# Implement a Truth-Machine

A truth-machine (credits goes to this guy for coming up with it) is a very simple program designed to demonstrate the I/O and control flow of a language. Here's what a truth-machine does:

• Gets a number (either 0 or 1) from STDIN.

• If that number is 0, print out 0 and terminate.

• If that number is 1, print out 1 forever.

# Challenge

Write a truth-machine as described above in your language of choice. The truth-machine must be a full program that follows these rules:

• take input from STDIN or an acceptable alternative
• If your language cannot take input from STDIN, it may take input from a hardcoded variable or suitable equivalent in the program
• must output to STDOUT or an acceptable alternative
• If your language is incapable of outputting the characters 0 or 1, byte or unary I/O is acceptable.
• when the input is 1, it must continually print 1s and only stop if the program is killed or runs out of memory
• the output must only be either a 0 followed by either one or no newline or space, or infinite 1s with each 1 followed by either one or no newline or space. No other output can be generated, except constant output of your language's interpreter that cannot be suppressed (such as a greeting, ANSI color codes or indentation). Your usage of newlines or spaces must be consistent: for example, if you choose to output 1 with a newline after it all 1s must have a newline after them.

• if and only if your language cannot possibly terminate on an input of 0 it is acceptable for the code to enter an infinite loop in which nothing is outputted.

Since this is a catalog, languages created after this challenge are allowed to compete. Note that there must be an interpreter so the submission can be tested. It is allowed (and even encouraged) to write this interpreter yourself for a previously unimplemented language. Other than that, all the standard rules of must be obeyed. Submissions in most languages will be scored in bytes in an appropriate preexisting encoding (usually UTF-8).

# Catalog

The Stack Snippet at the bottom of this post generates the catalog from the answers a) as a list of shortest solution per language and b) as an overall leaderboard.

## Language Name, N bytes

where N is the size of your submission. If you improve your score, you can keep old scores in the headline, by striking them through. For instance:

## Ruby, <s>104</s> <s>101</s> 96 bytes

If there you want to include multiple numbers in your header (e.g. because your score is the sum of two files or you want to list interpreter flag penalties separately), make sure that the actual score is the last number in the header:

## Perl, 43 + 2 (-p flag) = 45 bytes

You can also make the language name a link which will then show up in the snippet:

## [><>](http://esolangs.org/wiki/Fish), 121 bytes

• Can we assume that the program halts when the processor finishes executing the written code, for a machine code entry? – lirtosiast Nov 3 '15 at 16:58
• Assuming any behaviour is fine for all invalid inputs? – Cruncher Nov 3 '15 at 17:33
• @Cruncher Yes, the only inputs you should expect to get are 0 and 1. – a spaghetto Nov 3 '15 at 17:38
• Catalog is borked. – Addison Crump Nov 6 '15 at 15:18
• Catalog appears to consider Bf and bf to be different languages. – Mooing Duck Nov 10 '15 at 1:13

# !@#$%^&*()_+, 10 bytes *0_+!#(!#) Try it online! ## Explanation *0_+!#(!#) * take byte of input 0 push 48 _+ subtract !# duplicate and output ( ) while the top of the stack is not 0: !# duplicate and output # Alchemist, 24 bytes _->In_a+Out_a a->a+Out_a Try it online! I couldn't figure out a shorter way using only one output section. The closest I came was 25 bytes ### Explanation: _->In_a # a = input +Out_a # Output a a->a # While a +Out_a # Output a ## Gol><>, 5 bytes IZh1n Thanks to JoKing for helping golf this down further!!! Try it online! # Tamsin, 19 bytes main="0"|{print 1}. ## Explanation The main production first tries to read a 0. If it succeeds, it is returned, and, because this is the main production, printed. Otherwise, it ignores the input and tries the alternative, which is {print 1}. The { } brackets denote a loop that continues running as long as its content succeeds, and a print statement always succeeds, so this loops forever. # Python 3, 41 bytes a=input() while(a=='1'):print(1) print(0) Try it online! # INTERCAL, 89 bytes DOWRITEIN.1PLEASE.3<-#1$.1DOCOMEFROM(2)DOREADOUT.1(2)PLEASE(1)NEXT(1)DO(1022)NEXTDOGIVEUP

Try it online!

Looking at some answers to this challenge, I noticed that the current INTERCAL answer was barely golfed at all, with 141 bytes 30 of which are whitespace. Not hard to outdo!

Uses native INTERCAL-72 integer I/O, that is, the input is ZERO or ONE (or some translation thereof) with a trailing newline, and the output is in "butchered Roman numerals".

Explanation:

DO WRITE IN .1

Inputs a number to the 16-bit variable .1.

PLEASE .3 <- #1 $.1 Sets .3's value to 1 mingled with .1's value. Since .1 holds either 0 or 1, .3 is set to either 2 or 3. DO COME FROM (2) If execution would move on normally from statement (2), it goes here instead. DO READ OUT .1 Prints the value in .1. (2) PLEASE (1) NEXT This is statement (2). It places itself on the NEXT stack and moves execution to statement (1). It does not trigger the COME FROM unless RESUMEd to. (1) DO (1022) NEXT This is statement (1). It NEXTS to statement (1022) in syslib, which then itself NEXTS to statement (1023) in syslib, which reads PLEASE RESUME .3. This pops a non-zero number of entries equal to .3 off the NEXT stack and resumes execution at the last one popped. Since there are at this point three entries on the NEXT stack, corresponding to (2), (1), and (1022) (RESUME works just fine without labels, it just so happens that all of the NEXTs here have them), and .3 is 2 or 3 depending on whether .1 is 0 or 1, this goes to and moves on from either (1) if .1 is 0, or (2) if .1 is 1. If this RESUMEs to (2), the COME FROM takes effect and begins an infinite loop (and since this outcome leaves the NEXT stack empty, the program does not disappear into the black lagoon after 80 loops). DO GIVE UP If (1) was RESUMEd to, execution moves here, and this terminates the program without an error. (I'll edit in a version with Turing Tape string I/O some time later.) # NuStack, 82 bytes putchar(c:char):int;f(i:int):int{if(i>0)putchar('1');else while(1>0)putchar('0');} Been working on this compiler for a month and a half. Still very much a WIP, but finally at a point where I can do some PPCG with it :D Unlike all of my other languages, this isn't some toy, esoteric, interpreted language. It's a serious language that compiles directly to assembly (nasm syntax). I'll probably opt to get NuStack on TIO once it's notably more feature-complete than it currently is. ## Explanation: Ungolfed version: putchar(c: char): int; f(i: int): int { if(i > 0) putchar('1'); else while(1 > 0) putchar('0'); } The first line is a function prototype that allows you to call a function defined elsewhere (think C header files) putchar in particular is currently the only function in libns, the language's standard library. It takes a single char, and outputs it to stdout. libns is currently available for 64bit linux, so it just makes a syscall using sys_write(), and 32bit "raw", ie not running on a kernel or operating system, where putchar will use the int 0x10 interrupt to print the character in TTY mode NuStack uses post-fix types, similar to TypeScript, and there is no type inference, so types must always be explicitly declared. NuStack also doesn't have type casts yet, or do..while loops, so this is, as far as I know, the golfiest way to do it. # GolfScript, 13 bytes ~{.}{.p}while ### Explanation ~ # convert STDIN to an integer {.} # while loop condition: remains the same--the input {.p} # while loop body: print the input while This is one of my first golfs! # Ruby + -np, 16 12 bytes p 1while~/1/ Try it online! ~/1/ matches the last line of input to the regular expression /1/ Edit: replaced ;p 0 with the -p flag making the answer -np complete, thanks to @ValueInk • you don't have to count flags any longer; just note in the body which one you've used (as you already have done) – Giuseppe Mar 5 '18 at 23:14 • @Giuseppe thanks, is this the consensus opinion now? – Asone Tuhid Mar 5 '18 at 23:16 • Yeah, see here. – Giuseppe Mar 5 '18 at 23:18 • @Giuseppe so should I call it Ruby + -n? – Asone Tuhid Mar 5 '18 at 23:28 • Use the -p flag instead to implicit print the zero ;p – Value Ink Jun 5 '19 at 2:10 ## Enterprise, 128 bytes /©©//NDANDA/final disruptive class fdcBit{final immutable void main(){var Money i=read();;;write(i);;;while(i==1){write(i);;;}}} It's really annoying how the specification isn't very clear, so I had to guess a lot of things. # Rust compiler (nightly), 86 bytes #![feature(log_syntax)]macro_rules!t{(0)=>{log_syntax!{0}};(1)=>{log_syntax!{1}t!{1}}} (playground) Readable version: #![feature(log_syntax)] macro_rules! truth_machine { (0) => { log_syntax!(0); }; (1) => { log_syntax!(1); truth_machine!(1); }; } truth_machine!(0); truth_machine!(1); ## Explanation As the only way to iterate over anything other than the inputs of a macro is recursion, an infinite loop will always end in a stack overflow. I'm considering this error message "constant output of your language's interpreter that cannot be suppressed," as there's no other way to "infinitely" loop. The time for this to occur is dependent on the stack size, and thus most real-world macros will attempt to perform multiple operations per iteration to avoid this. Since this takes more space, I didn't do this. The only ways for a macro to give output are: • compile_error!, a builtin which will immediately terminate compilation with a message, and is thus not valid for this challenge • log_syntax!, an unstable builtin which will print its input to the compiler's stdout, with a newline appended • Returning a value from the macro directly, which directly conflicts with the concept of an infinite loop I chose to use log_syntax!, as I decided it was the only option that would follow the rules of the challenge. However, it requires a nightly compiler, and 23 additional characters in the source to enable. The syntax of macros is pretty fixed, but one trick I found is that macros called with macro!{} instead of macro!(); do not require a semicolon after, and are thus one character shorter. # Cascade, 10 bytes ?1/ #? &#| Try it online! My first submission with my new language Cascade! Cascade is a tree-like esolang, similar to Pyramid Scheme, but a bit terser and with wrapping boundaries. Ungolfed, this looks more like: @ | ? / #| &| ^ # | 1 | First, the ? checks if the printed (#) input (&) is positive. If not, it goes left and terminates since it finds no more instructions. If so, it goes right to the ^, which first executes the left (#1, print 1), then goes to the right, which wraps around back to the ^ for an infinite loop. In the golfed code we use another ? which does a similar thing except with the center and the right, since we know the return value of # is always positive. # MathGolf, 5 4 3 2 bytes -1 for dropping block marker q▲ ## Explanation Implicit input Starts a block q Output TOS without a newline. ▲ Execute the code block while the TOS is true. Executes at least once. Implicit output Try it online! • I think you can drop the Ä since there's nothing before it now. – Unrelated String Oct 13 '19 at 3:16 # Gaia, 3 bytes :p↺ Try it online! ### Explanation ↺ While... : Duplicating the top of stack leaves a true value on top: p Print the top of the stack Input is taken implicitly in Gaia. Every iteration leaves the stack empty, but if there is no input left when it attempts to grab some, it simply takes the most recently given input again. ## Keg, 1287 4 bytes {:|④ Try it online! ## Explanation # Implicit input {:| # While the duplicated item is nonzero: ④ # Output the item without popping # After that, Output the item remaining on the stack and stop the program • This can now be 8 bytes due to implicit input – EdgyNerd Aug 10 '19 at 9:30 • 4 bytes Try it online! – Jono 2906 Dec 15 '19 at 21:11 # Python 3, 35 bytes a=input() while a:a=int(a);print(a) This is the best I can do, don't know if it's the optimal solution though. ## Explanation In Python 3, input() stores the input as a string, so even if the user types 0, it will be stored as '0', which is truthy. This allows me to make a kind of do-while loop, which will always run at least once since '0' and '1' are both truthy. However, I then convert a to an integer ('0' becomes 0 and '1' becomes 1). Now, if the user typed 0, it will not run again since the while loop is now while 0 and 0 is falsy. However, if the user typed 1, it will run forever since the while loop is now while 1 and 1 is truthy. • 34 bytes – a'_' Dec 26 '19 at 15:17 • @a'_' oh nice, didn't know that was a thing in Python 3.8 – Sagittarius Dec 27 '19 at 1:22 ## JavaScript, 39 32 Bytes x=+prompt();do{alert(x)}while(x) x=+prompt() gets input from user, and transforms it from a string to a number do{alert(x)}while(x) is a post test loop, so it always alerts X, then executes again if x not = 0 New contributor Benji is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct. • It might... I didn't think of that (this is my first time doing any kind of code golfing, so it's probably not the best) – Benji Jan 15 at 11:26 # Shakespeare Programming Language, 123 bytes (Whitespace added for readability) T.Ajax,.Puck,.Act I:.Scene I:.[Exeunt][Enter Ajax and Puck] Ajax:Listen tothy.Open heart. Is zero worse you?If solet usAct I. Try it online! I make use of a trick to reuse Scene I. For some reason, it doesn’t error to keep taking input, so I do that. Then I just compare the input to zero, and loop if it’s greater. • You can use Is I worse you? instead of Is zero worse you?, since all characters are implicitly initialized at 0. – Robin Ryder 5 hours ago # Lua For windows, 70 bytes Use lua for windows for this I=io.read() If I=="1" then while 1 do print"1" end elseif print"0" end This program works because it takes a 0 or 1 from stdin then if it's a one it makes a while loop that prints one if it's a 0 it prints 0 then the program ends • Does this rely on functionality specific to Windows? Why couldn't this be run with Lua on other systems? – Alex A. Nov 4 '15 at 3:16 • This could be shorter if you just called If io.read()=="1" instead of assigning it to an alias. – James Murphy Nov 4 '15 at 5:41 ## Ceylon, 91 88 bytes Improved (explanation at the end): shared void run()=>("0"<(process.readLine()else"")then{1}.cycled else{0}).each(print); This was the original (91 bytes): shared void run(){if("0"<(process.readLine()else"")){while(0<1){print(1);}}else{print(0);}} Unfortunately the most part goes to reading the input and making sure it is defined. shared void run() { if("0"<(process.readLine() else "")) { while(0<1) { print(1); } } else { print(0); } } I merged this null check (in the else "" form) with the decision into one if, so when the input is ended without a line being read, it also prints 0. Also, any string sorting less than "0" lexicographically prints 0, and any string lexicographically > "0" will produce the 1-loop. The 0<1 is used instead of true because it is shorter. Here is a functional approach, unfortunately it can't compete (107 bytes after whitespace removal) due to the length of parseInteger: shared void run() { value i = parseInteger(process.readLine() else "") else 0; (1:i).cycled.follow(i).each(print); } It cycles a range starting at 1 of length i (i.e. either {} or {1}), resulting in {} or {1,1,1,1,...}, prepends i (which results in {0} or {1,1,1,1,1,1,...}), and prints each element. A different functional approach without variables and parsing is this one (88 bytes, shrinked version at the top): shared void run() => ("0" < (process.readLine() else "") then { 1 }.cycled else { 0 }).each(print); This does the case distinction in an expression, using the then and else operators to produce either the {1,1,1,...} stream or the {0} singleton, and then calls print(...) on each element of that stream. ## C#, 120 98 bytes using c=System.Console;class o{static void Main(){if(c.Read()=='1')for(;;c.Write(1));c.Write(0);}} Ungolfed: using c=System.Console; class o { static void Main() { if(c.Read()=='1') for(;;c.Write(1)); c.Write(0); } } • 2-byte gain: int a=c.Read();do c.Write(a-48);while(a>48); – LegionMammal978 Dec 6 '15 at 16:10 • 95 bytes based on @LegionMammal978 answer using c=System.Console;class P{static void Main(){int a=c.Read()-48;do c.Write(a);while(a>0);}} – krontogiannis Aug 29 '16 at 10:26 ## x86 (Linux / NASM syntax), 86 bytes mov dx,1 ; 3rd argument to read(): length of buffer mov ecx,esp ; 2nd argument to read(): the buffer ; 1st argument to read(): the FD is already 0 mov ax,3 ; read system call number int 128 ; invoke syscall, returns eax=1 == num of read bytes l:mov ax,4 ; start of loop, eax=syscall number of write() int 128 ; invoke syscall, returns eax=1 == num of written bytes cmp byte[esp],49 ; compare buffer against '1' jz l ; loop of it's the same int 128 ; invoke syscall otherwise (syscall 1 == exit) Compile with nasm -f elf h.asm && ld -o h h.o -m elf_i386 and ignore all warnings. :-) ## Swift, 187 bytes import Foundation let i = Int("\(String(data: NSFileHandle.fileHandleWithStandardInput().availableData, encoding: NSUTF8StringEncoding)!.characters.first!)")! print(i) while i>0{print(i)} Sadly this has to read from stdin if it can :( Would be so much smaller if I could just declare it inside... • Can you not remove the spaces around =, after :, and after ,? – Esolanging Fruit Dec 14 '17 at 0:36 ## Scala, 46 bytes if(readInt==0)print(0)else while(2>1)print(1) Kinda uninteresting and obvious, but there we go • ==0 can be replaced with <1 – V.G. Nov 12 '15 at 17:18 • val k=readInt;do{print(k)}while(k>0) – V.G. Nov 12 '15 at 17:24 # Atari Basic, 47 bytes If I remember the syntax correctly, it will be something like this: 1 input a$
2 IF a$="0" THEN end 3 ? a$
4 goto 3

Please correct me if I'm wrong.

• Do you need some spaces? ("0" THEN, ? a$) – Erik the Outgolfer Jul 22 '16 at 12:35 • @EʀɪᴋᴛʜᴇGᴏʟғᴇʀ I'm not sure, i produced this snippet from memory, i did't use atari basic for over 25 years. – user902383 Jul 22 '16 at 12:41 ## Python 2, 46 bytes 34 bytes x=input() print x while x:print x Basically, it takes in the input, when 1, prints forever (while loop), but if it isn't 1 (when it is zero) it prints and then exits. This is my first answer! Yay! ## Java, 90 97 bytes class T{public static void main(String[]a){while(a[0].equals("1"))System.out.println(1);}} I'm not sure if this counts because it takes 1 or 0 as an argument rather than STDIN. It's been emotional, either way. Edit: The above does not print zero, the below does: class T{public static void main(String[]a){do System.out.println(a[0]);while(a[0].equals("1"));}} • You can save two characters by using print instead of println. (There was no requirement saying each 1 must be on a new line). Also, the command line can be added easily as while(a[0].equals(new java.util.Scanner(System.in).nextLine()). However, I don't think this meets the requirement, since it doesn't print 0. – Xynariz Nov 10 '15 at 1:42 • Fixed to print zero in 97 bytes. I had to use println because print must just be appending to the string so never actually prints a line for me. – ESP Nov 11 '15 at 9:45 • "print must just be appending to the string" .. .what? print and println do exactly the same thing, except that println prints the text plus the current OS's line delimiter. See the documentation: docs.oracle.com/javase/8/docs/api/java/io/… and docs.oracle.com/javase/8/docs/api/java/io/… – Xynariz Nov 13 '15 at 0:42 • @Xynariz You should only read the input once – SuperJedi224 Jan 3 '16 at 13:35 • @SuperJedi224 Um ... how is that even relevant to what I said? – Xynariz Jan 4 '16 at 20:57 ## VB.NET, 106 Bytes Imports c=System.Console Module m Sub Main i=c.readline 1:c.Write(i) if i=1 then goto 1 end sub end module Straightforward, using goto instead of a loop. Compiles with Option Explicit and Option Strict Off, and Option Infer On ## STATA, 31 bytes di _r(a) m do{$a
}while(\$a)
end

First, get input from the user and store it in global variable a. Then switch to Mata for better commands. Print the value of a and then if it is truthy (i.e. 1), go to the start of the loop. If the end keyword is omitted, unexpected end of file (end statement missing) is output to standard error. I'm not sure if that's okay.