Reverse the truth

Question

Write a program or function that:

1. takes in a string from stdio or arguments
2. replaces all occurrences of true with false and false with true
3. reverses it, but does not reverse true and false
4. returns or prints result

Examples (left side is input):

"true"                     "false"
"2false"                   "true2"
"true is false"            "true si false"
"false,true,undefined"     "denifednu,false,true"
"stressed-false"           "true-desserts"
"falstrue"                 "falseslaf"
"true false true x"        "x false true false"

Standard loopholes apply. This is code-golf, so shortest code in bytes wins.

Leaderboard

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Answer 1

C# 6, 144 bytes

string R(string t)=>string.Concat(new System.Text.RegularExpressions.Regex("true|false").Replace(t,m=>m.Value[0]<'g'?"eurt":"eslaf").Reverse());

It uses a regular expression to match true|false, and if it matches true it will be replaced by eslaf, otherwise by eurt. m.Value[0]<'g' is a shorter way to say m.Value[0]=="false", because the only possible values for m.Value are "true" or "false", so if the char code of the first char is smaller than the char code of 'g', it's "false".

Old version, 95 bytes

This one had a bug, it didn't return correct output for falstrue.

string R(string t)=>string.Concat(t.Reverse()).Replace("eurt","false").Replace("eslaf","true");

Answer 2

TeaScript, 36 25 24 bytes

xv¡g("eurt|eslaf",#ln>4)   

TeaScript is JavaScript for golfing.

Edits: Saved 11 bytes thanks to @Vɪʜᴀɴ. Fixed for input falstrue and saved a byte.

Old version (Invalid):

xv¡g("eurt",f)g(f.T¡v¡,t)

Explanation:

x    // Input
 v¡    // Reverse
   g("eurt",    // Global replace "eurt" with "false".
            f)    // f is predefined to false.
              g(f.T¡v¡, // Convert false to string, then reverse.
                       t) // t is predefined to true.

Answer 3

Bash+GNU, 45 38 73 bytes

Edit: works with both trufalse and falstrue

sed s/false/%eurt%/g\;s/true/%eslaf%/g|rev|sed "s/%\(true\|false\)%/\1/g"

Old version, 38 bytes (shortened, thanks to Digital Trauma):

rev|sed s/eurt/false/g\;s/eslaf/true/g

Answer 4

JavaScript ES6, 59

As an anonymous function.

Note, replace is used just as a shorthand for match().map(). The replaced string is discarded, and the output string is made piece by piece backwards (so no need to reverse).

s=>s.replace(/false|true|./g,x=>s=(x[1]?x<'t':x)+s,s='')&&s

Test running the snippet below in an EcmaScript 6 compliant browser.

f=s=>s.replace(/false|true|./g,x=>s=(x[1]?x<'t':x)+s,s='')&&s

//test

console.log=x=>O.innerHTML+=x+'\n'

;[
 ["true","false"]
,["falstrue","falseslaf"]  
,["1false","true1"]
,["true is false","true si false"]
,["false,true,undefined","denifednu,false,true"]
,["stressed-false","true-desserts"]
,["true false true x","x false true false"]
].forEach(t=>console.log(t[0]+' -> '+f(t[0])))

<pre id=O></pre>

Answer 5

Windows Batch, 184 213 bytes

Fixed the bug, falstrue -> falseslaf and trufalse -> trueurt

Probably one of the less popular languages:

setlocal enabledelayedexpansion
set /p Q=
set N=0
:L
call set T=%%Q:~%N%,1%%%
set /a N+=1
if not "%T%" equ "" (
set R=%T%%R%
goto L
)
set R=%R:eurt=false%
set R=%R:eslaf=true%
set R=%R:falstrue=falseslaf%
echo %R%

Answer 6

Haskell, 94 bytes

Performs pattern matching on the input string, looking for "false" or "true" and appends the opposite to the result of applying the function on the remainder of the string. If true or false isn't found, it uses recursion to reverse the string in the same fashion.

f[]=[]
f('t':'r':'u':'e':s)=f s++"false"
f('f':'a':'l':'s':'e':s)=f s++"true"
f(x:s)=f s++[x]

Answer 7

JavaScript ES6, 95 93 bytes

Unnamed function. Add f= to the beginning to use it. Thanks Ismael! Also assumes that the input does not contain tabs.

x=>[...x[r="replace"](/false/g,"\teslaf")[r](/(\t)*true/g,"eurt")[r](/\t/g,"")].reverse().join``

Answer 8

Pyth, 30 bytes

::_z"eurt""false""eslaf""true"

This reverses the input (_z), substitutes "eurt" for "false" and "eslaf" for "true". Replacement is done using :.

Try it online

Answer 9

rs, 56 bytes

\t
+(.*\t)(.)(.*)/\2\1\3
\t/
eurt/fals\t
eslaf/true
\t/e

Live demo and test cases.

Answer 10

Julia, 59 55 46 bytes

s->replace(reverse(s),r"eurt|eslaf",i->i<"et")

This creates an unnamed function that accepts a string and returns a string. To call it, give it a name, e.g. f=s->....

The input is reversed using reverse. We match on the regular expression eurt|eslaf which matches true or false backwards. To the match we apply a function that returns true if the match is lexicographically smaller than et (i.e. eslaf) and false otherwise. The boolean literals are converted to string in the output.

Saved 9 bytes and fixed an issue thanks to Glen O!

Answer 11

Javascript, 135 Bytes

function(s){return s.split("").reverse().join("").replace(/eslaf/i,'☺').replace(/eurt/i,'☻').replace(/☻/g,!1).replace(/☺/g,!1)}

Test:

=>"false is the opposite of true"

<="true fo etisoppo eht si false"

Thanks ProgramFOX and edc65 for pointing out a bug!

Answer 12

Java, 162 98 92 bytes

Thanks (and sorry!~) to @DanielM. for telling me about StringBuffer and the fact that we can use functions!

Because, you know, Java.

s->(""+new StringBuffer(s.replaceAll("false","eurt")).reverse()).replaceAll("eurt","false");

Returns the correct, reversed string.

Ungolfed Version:

s->new StringBuilder(
    s.replaceAll("false","eurt"))
    .reverse().toString().replaceAll("eurt","false");

Basically, I replace all instances of "false" with a backwards "true", then reverse the entire string, and then replace the now backwards versions of "true" (not the ones I just replaced) with "false". Easy peasy.

Answer 13

Mathematica, 64 bytes

StringReverse@#~StringReplace~{"eurt"->"false","eslaf"->"true"}&

Answer 14

Python 3, 68 100 bytes

I'm still golfing it, but it's fixed to the bug, so falstrue -> falselsaf and trufalse -> trueurt

Pretty straightforward:

print(input()[::-1].replace("eurt","false").replace("eslaf","true").replace("falstrue","falseslaf"))

Answer 15

Japt, 26 bytes

Note: This may be invalid, as it requires bug fixes made after this challenge was posted.

Uw r"eurt|eslaf",X=>X<"et"

Try it in the online interpreter! (Arrow function requires ES6-compliant browser, such as Firefox.)

How it works

             // Implicit: U = input string
Uw r         // reverse U, then replace:
"eurt|eslaf" // occurrences of either "eurt" or "eslaf"
X=>X<"et"    // with "false" or "true", respectively
             // Implicit: output last expression

Here's a version that worked before the bug fixes: (38 bytes)

Uw $.replace(/eurt|eslaf/g,X=>X<"et")$

Answer 16

Gema, 43

*=@a{@reverse{*}};a:eurt=false;a:eslaf=true

(Gema is an obscure macro language.)

Answer 17

Pyth, 28 22

Amr`!dZ2jHjLGcR_Hc_z_G

6 bytes thanks to Jakube

Works correctly for falstrue, as shown in the suite below.

Test suite

Answer 18

Haskell, 102 bytes

h('t':'r':'u':'e':s)="eslaf"++h s
h('f':'a':'l':'s':'e':s)="eurt"++h s
h(x:s)=x:h s
h[]=""
r=reverse.h

The replacement of "true" by "false" and vice-versa is quite lengthy with the pattern-matching, but at least it deals correctly with "falstrue" and the like. And besides, I suspect that a correct regex-based version would be a bit longer.

Answer 19

Python 3 - 108 92 bytes

import re
print(re.sub("eslaf|eurt",lambda m:repr(len(m.group(0))>4).lower(),input()[::-1]))

Uses a regex to match on "true" or "false" and uses a lambda to process matches and choose what to use as a replacement string. Using repr gets the string representation of (len(match)>4) which gives "True" when "false" is matched and vice versa (and use .lower() because repr(bool) gives a capitalized string) to get the inverse of the match and finish up by reversing the replacement and then the processed input using [::-1]

Managed to get the length down 16 bytes from TFelds suggestions.

Edit: Python is back in front of java, no need for alarm.

Answer 20

𝔼𝕊𝕄𝕚𝕟, 40 chars / 65 bytes

ô[…ïċ/false|true/g,⇏[…⬯+!שúa]ù⬮ø⬯)]ù⬮ø⬯)

Try it here (Firefox only).

Thanks to @feersum for pointing out a bug!

21 chars / 43 bytes, non-competitive

ôᴙ(ïċ/⊭|⊨⌿,↪ᴙ(⬯+!ë$⸩⸩

Try it here (Firefox only).

Answer 21

Prolog, 225 bytes

p(X):-string_to_list(X,L),reverse(L,B),q(B,C),string_to_list(Z,C),write(Z),!.
q([],[]).
q([101,117,114,116|T],[102,97,108,115,101|L]):-q(T,L).
q([101,115,108,97,102|T],[116,114,117,101|L]):-q(T,L).
q([H|T],[H|L]):-q(T,L).

Try it out online here
Run by querying in the following way:

p("falstrue").

Answer 22

Japt, 15 bytes

Ôr`laf|e¨t`Èøf

Try it

Answer 23

Ruby, 55 bytes

->s{s.gsub(/true|false/){$&[?t]?:eslaf: :eurt}.reverse}

Test:

->s{s.gsub(/true|false/){$&[?t]?:eslaf: :eurt}.reverse}["false,true,undefined"]
=> "denifednu,false,true"

Answer 24

Perl 5, 68 bytes

67 plus 1 for -E instead of -e

%a=(false,eurt,true,eslaf);say~~reverse<>=~s/false|true/$a{$&}/gr

Answer 25

OpenSCAD, 178 bytes

(Note that this uses the String Theory library, as OpenSCAD doesn't exactly have a standard library. Additionally, this is a function because the only allowed input is to hard-code it.

use <Strings.scad>;function f(a)=contains(g(a),"eurt")?replace(g(a),"eurt","false"):g(a);function g(a)=contains(reverse(a),"eslaf")?replace(reverse(a),"eslaf","true"):reverse(a);

Answer 26

C#, 260 bytes

using System;class P{static void Main(){var y=Console.ReadLine();char[] n=y.ToCharArray();Array.Reverse(n);var s=new string(n);if(s.Contains("eslaf")){s=s.Replace("eslaf","true");}if(s.Contains("eurt")){s=s.Replace("eurt","false");}Console.WriteLine(s);}}

Answer 27

PHP, 60 bytes

Simple, reverses the string first, then replaces the reversed versions with their respective swaps.

"falstrue" becomes "eurtslaf" becomes "falseslaf".

<?=strtr(strrev($argv[1]),{'eurt'=>'false','eslaf'=>'true'})

Answer 28

Perl 5.10, 54 bytes

$_=reverse<>;s/(eurt)|(eslaf)/$1?"false":"true"/eg;say

Reverse, then replace. A different way of doing it besides the hash table used for the other Perl answer, which ends up being shorter!

Try it online.

Answer 29

Pyth - 18 bytes

Can be golfed a lot more.

Amr`!dZ2::_z_GH_HG

Test Suite.