Reverse a string
From Rosetta Code
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".
[edit] 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;
}
[edit] 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;
[edit] 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
[edit] APL
⌽'asdf'
fdsa
[edit] AutoHotkey
MsgBox % reverse("asdf")
reverse(string)
{
Loop, Parse, string
reversed := A_LoopField . reversed
Return reversed
}
[edit] AWK
function reverse(s)
{
p = ""
for(i=length(s); i > 0; i--) { p = p substr(s, i, 1) }
return p
}
BEGIN {
print reverse("edoCattesoR")
}
[edit] 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
[edit] 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.
[edit] Brainf***
[-]>,+[->,+]<[.<]
[edit] C
#include <stdio.h>
/* This function assumes the passed pointer points to a valid, zero-terminated string */
void reverse (char *s)
{
char *t = s;
while (*t != '\0') t++;
while (s < t) { int c = *s; *s++ = *--t; *t = c; }
}
int main ()
{
char text1[] = "asdf", text2[] = "";
reverse (text1); printf ("'%s'\n", text1);
reverse (text2); printf ("'%s'\n", text2);
return 0;
}
[edit] C++
#include <iostream>
#include <string>
#include <algorithm>
int main()
{
std::string s;
std::get_line(std::cin, s);
std::reverse(s.begin(), s.end()); // modifies s
std::cout << s << std::endl;
return 0;
}
[edit] 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);
}
[edit] Clojure
[edit] 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)))
[edit] 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)))))
And the test result:
user=> s "as⃝df̅" user=> (str-reverse s) "f̅ds⃝a"[ user=> (str-reverse (str-reverse s)) "as⃝df̅" user=>
[edit] 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 ) />
[edit] Common Lisp
(reverse my-string)
[edit] D
D has a built-in reverse function for array types (including string=char[]). It is an in-place function.
string s ;
s.reverse ;
s.dup.reverse ; // preserve original array
[edit] E
pragma.enable("accumulator")
def reverse(string) {
return accum "" for i in (0..!(string.size())).descending() { _ + string[i] }
}
[edit] 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
[edit] 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>>.
Result:
1> test:reverse(<<"hello">>). <<"olleh">>
[edit] 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" = .
[edit] Forth
: cexch { a1 a2 -- }
a1 c@ a2 c@ a1 c! a2 c! ;
: reverse ( caddr len -- )
1- bounds
begin 2dup >
while 2dup cexch
1+ swap 1- swap
repeat
2drop ;
s" testing" 2dup reverse type \ gnitset
This one doesn't require locals:
: reverse
2dup 1- chars over + \ save string, calculate addresses
begin
over over < \ as long we didn't pass the middle
while \ swap the characters
over c@ over c@ >r over c! over r> swap c!
1- swap 1+ swap \ increment pointers
repeat 2drop \ drop the addresses
;
s" testing" reverse type
[edit] 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
[edit] F#
let ReverseString (s:string) = new string(Array.rev (s.ToCharArray()))
[edit] Groovy
Solution:
println "Able was I, 'ere I saw Elba.".reverse()
Output:
.ablE was I ere' ,I saw elbA
[edit] Haskell
reverse = foldl (flip (:)) []
This function in defined in the Haskell Prelude.
[edit] J
Reverse (|.) reverses items of any shape or type.
|.'asdf'
fdsa
[edit] Java
public static String reverseString(String s) {
return new StringBuffer(s).reverse().toString();
}
[edit] JavaScript
var a = "cat".split("");
a.reverse();
print(a.join("")); // tac
[edit] Logo
REVERSE works on both words and lists.
print reverse "cat ; tac
[edit] Lua
Built-in string.reverse(s) or s:reverse().
theString = theString:reverse()
[edit] M4
define(`invert',`ifelse(len(`$1'),0,,`invert(substr(`$1',1))'`'substr(`$1',0,1))')
[edit] Mathematica
StringReverse["asdf"]
[edit] MAXScript
fn reverseString s =
(
local reversed = ""
for i in s.count to 1 by -1 do reversed += s[i]
reversed
)
[edit] 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
[edit] Nial
reverse 'asdf'
=fdsa
[edit] 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
{
NSInteger l;
NSMutableString *ostr = [NSMutableString stringWithCapacity:[self length] ];
for(l=[self length]-1; l>=0; l--)
{
[ostr appendFormat:@"%C", [self characterAtIndex:l] ];
}
return ostr;
}
@end
Usage example:
int main()
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSString *test = [@"!A string to be reverted!" reverseString];
NSLog(test);
[pool release];
return 0;
}
[edit] 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
[edit] Octave
s = "a string";
rev = s(length(s):-1:1)
[edit] 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.
[edit] Pascal
The following examples handle correctly only single-byte encodings.
[edit] 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;
[edit] Extended Pascal, Turbo Pascal, Delphi and compatible compilers
function reverse(s : String) : String;
var
i : Integer;
begin
for i := 1 to length(s) do
s[i] := s[length(s)-(i-1)];
reverse := s;
end;
[edit] Perl
reverse $string
[edit] Perl 6
Works with: Rakudo version #21 "Seattle"
flip $string
[edit] PHP
strrev($string)
[edit] PicoLisp
(pack (flip (chop "äöüÄÖÜß")))
Output:
-> "ßÜÖÄüöä"
[edit] PL/I
s = reverse(s);
[edit] 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;
[edit] PowerShell
Test string
$s = "asdf"
[edit] 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])
[edit] 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 { "`$$_" }))
Works with: PowerShell version 2
$s -replace
('(.)' * $s.Length),
-join ($s.Length..1 | ForEach-Object { "`$$_" } )
[edit] PureBasic
Debug ReverseString("!dekrow tI")
[edit] Python
[edit] Optimized for user input
raw_input()[::-1]
[edit] Already known string
string[::-1]
[edit] 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)
[edit] 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="")
)
}
revstring("asdf")
revstring("m\u00f8\u00f8se")
Encoding("m\u00f8\u00f8se") # just to check if on your system it's something
# different!
Outputs
[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.
[edit] 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.
[edit] REXX
say reverse('asdf')
Output:
fdsa
[edit] Ruby
str = "asdf"
reversed = str.reverse
[edit] Scala
Easy way:
"asdf".reverse
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)
[edit] Scheme
(define (string-reverse s)
(list->string (reverse (string->list s))))
> (string-reverse "asdf") "fdsa"
[edit] 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
[edit] Slate
In-place reversal:
'asdf' reverse
Non-destructive reversal:
'asdf' reversed
[edit] Smalltalk
'asdf' reverse
[edit] Standard ML
val str_reverse = implode o rev o explode;
val string = "asdf";
val reversed = str_reverse string;
[edit] Tcl
package require Tcl 8.5
string reverse asdf
[edit] 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).
[edit] Ursala
#import std
#cast %s
example = ~&x 'asdf'
verbose_example = reverse 'asdf'
output:
'fdsa'
[edit] 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)
}
