# Shortest way to reverse a number

Write a function (or equivalent subprogram) to accept a single integer valued argument and return a (similarly typed) value found by reversing the order of the base-10 digits of the argument.

For example given 76543 return 34567

• Go back to the time the number was a string, then reverse the string – pmg Jun 11 '11 at 10:54
• The idea of a "shortest algorithm" is somewhat specious, especially if you'll allow "any language." Think up an algorithm, and I'll give you a DSL with an appropriate "~" operator ... – Pointy Jun 11 '11 at 10:56
• Just a notice: any number ending with 0 becomes a shorter number of digits when reversed... – powtac Jun 11 '11 at 12:44
• I know an algorithm that takes no time at all, but only works on palindromic numbers ;) – schnaader Jun 11 '11 at 14:47
• Found time to do the re-write myself. I hope this remain the puzzle that eltond meant to pose. – dmckee Jun 12 '11 at 0:37

# HTML 21 7 chars (1 char if I'm cheeky...)

&#8238;n


replace n with your number

• This is just plain genius. I'd go for one char. Or 2, as it encodes to two bytes in UTF-16 :P – tomsmeding Dec 3 '12 at 13:33
• Hahaha I did a Google search on that tag and was rewarded with Your search -‮ - did not match any documents. – JoeFish Dec 4 '12 at 14:32
• U could try this link in browser: data:text/html,&%238238;egnahcxEkcatS olleH – F. Hauri Dec 1 '13 at 12:20
• Funny in google transate too. @JoeFish: I can't reproduce, please post a link! ‮ – F. Hauri Dec 1 '13 at 12:27
• @JoeFish When I look at the comment, your username is flipped and there is some text after it. ‮txet emos si ereH – Stefnotch Oct 22 '15 at 17:03

Python

int(str(76543)[::-1])

EDIT:

Shorter solution as suggested by @gnibbler:

int(76543[::-1])


or, if above is unclear:

x=76543
int(x[::-1])

• s[::-1] is a lot faster than ''.join(reversed(s)) – riza Jun 11 '11 at 12:56
• You can use backticks (for repr) instead of using str – gnibbler Jun 12 '11 at 11:11
• @gnibbler Thanks for suggestion. I've updated my answer. – Vader Jun 12 '11 at 13:03
• TBH, that ain't a function/proceduce/whatever you want to call it, and the specs require it. – Thomas Eding Aug 19 '11 at 20:39
• Also, it doesn't even accept a value... – Exelian Aug 30 '11 at 18:43

Universal (language agnostic/independent)

If you want to use only numbers (avoid converting the number to string) and don't want to use some specific library (to be universal for any language):

x = 76543 # or whatever is your number
y = 0
while x > 0:
y *= 10
y += ( x %10 )
x /= 10 # int division


This is python, but it could be done in any language, because it's just a math method.

• If you replace mod with %, it's valid Python ;) – phihag Jun 11 '11 at 11:02
• You're right, actually :) 10x – Kiril Kirov Jun 11 '11 at 11:05
• Not the shortest, but the most common and universal. – Kiril Kirov Jun 11 '11 at 12:53
• y=y*10+x%10.... – st0le Jun 11 '11 at 13:48
• BrainFuck doesn't, though it can be calculated. Any language that doesn't have it can use a - (n * int(a/n)) instead of a mod n. Also, if you look here, the modulus operation is implemented differently in every language. (See the table on the right.) – mbomb007 Jun 24 '15 at 14:25

## Perl 6

+$n.flip  or: $n.flip


for dynamically typed code.

Numbers got string methods due to language design.

# J - 6 characters + variable

".|.":y


• As a function: |.&.": "reverse under do" which is pretty much a literal translation of the task. – FireFly Aug 8 '14 at 20:40

## APL (3)

⍎⌽⍕


Usage:

⍎⌽⍕12345 => 54321


# PHP, 9 chars

(int)strrev(123);


To do it short where N is a constant:

strrev(N)


# Befunge (3 characters)

Complete runnable program:

N.@


Where N is your number. Rules say "accept a single integer valued argument"; In Befunge you can only enter integers from 0 to 9.

• Those are the only literals, but other numbers could certainly be represented. Otherwise, the winning answer would be Brainfuck with the empty program. ;-) – FireFly Dec 1 '13 at 10:07

### Language-independent/mathematics

Inspired by Kiril Kirov's answer above. I got curious about the mathematical properties of reversing a number, so I decided to investigate a bit.

Turns out if you plot the difference n - rev(n) for natural numbers n in some base r, you get patterns like this ((n - rev(n)) / (r - 1), for r=10, wrapped at r columns, red denotes negative number): This sequence could be generated as such (pseudocode):

for i=1 to r:
output 0

for m=0, 1, …
for k=1 to (r-1):
for d=1 to r^m:
for i=0 to (r-1):
output (r-1) * (r+1)^m * (k - i)


If you store these values in a list/array, then n - arr[n] would get you the reversed form of n. Now, to "mathematically golf" this, we'd ideally want a closed-form expression that gives us the n:th value in the sequence, so that we could have a closed-form expression for solving the entire task. Unfortunately, I haven't been able to find such an expression... but it looks like it should be possible. :(

So yeah, not so much a code-golf as a mathematical curiosity, but if there is a closed-form expression of the above sequence it might actually be useful in proper PL golf submissions.

f=read.reverse.show.(+0)

• How about f=read.reverse.show.(+0)? – FUZxxl Jun 11 '11 at 13:43
• (+0): Legit man! Though technically you don't need the .(+0) at all, as f would be more polymorphic than what the problem requires (it is allowed to return a 'similarly typed' output). I would shave off those 5 characters. – Thomas Eding Aug 10 '11 at 22:14

# Vim

17 chars

:se ri<CR>C<C-R>"

• I would say that's 10 chars (keystrokes) if you type the command directly in vim. Btw, I learned something new in vim today, thanks :) – daniero Jan 4 '13 at 18:35

Scala - 33 Chars

def r(a:Int)=(a+"").reverse.toInt

• +1 for scala, nice to see something else than python/ruby/perl – lhk Dec 12 '12 at 10:19
• This will fail on negative Int. -123 should return -321 – samach Aug 12 at 21:04

# Ruby (14)

x = 13456
x.to_s.reverse

• "no" is undefined. I think you meant to put "x" there. – David Rivers Nov 29 '11 at 3:59
• 123456.to_s.reverse is even shorter. – Steffen Roller Nov 28 '12 at 5:33
• @mmdemirbas - thanks for fixing the typo – bodacious Nov 28 '12 at 15:39
• Needs to be .to_s.reverse.to_i to comply with spec. – histocrat Dec 29 '12 at 16:48
• A number that starts with 0 doesnt seem to work. 0112.to_s.reverse.to_i => 47 – Joel Feb 16 '15 at 15:35

It is possible to convert a number a string, then reverse the string and then convert that string back to number. This kind of feature is probably available in all language. If you are looking for a more mathematical method then this might help:

int n = 76543;
int r = 0;

while (n > 0) {
r *= 10;
r += n % 10;
n /= 10;
}

• Mine is absolutely the same (: – Kiril Kirov Jun 11 '11 at 11:01
• Ya, only difference is your code looks like Python. – taskinoor Jun 11 '11 at 11:08
• This method overflow's on languages with limited precision. try 1111111119 – st0le May 16 '12 at 6:46

## Python 3+

### Function form: 28 characters

r=lambda i:int(str(i)[::-1])


### (Sub)program form: 25 characters

print(input()[::-1])


I consider some of the other Python examples to be cheating, or at least cheap, due to using hardcoded input and/or not fully satisfying the requirements.

## Golfscript, 5 chars

-1%~


This takes an argument on the stack and leaves the result on the stack. I'm exploiting the "subprogram" option in the spec: if you insist on a function, that's four chars more leaving it on the stack:

{-1%~}:r

• I think you must've meant -1%~ rather than -1$~ (and I've taken the liberty of editing your answer to say so). – Ilmari Karonen Mar 7 '12 at 19:47 In shell scripting :  echo "your number"|rev  Hope this was useful :) • good one! didn't know bash was capable to that also! – Pranit Bauva Jan 8 '13 at 16:17 • I guess technically it does return a similarly-typed "number"... could be shortened further with rev<<<yournumber, e.g. rev<<<132 (for bash/zsh, not per POSIX though) – FireFly Dec 1 '13 at 1:26 • Just rev is enough, the question doesn't say it has to be a function. You could compare rev to a built-in function, even though it's not one. – nyuszika7h Jul 1 '14 at 20:43 • this is invalid: 'rev' is not a builtin, but an external program call. – Bastian Bittorf Jul 22 '18 at 19:45 • 67 Bytes pure POSIX shell: X=$1;while [ $X != 0 ];do Y=$((Y*10+X%10));X=$((X/10));done;echo$Y – Bastian Bittorf Jul 22 '18 at 19:46

Kinda late but

## APL, 3

⍎⌽⍞


If you insists on a function

⍎∘⌽∘⍕

• Well looks like I couldn't spot a duplicate above...(due to it being on the 2nd page) – TwiNight Dec 29 '12 at 16:26
• I'm sad, that nobody gave brainfu*k or whitespace solution :( (one more vote and you're on the first page ) – Kiril Kirov Nov 30 '13 at 11:38
• @KirilKirov I've a brainfu*k solution : codegolf.stackexchange.com/a/32826/24829 – rpax Jul 2 '14 at 13:08

# Mathematica, 14 bytes

IntegerReverse


This is not competing, because this function was only added in last week's 10.3 release, but for completeness I thought I'd add the only ever (I think?) built-in for this task.

You could do the following in Java. Note that this converts to String and back and is not a mathematical solution.

public class test {
public static int reverseInt(int i) {
return Integer.valueOf((new StringBuffer(String.valueOf(i))).reverse().toString());
}

public static void main(String[] args) {
int i = 1234;
System.out.println("reverse("+i+") -> " + reverseInt(i));
}
}

• It is a mathematical solution. Mathematics is not numbers is not arithmetics. Mathematics also deals with strings of symbols. And in this special case, the conversion to and from string is just conversion to and from base-10. – R. Martinho Fernandes Jun 11 '11 at 14:20
• What I meant by "not a mathematical solution" is that we're not doing any math ourselves. The methods are doing all of the parsing and mathematics for us. As opposed to e.g. Kiril Kirov's answer. – Victor Jun 13 '11 at 7:38
• Will Overflow... – st0le May 16 '12 at 6:46

# Lua

Numbers and strings are interchangeable, so this is trivial

string.reverse(12345)


This one ACTUALLY takes an input, unlike some of the rest:

printinput()[::-1]


Python btw.

## Actionscript

43 characters. num as the parameter to the function:

num.toString().split('').reverse().join('')


r={"$it".reverse() as BigDecimal} assert r(1234) == 4321 assert r(345678987654567898765) == 567898765456789876543 assert r(345346457.24654654) == 45645642.754643543  # Perl, 11 chars The p flag is needed for this to work, included in the count. Usage: $ echo 76543 | perl -pE '$_=reverse'  • I count 10 chars – F. Hauri Dec 1 '13 at 13:20 • The p flag is included in the count – Zaid Dec 1 '13 at 15:54 # Clojure (42 chars) #(->> % str reverse(apply str)read-string)  Example usage: (#(->> % str reverse(apply str)read-string) 98321)  returns 12389 # Common Lisp - 60 chars (first(list(parse-integer(reverse(write-to-string '4279)))))  will get you 9724. • Why (first(list? parse-integer already returns the number. – Florian Margaine Jul 4 '15 at 18:26 # K, 3 bytes: .|$


Evaluate (.) the reverse (|) of casting to a string ($). Usage example:  .|$76543
34567


# rs, 20 bytes

#
+#(.*)(.)/\2#\1
#/


Technically, this doesn't count (rs was created earlier this year), but I didn't see any other regex-based answers, and I thought this was neat.

Live demo.

## Explanation:

#


Insert a pound character at the beginning of the string. This is used as a marker.

+#(.*)(.)/\2#\1


Continuously prepend the last character of the main string to the area before the marker until there are no characters left.

#/


Remove the marker.

## mIRC 4.45 (35 Bytes)

$regsubex(12,/(.)/g,$mid(\A,-\n,1))