-in string.reverse(s) or s:reverse().

theString = theString:reverse()

M4

define(`invert',`ifelse(len(`$1'),0,,`invert(substr(`$1',1))'`'substr(`$1',0,1))')

Maple

> StringTools:-Reverse( "foo" );
                                 "oof"

Mathematica

StringReverse["asdf"]

MATLAB

A built-in function, "fliplr(string)" handles reversing a string of ASCII characters. Unicode is a whole other beast, if you need this functionality test to see if "fliplr()" properly handles the unicode characters you use. If it doesn't then you will need to code a function that is specific to your application.

Sample Usage:

>> fliplr(['She told me that she spoke English and I said great. '...
'Grabbed her hand out the club and I said let''s skate.'])
 
ans =
 
.etaks s'tel dias I dna bulc eht tuo dnah reh debbarG .taerg dias I dna hsilgnE ekops ehs taht em dlot ehS

Maxima

sreverse("abcdef");         /* "fedcba" */
 
sreverse("rats live on no evil star");   /* not a bug :o) */

MAXScript

fn reverseString s =
(
    local reversed = ""
    for i in s.count to 1 by -1 do reversed += s[i]
    reversed
)

Mirah

def reverse(s:string) 
    StringBuilder.new(s).reverse
end
 
puts reverse('reversed')

Modula-3

MODULE Reverse EXPORTS Main;
 
IMPORT IO, Text;
 
PROCEDURE String(item: TEXT): TEXT =
  VAR result: TEXT := "";
  BEGIN
    FOR i := Text.Length(item) - 1 TO 0 BY - 1 DO
      result := Text.Cat(result, Text.FromChar(Text.GetChar(item, i)));
    END;
    RETURN result;
  END String;
 
BEGIN
  IO.Put(String("Foobarbaz") & "\n");
END Reverse.

Output:

zabrabooF

MUMPS

REVERSE
 ;Take in a string and reverse it using the built in function $REVERSE
 NEW S
 READ:30 "Enter a string: ",S
 WRITE !,$REVERSE(S)
 QUIT

Output:

USER>D REVERSE^ROSETTA
Enter a string: Hello, World!
!dlroW ,olleH

Nemerle

Supporting Combining Characters

Compile with:

ncc -reference:System.Windows.Forms reverse.n

using System;
using System.Globalization;
using System.Windows.Forms;
using System.Console;
using Nemerle.Utility.NString;
 
module StrReverse
{
    UReverse(text : string) : string
    {
        mutable output = [];
        def elements = StringInfo.GetTextElementEnumerator(text);
        while (elements.MoveNext())
            output ::= elements.GetTextElement().ToString();
        Concat("", output.Reverse());
    }
 
    Main() : void
    {
        def test = "as⃝df̅";
        MessageBox.Show($"$test --> $(UReverse(test))");  //for whatever reason my console didn't display Unicode properly, but a MessageBox worked
    }
}

Basic Reverse

Doesn't require the System.Globalization namespace, probably a little less overhead.

Reverse(text : string) : string
{
    mutable output = [];
    foreach (c in text.ToCharArray())
        output ::= c.ToString();
    Concat("", output)
}

NetRexx

/* NetRexx */
 
options replace format comments java crossref savelog symbols nobinary
 
reverseThis = 'asdf'
sihTesrever = reverseThis.reverse
 
say reverseThis
say sihTesrever
 
return

Output:

asdf
fdsa

Nial

reverse 'asdf'
=fdsa

Nimrod

 
var
  str1 = "Reverse This!"
 
proc reverse(s: string): string =
  result = ""
  for i in countdown(high(str1), 0):
    result.add str1[i]
 
echo "Original string: ", str1, "\nReversed: ", reverse(str1)
 

Output:

Original string: Reverse This!
Reversed: !sihT esreveR

NewLISP

(reverse "!dlroW olleH")

Objeck

 
result := "asdf"->Reverse();
 

Objective-C

This extends the NSString object adding a reverseString class method.

#import <Foundation/Foundation.h>
 
@interface NSString (Extended)
-(NSString *)reverseString;
@end
 
@implementation NSString (Extended)
-(NSString *) reverseString
{
    NSUInteger len = [self length];
    NSMutableString *rtr=[NSMutableString stringWithCapacity:len];
    //        unichar buf[1];
 
    while (len > (NSUInteger)0) { 
        unichar uch = [self characterAtIndex:--len]; 
        [rtr appendString:[NSString stringWithCharacters:&uch length:1]];
    }
    return rtr;
}
@end

Usage example:

int main()
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
 
    NSString *test = [@"!A string to be reverted!" reverseString];
 
    NSLog(@"%@", test);
 
    [pool release];
    return 0;
}

Supporting combining characters

Extra credit

#import <Foundation/Foundation.h>
 
@interface NSString (Extended)
-(NSString *)reverseString;
@end
 
@implementation NSString (Extended)
-(NSString *)reverseString
{
	NSInteger l = [self length] - 1;
	NSMutableString *ostr = [NSMutableString stringWithCapacity:[self length]];
	while (l >= 0)
	{
		NSRange range = [self rangeOfComposedCharacterSequenceAtIndex:l];
		[ostr appendString:[self substringWithRange:range]];
		l -= range.length;
	}
	return ostr;
}
@end

Usage example:

int main()
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
 
    NSString *test = [@"as⃝df̅" reverseString];
 
    NSLog(@"%@", test);
 
    [pool release];
    return 0;
}

OCaml

Here a version that returns a new allocated string (preserving the original one):

let rev_string str =
  let len = String.length str in
  let res = String.create len in
  let last = len - 1 in
  for i = 0 to last do
    let j = last - i in
    res.[i] <- str.[j];
  done;
  (res)

and here with in place modification:

let rev_string str =
  let last = String.length str - 1 in
  for i = 0 to last / 2 do
    let j = last - i in
    let c = str.[i] in
    str.[i] <- str.[j];
    str.[j] <- c;
  done

here is a 100% functionnal string reversing function:

let rec revs strin list index =
  if List.length list = String.length strin
  then String.concat "" list
  else revs strin ((String.sub strin index 1)::list) (index+1)

revs "Hello World!" [] 0 will return "!dlroW olleH"

Octave

s = "a string";
rev = s(length(s):-1:1)

OpenEdge/Progress

FUNCTION reverseString RETURNS CHARACTER (
   INPUT i_c AS CHARACTER
):
 
   DEFINE VARIABLE cresult AS CHARACTER   NO-UNDO.
   DEFINE VARIABLE ii      AS INTEGER     NO-UNDO.
 
   DO ii = LENGTH( i_c ) TO 1 BY -1:
      cresult = cresult + SUBSTRING( i_c, ii, 1 ).
   END.
   RETURN cresult.
 
END FUNCTION.
 
MESSAGE reverseString( "asdf" ) VIEW-AS ALERT-BOX.

OxygenBasic

 
 
'8 BIT CHARACTERS
 
string s="qwertyuiop"
sys a,b,i,j,le=len s
'
for i=1 to le
  j=le-i+1
  if j<=i then exit for
  a=asc s,i
  b=asc s,j
  mid s,j,chr a
  mid s,i,chr b
next
'
 
print s
 
'16 BIT CHARACTERS
 
wstring s="qwertyuiop"
sys a,b,i,j,le=len s
'
for i=1 to le
  j=le-i+1
  if j<=i then exit for
  a=unic s,i
  b=unic s,j
  mid s,j,wchr a
  mid s,i,wchr b
next
'
print s
 

OxygenBasic x86 Assembler

32 bit code, 8-bit characters:

 
 
string s="qwertyuiop"
sys p=strptr s, le=len s
mov esi,p
mov edi,esi
add edi,le
dec edi
(
 cmp esi,edi
 jge exit
 mov al,[esi]
 mov ah,[edi]
 mov [esi],ah
 mov [edi],al
 inc esi
 dec edi
 repeat
)
 
print s
 

Oz

Strings are lists. A function "Reverse" defined on lists is part of the implementation.

{System.showInfo {Reverse "!dlroW olleH"}}

An efficient (tail-recursive) implementation could look like this:

local
   fun {DoReverse Xs Ys}
      case Xs of nil then Ys
      [] X|Xr then {DoReverse Xr X|Ys}
      end
   end
in
   fun {Reverse Xs} {DoReverse Xs nil} end
end

Oz uses a single-byte encoding by default. If you decide to use a multi-byte encoding, Reverse will not work correctly.

PARI/GP

reverse(expr)=my(v=Vec(Str(expr)),n=length(v));concat(vector(n,i,v[n-i+1]));

Pascal

The following examples handle correctly only single-byte encodings.

Standard Pascal

The following only works on implementations which implement Level 1 of standard Pascal (many popular compilers don't).

Standard Pascal doesn't have a separate string type, but uses arrays of char for strings. Note that Standard Pascal doesn't allow a return type of char array, therefore the destination array is passed through a var parameter (which is more efficient anyway).

{ the result array must be at least as large as the original array }
procedure reverse(s: array[min .. max: integer] of char, var result: array[min1 .. max1: integer] of char);
 var
  i, len: integer;
 begin
  len := max-min+1;
  for i := 0 to len-1 do
   result[min1 + len-1 - i] := s[min + i]
 end;

{Copy and paste it in your program}
function revstr(my_s:string):string;
    var out_s:string;
    ls,i:integer;
    begin
    ls:=length(my_s);
    for i:=1 to ls do
    out_s:=out_s+my_s[ls-i+1];
    revstr:=out_s;
    end;

Extended Pascal, Turbo Pascal, Delphi and compatible compilers

function reverse(s:string):string;
var i:integer;
var tmp:char;
begin
    for i:=1 to length(s) div 2 do
      begin
       tmp:=s[i];
       s[i]:=s[length(s)+1-i];
       s[length(s)+1-i]:=tmp;
       reverse:=s;
      end;
end;

Perl

This reverses characters (code points):

$string = "visor";
$flip = reverse $string;  # becomes "rosiv"

This reverses graphemes:

$string = "Jose\x{301}"; # "José" in NFD
$flip = join("", reverse $string =~ /\X/g); # becomes "ésoJ"

Perl 6

# Not transformative.
my $reverse = flip $string;
# or
say "hello world".flip;

Perl 6 is specced to work on graphemes by default, but current implementations are limited to processing codepoints.

PHP

strrev($string);

If you want Unicode support, you have to use some multibyte function. Sadly, PHP doesn't contain mb_strrev(). One of functions which support Unicode and is useful in this case is preg_split().

// Will split every Unicode character to array, reverse array and will convert it to string.
join('', array_reverse(preg_split('""u', $string, -1, PREG_SPLIT_NO_EMPTY)));

PicoLisp

(pack (flip (chop "äöüÄÖÜß")))

Output:

-> "ßÜÖÄüöä"

PL/I

s = reverse(s);

Pop11

define reverse_string(s);
    lvars i, l = length(s);
    for i from l by -1 to 1 do
        s(i);
    endfor;
    consstring(l);
enddefine;

PostScript

The following implementation works on arrays of numerics as well as characters ( string ).

/reverse{
/str exch def
/temp str 0 get def
/i 0 def
str length 2 idiv{
/temp str i get def
str i str str length i sub 1 sub get put
str str length i sub 1 sub temp put
/i i 1 add def
}repeat
str pstack
}def

Output is as below:

[1 2 3] reverse % input
[3 2 1]
 
(Hello World) reverse % input
(dlroW olleH)

PowerShell

Test string

$s = "asdf"

Array indexing

Creating a character array from the end to the string's start and join it together into a string again.

Works with: PowerShell version 1

[string]::Join('', $s[$s.Length..0])

Works with: PowerShell version 2

-join ($s[$s.Length..0])

Works with: PowerShell version 2

[array]::Reverse($s)

Regular expressions

Creating a regular expression substitution which captures every character of the string in a capture group and uses a reverse-ordered string of references to those to construct the reversed string.

Works with: PowerShell version 1

$s -replace
      ('(.)' * $s.Length),
      [string]::Join('', ($s.Length..1 | ForEach-Object { "`$$_" }))

Works with: PowerShell version 2

$s -replace
      ('(.)' * $s.Length),
      -join ($s.Length..1 | ForEach-Object { "`$$_" } )

Prolog

Works with: SWI Prolog

reverse("abcd", L), string_to_list(S,L).

output :
L = [100,99,98,97],
S = "dcba". 

The main workings are hidden inside the reverse/2 predicate, so lets write one to see how it works:

accRev([H|T], A, R) :- accRev(T, [H|A], R).
accRev([], A, A).
 
rev(L,R) :- accRev(L,[],R).

Protium

Padded out, variable length Chinese dialect

<# 显示 指定 变量 反转顺序 字串>集装箱|猫坐在垫子</#>

This assigns the reverse of 'the cat sat on the mat' to the variable 'container' and displays the result which is

子垫在坐猫

which Google Translate renders as

Sub-pad sitting cat

.

The same again but with everything in Korean.

<# 보이십 할당하 변물건 열거꾸 문자그>컨테이너|고양이가 매트 위에 앉아</#>

Reversing the Korean makes an untranslatable-by-Google mess of the sentence, viz

아앉 에위 트매 가이양고

.

The short-opcode version in English dialect is

<@ SAYLETVARREVLIT>集装箱|猫坐在垫子</@>

Protium works in Unicode.

PureBasic

Debug ReverseString("!dekrow tI")

Python

Optimized for user input

raw_input()[::-1]

Already known string

string[::-1]

Unicode reversal

(See this article for more information)

'''
  Reverse a Unicode string with proper handling of combining characters
'''
 
import unicodedata
 
def ureverse(ustring):
    '''
    Reverse a string including unicode combining characters
 
    Example:
        >>> ucode = ''.join( chr(int(n, 16))
                             for n in ['61', '73', '20dd', '64', '66', '305'] )
        >>> ucoderev = ureverse(ucode)
        >>> ['%x' % ord(char) for char in ucoderev]
        ['66', '305', '64', '73', '20dd', '61']
        >>> 
    '''
    groupedchars = []
    uchar = list(ustring)
    while uchar:
        if 'COMBINING' in unicodedata.name(uchar[0], ''):
            groupedchars[-1] += uchar.pop(0)
        else:
            groupedchars.append(uchar.pop(0))
    # Grouped reversal
    groupedchars = groupedchars[::-1]
 
    return ''.join(groupedchars)
 
if __name__ == '__main__':
    ucode = ''.join( chr(int(n, 16))
                     for n in ['61', '73', '20dd', '64', '66', '305'] )
    ucoderev = ureverse(ucode)
    print (ucode)
    print (ucoderev)

Qi

It's simplest just to use the common lisp REVERSE function.

(REVERSE "ABCD")

R

Works with: R version 2.8.1

The following code works with UTF-8 encoded strings too.

revstring <- function(stringtorev) {
   return(
      paste(
           strsplit(stringtorev,"")[[1]][nchar(stringtorev):1]
           ,collapse="")
           )
}

Alternatively (using rev() function):

 revstring <- function(s) paste(rev(strsplit(s,"")[[1]]),collapse="")

revstring("asdf")
revstring("m\u00f8\u00f8se")
Encoding("m\u00f8\u00f8se")   # just to check if on your system it's something
                              # different!

Output:

[1] "fdsa"
[1] "esøøm"
[1] "UTF-8"

R can encode strings in Latin1 and UTF-8 (the default may depend on the locale); the Encoding(string) can be used to know if the string is encoded in Latin1 or UTF-8; the encoding can be forced (Encoding(x) <- "latin1"), or we can use iconv to properly translate between encodings whenever possible.

Rascal

import String;
reverse("string")

Racket

As in Scheme:

#lang racket
 
(define (string-reverse s)
  (list->string (reverse (string->list s))))
 
(string-reverse "aoeu")

Output:

Welcome to DrRacket, version 5.3.3.5--2013-02-20(5eddac74/d) [3m].
Language: racket; memory limit: 512 MB.
"ueoa"
> 

Raven

"asdf" reverse

Output:

fdsa

REBOL

print reverse "asdf"

Note the string is reversed in place. If you were using it anywhere else, you would find it reversed:

x: "asdf"
print reverse x
print x ; Now reversed.

REBOL/View 2.7.6.3.1 14-Mar-2008 does not handle Unicode strings. This is planned for REBOL 3.

Retro

with strings'
"asdf" reverse puts

REXX

All methods shown below also work with   NULL   values.

using REVERSE bif

/*REXX program to reverse a string  (and show before and after strings).*/
 
string1 = 'A man, a plan, a canal, Panama!'
string2 = reverse(string1)
 
say ' original string: '  string1
say ' reversed string: '  string2
                                       /*stick a fork in it, we're done.*/

output

 original string:  A man, a plan, a canal, Panama!
 reversed string:  !amanaP ,lanac a ,nalp a ,nam A

using SUBSTR bif, left to right

/*REXX program to reverse a string  (and show before and after strings).*/
 
string1 = 'A man, a plan, a canal, Panama!'
string2 =
                                   do j=1  for length(string1)
                                   string2 = substr(string1,j,1)string2
                                   end   /*j*/
say ' original string: '  string1
say ' reversed string: '  string2
                                       /*stick a fork in it, we're done.*/

output is identical to the 1^st version.

(Regarding the previous example)   Another method of coding an abutment (an implied concatenation) is:

                                   string2 = substr(string1,j,1) || string2
                                                /*───── or ─────*/
                                   string2=substr(string1,j,1)||string2

using SUBSTR bif, right to left

/*REXX program to reverse a string  (and show before and after strings).*/
 
string1 = 'A man, a plan, a canal, Panama!'
string2 =
                                  do j=length(string1)  to 1  by -1
                                  string2 = string2 || substr(string1,j,1)
                                  end   /*j*/
say ' original string: '  string1
say ' reversed string: '  string2
                                       /*stick a fork in it, we're done.*/

output is identical to the 1^st version.

RLaB

>> x = "rosettacode"
rosettacode
 
// script
rx = "";
for (i in strlen(x):1:-1)
{
  rx = rx + substr(x, i);
}
 
>> rx
edocattesor

Ruby

str = "asdf"
reversed = str.reverse

or

#encoding: utf-8
"résumé niño".reverse #=> "oñin émusér"

Run BASIC

string$ = "123456789abcdefghijk"
for i = len(string$) to 1 step -1
 print mid$(string$,i,1);
next i

SAS

data _null_;
length a b $11;
a="I am Legend";
b=reverse(a);
put a;
put b;
run;

Sather

class MAIN is
  main is
    s ::= "asdf";
    reversed ::= s.reverse; 
    -- current implementation does not handle multibyte encodings correctly
  end;
end;

Scala

Easy way:

"asdf".reverse

Slightly less easy way:

"asdf".foldRight(""){ (a,b)=> b+a } 

Unicode-aware. I can't guarantee it get all the cases, but it does work with combining characters as well as supplementary characters. I did not bother to preserve the order of newline characters, and I didn't even consider directionality beyond just ruling it out.

def reverseString(s: String) = {
  import java.lang.Character._
 
  val combiningTypes = List(NON_SPACING_MARK, ENCLOSING_MARK, COMBINING_SPACING_MARK)
  def isCombiningCharacter(c: Char) = combiningTypes contains c.getType
  def isCombiningSurrogate(high: Char, low: Char) = combiningTypes contains getType(toCodePoint(high, low))
  def isCombining(l: List[Char]) = l match {
    case List(a, b) => isCombiningSurrogate(a, b)
    case List(a) => isCombiningCharacter(a)
    case Nil => true
    case _ => throw new IllegalArgumentException("isCombining expects a list of up to two characters")
  }
 
  def cleanSurrogate(l: List[Char]) = l match {
    case List(a, b) if a.isHighSurrogate && b.isLowSurrogate => l
    case List(a, b) if a.isLowSurrogate => Nil
    case List(a, b) => List(a)
    case _ => throw new IllegalArgumentException("cleanSurrogate expects lists of two characters, exactly")
  }
 
  def splitString(string: String) = (string+" ").iterator sliding 2 map (_.toList) map cleanSurrogate toList
 
  def recurse(fwd: List[List[Char]], rev: List[Char]): String = fwd match {
    case Nil => rev.mkString
    case c :: rest =>
      val (combining, remaining) = rest span isCombining
      recurse(remaining, c ::: combining.foldLeft(List[Char]())(_ ::: _) ::: rev)
  }
  recurse(splitString(s), Nil)
}

REPL on Windows doesn't handle Unicode, so I'll show the bytes instead:

scala> res71 map ("\\u%04x" format _.toInt)
res80: scala.collection.immutable.IndexedSeq[String] = IndexedSeq(\u0061, \u0073, \u20dd, \u0064, \u0066, \u0305)

scala> reverseString(res71) map ("\\u%04x" format _.toInt)
res81: scala.collection.immutable.IndexedSeq[String] = IndexedSeq(\u0066, \u0305, \u0064, \u0073, \u20dd, \u0061)

Scheme

(define (string-reverse s)
  (list->string (reverse (string->list s))))

> (string-reverse "asdf")
"fdsa"

Scratch

Sed

#!/bin/sed -f
 
/../! b
 
# Reverse a line.  Begin embedding the line between two newlines
s/^.*$/\
&\
/
 
# Move first character at the end.  The regexp matches until
# there are zero or one characters between the markers
tx
:x
s/\(\n.\)\(.*\)\(.\n\)/\3\2\1/
tx
 
# Remove the newline markers
s/\n//g

Seed7

Seed7 strings are encoded with UTF-32 therefore no special Unicode solution is necessary

$ include "seed7_05.s7i";
 
const func string: reverse (in string: stri) is func
  result
    var string: result is "";
  local
    var integer: index is 0;
  begin
    for index range length(stri) downto 1 do
      result &:= stri[index];
    end for;
  end func;
 
const proc: main is func
  begin
    writeln(reverse("Was it a cat I saw"));
  end func;

Output:

was I tac a ti saW

Slate

In-place reversal:

'asdf' reverse

Non-destructive reversal:

'asdf' reversed

Smalltalk

'asdf' reverse

Works with: Smalltalk/X

the above does inplace, destructive reverse. It is usually better to use

'asdf' reversed

which returns a new string.

SNOBOL4

ASCII-only

	output = reverse(reverse("reverse"))
end

{{out}

reverse

Standard ML

val str_reverse = implode o rev o explode;
val string = "asdf";
val reversed = str_reverse string;

Tcl

package require Tcl 8.5
string reverse asdf

TI-83 BASIC

The following program will place the reverse of Str1 in Str0. Note: length( and sub( can be found in the catalog.

:"ASDF"→Str1
:
:" "→Str0
:length(Str1)→B
:For(A,B,1,-1)
:Str0+sub(Str1,A,1)→Str0
:End
:sub(Str0,2,B)→Str0

The reason a single space must be placed in Str0 and then later removed is that for some reason, the TI-83 will not allow concatenation with an empty string (so Str0+sub... would fail).

Turing

 
% Reverse a string
 
var input : string (100)
 
put "Enter a string to reverse: " ..
get input
 
var count : int := length(input)
loop
    if count >= 1 then
        put input(count) ..
    else
        exit
    end if
    count := count - 1
end loop
 

TUSCRIPT

$$ MODE TUSCRIPT
SET input="was it really a big fat cat i saw"
SET reversetext=TURN (input)
PRINT "before: ",input
PRINT "after:  ",reversetext

Output:

before: was it really a big fat cat i saw
after:  was i tac taf gib a yllaer ti saw   

UNIX Shell

 
#!/bin/bash
str=abcde
 
for((i=${#str}-1;i>=0;i--)); do rev="$rev${str:$i:1}"; done
 
echo $rev
 

Unlambda

Reverse the whole input:

``@c`d``s`|k`@c

Ursala

#import std
 
#cast %s
 
example = ~&x 'asdf'
 
verbose_example = reverse 'asdf'

Output:

'fdsa'

VBA

Non-recursive version

Public Function Reverse(aString as String) as String
' returns the reversed string
dim L as integer        'length of string
dim newString as string
 
newString = ""
L = len(aString)
for i = L to 1 step -1
 newString = newString & mid$(aString, i, 1)
next
Reverse = newString
End Function

Recursive version

Public Function RReverse(aString As String) As String
'returns the reversed string
'do it recursively: cut the string in two, reverse these fragments and put them back together in reverse order
Dim L As Integer    'length of string
Dim M As Integer    'cut point
 
L = Len(aString)
If L <= 1 Then   'no need to reverse
  RReverse = aString
Else
  M = Int(L / 2)
  RReverse = RReverse(Right$(aString, L - M)) & RReverse(Left$(aString, M))
End If
End Function

Example dialogue

print Reverse("Public Function Reverse(aString As String) As String")
gnirtS sA )gnirtS sA gnirtSa(esreveR noitcnuF cilbuP

print RReverse("Sunday Monday Tuesday Wednesday Thursday Friday Saturday Love")
evoL yadrutaS yadirF yadsruhT yadsendeW yadseuT yadnoM yadnuS

print RReverse(Reverse("I know what you did last summer"))
I know what you did last summer

Vedit macro language

This routine reads the text from current line, reverses it and stores the reversed string in text register 10:

Reg_Empty(10)
for (BOL; !At_EOL; Char) {
    Reg_Copy_Block(10, CP, CP+1, INSERT)
}

This routine reverses the current line in-place:

BOL
while (!At_EOL) {
    Block_Copy(EOL_pos-1, EOL_pos, DELETE)
}

Visual Basic .NET

  Function Reverse(ByVal s As String) As String
    Dim t() as Char = s.toCharArray
    Array.reverse(t)
    Return new String(t)
  End Function

XPL0

include c:\cxpl\codes;  \intrinsic 'code' declarations
string 0;               \use zero-terminated strings, instead of MSb terminated
 
func StrLen(Str);       \Return the number of characters in an ASCIIZ string
char Str;
int  I;
for I:= 0 to -1>>1-1 do
        if Str(I) = 0 then return I;
 
func RevStr(S);         \Reverse the order of the bytes in a string
char S;
int  L, I, T;
[L:= StrLen(S);
for I:= 0 to L/2-1 do
        [T:= S(I);  S(I):= S(L-I-1);  S(L-I-1):= T];
return S;
];
 
[Text(0, RevStr("a")); CrLf(0);
 Text(0, RevStr("ab")); CrLf(0);
 Text(0, RevStr("abc")); CrLf(0);
 Text(0, RevStr("Able was I ere I saw Elba.")); CrLf(0);
]

Output:

a
ba
cba
.ablE was I ere I saw elbA

Yorick

This only handles ASCII characters. It works by converting a string to an array of char; dropping the last character (which is the null byte); reversing the order of the characters; then converting back to a string.

strchar(strchar("asdf")(:-1)(::-1))