Default for Code Golf: Input/Output methods

Question

It looks like we have a consensus that we want certain defaults for the format which answers are expected in for code-golf. On that poll, the question arose twice, which input/output formats should be allowed for programs and functions.

So here is another poll. This one works different though. All the input/output methods are independent of each other, so there will be one answer per method. Upvote all you think are reasonable for the default. Downvote those which you think shouldn't be allowed unless the OP explicitly permits them.

To keep this remotely manageable, I have not posted individual answers for all possible inputs for functions. So there are only four: functions can take input/output via their arguments and return values. Or functions can use any method full programs can. I don't think there is any point in (say) allowing programs to take input from STDIN (only) and to allow functions to take input from ARGV (only) or something like that. If you disagree, please leave a comment.

If I've overlooked an I/O method, feel free to add your own answer.

Note: Some votes have been reverted because they were detected as serial voting. If you vote on multiple answers, please leave some time between votes.

A method is allowed if it has 5 net votes and at least twice as many upvotes as downvotes.

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• mbomb007: Related (Can numeric input/output be in the form of byte values)
• Sparr: "The current results of the polls are now part of the tag wiki" which tag wiki? meta wiki has nothing for code-golf tag. ppcg wiki has codegolf.stackexchange.com/tags/code-golf/info but it's missing a lot of answers here.

Answer 1

Functions may output via their return value(s)

• user10766: How many languages support this? I support outputting to STDOUT.
• Martin Ender: @hosch250 Wait, are there languages that have functions without return values? o.O
• user10766: Oh, that is not considered outputting the last I heard - that is returning a value. If the question asks for a function, this is correct. If the question asks for a program, I support outputting to STDOUT or a file.
• Martin Ender: @hosch250 This particular answer only concerns functions. Whether functions and/or programs are allowed is another question (see the linked meta question, but of course it's ultimately up to the challenge author). But this answers says that if functions are allowed, then they should be allowed to use their return value for the result - as are all other answers here that start with the word Functions.
• user10766: Oh, sorry. I definitely will vote on this then.
• Shaun Bebbers: @MartinEnder Most (all?) 8-bit BASIC interpreters do not specifically return a value from a sub-routine, whilst many will allow you to define a mathematical function with the DEF keyword, this is quite limited. I also don't recall Amiga BASIC included with WorkBench having return values from sub-routines but it's been many years since I did any Amiga BASIC so don't quote me on that. 6502/10 assembly does not return values from sub-routines either. If you know that you've stored your result in the accumulator, then you simply use that on return from your sub-routine.
• The Fifth Marshal: @MartinEnder Yes, scratch has no return values for it's functions
• Luvexina: @Hosch250 There are languages that don't have STDOUT, like JS
• FZs: @VFDan In case of JS, console.log-ing is often regerded as STDOUT. Also, Node.js does have a STDOUT.

Answer 2

Functions may take input via function arguments

• Joe Z.: I use this in a lot of my questions, so definitely.
• l4m2: Can it take array via repeatedly call a function, once a value?
• Martin Ender: @l4m2 that sounds like a special form of currying and would be worth asking about in a separate question. (In principle, this should be valid, but it's worth discussing explicitly.)

Answer 3

Programs may take input via STDIN

Answer 4

Programs may output to STDOUT

Answer 5

Programs may take input via GUI prompts

(This is for languages, for which this is the closest alternative to STDIN, like JavaScript's prompt(), Mathematica's Input[] or InputString[], Matlab's input() and VBScript's InputBox().)

• Ypnypn: What about for other languages?
• Martin Ender: @Ypnypn I'm not aware of a language which can read from STDIN, but where producing a graphical prompt is shorter than that. But in that case, I'd say either is fine.
• flawr: @MartinBüttner Matlabs input('') usually just reads from the command line, perhaps you mean inputdlg('')?
• LegionMammal978: Anyway, Mma's Input and InputString both take the next line of input when run in a script.
• wizzwizz4: @MartinEnder wscript.
• Stan Strum: Bash read <var>
• l4m2: So reading choice from confirm and msgbox etc. are also allowed

Answer 6

Programs may output by displaying it on screen.

This makes it possible to use languages like Vim script that can't print output directly to stdout.

Example from this challenge:

$ echo "This is a test line!" | vim - -c 'nm Q vEUWvEuWQ|norm Q'

Will display:

THIS is A test LINE!
~
~
~
~
~

Within a Vim session.

I think that this way of displaying the result should be valid. I am sure that there are other domain specific languages like Vim script that lacks full I/O support and could use another way to display the result.

• Joshua: Incidentally, vim's output is standard output, but if you processed standard output, there is extra garbage there.
• DLosc: It's not even just domain specific languages. I'd say QBasic falls in this category: it doesn't have any concept of an output (or input) stream, but rather writes characters to a "window."

Answer 7

Programs may output using their exit code...

Exit codes are basically a return value for programs. If functions can output using their return values, it makes sense that programs should be able to do the same.

Examples:

• Java exit codes
• C and C++ exit codes

• wchargin: Hmm...should this perhaps be "if and only if the challenge requires as output an integer whose value is guaranteed to be between 0 and 255, inclusive"? For example, if the challenge is "output the number of arguments passed to the program," then int main(int argc, char **argv) { return argc; } should not be a valid solution (IMO), because the exit code is masked by 0xFF. (Perhaps this is implicit in the rule as a part of correctness? Just want to be clear.)
• Rainbolt: @wchargin That seems like a limitation of whatever you are using to run your program. I can write a PowerShell script like exit 1000000, and execute it in PowerShell ISE. This gives me no problems.
• wchargin: Interesting—I didn't know that. As far as I know, this is true on all *nix OSes (including Mac), but I'd be happy to be corrected!
• Jasen: is there a reason to limit this to integers? i mean if there's an environment where a program can return something other than an integer why not allow that too?
• Rainbolt: @Jasen I don't see a reason to limit it to integers.
• Shmuel H.: I'm new to this site. Would a crashed program's exit code considered a valid way of output (for example, SIGSEGV)?
• user45941: @ShmuelH. If you intentionally crash a program (or just use the equivalent of exit(code) in that language) to get a specific exit code, it's valid.

Answer 8

Programs may take input via command-line arguments

Answer 9

The contents of the tape post-execution may be used as a Turing machine's output

• SuperJedi224: This has my upvote.
• Christopher King: Emphasis on the "post-execution". If you allow the final tape of a non-halting turing machine to be considered out, your program can calculate a non-computable function.

Answer 10

Functions may take multiple arguments via currying

For some functional programming languages like Haskell this is actually necessary, because only single-argument functions exist and functions with multiple arguments are (somewhat transparently) implemented as curried functions. (The alternative would be to take a list or tuple of the values, but that is not how one would naturally write a two-argument function in Haskell.)

As per this consensus the same should also be allowable for languages where multi-argument functions do exist. As an example from JavaScript, instead of defining

f=(a,b)=>...

and calling it like f(a,b) one could then also define

f=a=>b=>...

and call it like f(a)(b).

• Lucenaposition: Isn't that how lambda calculus works?
• Weird Glyphs: @Lucenaposition Yep, that's one of the reasons why it's allowed

Answer 11

Functions may output via the same methods as full programs

(This depends on how the poll goes, but could be any subset of STDOUT, STDERR and file.)

• John Dvorak: This violates the single-responsibility principle, the separation of concerns principle, and I don't think it's really useful in code golf. Moreover, functional programming languages won't let you do that at all, and I want some more love for Haskell.
• xnor: @JanDvorak One simple golf use is that print is shorter than return in Python.
• John Dvorak: Eugh. Not a fan of that optimisation. What it could be useful for is printing in a loop rather than collecting results in an array. Still not a fan of allowing that.
• xnor: @JanDvorak Yeesh, I did not consider that one could print repeatedly to "output" a list. I think that should not be allowed even if this vote passes because the output is not the list that is asked for.
• Martin Ender: @xnor Why not, full programs are allowed to do that, too, right?
• xnor: @MartinBüttner Hmm, you're right, what a program really "outputs" is the string representation of the result, so this rule should mean functions can too for consistency. But this is another awkward consequence that makes me disfavor this rule.
• Felipe Nardi Batista: what about exit codes? aren't functions suposed to be reusable? if my function exits the program it isn't reusable. that's what the title indicates

Answer 12

Programs may output to STDERR

• yyny: Can anyone give a comment as to why not to? For example, print("Hello, world!") in lua prints to STDERR.
• Dennis: Forbidding output to STDERR seems incompatible with allowing output to the screen.
• wizzwizz4: This should go as standar... Ah. I see.
• Esolanging Fruit: The only problem I see with this is vagueness between what is written to STDERR by the "language" and what is written by the "program." If a distinction between those is not made, this would permit error quines and answers like this, which is questionably valid.
• drolex: I think this shouldn't be allowed by default (but could be explicitly allowed on some challenges). I believe this goes against the spirit of some challenges such as this. On the other hand it could be explicitly forbidden but not everybody will think of this
• Ismael Miguel: This is game-breaking for PHP, where <?=a; outputs a to STDOUT and Notice: Use of undefined constant a - assumed 'a' in [...][...] on line 3 to STDERR.
• MilkyWay90: @IsmaelMiguel This means that outputting to STDERR only counts if that's your chosen output format.
• val - disappointed in SE: @YoYoYonnY Since when? All Lua 5.1-5.3 output to stdout...

Answer 13

Functions may output by modifying their arguments or writing to out arguments

Here, "out arguments" refers to arguments that are passed in by reference, so that modifying the value inside the function (as opposed to just overwriting the argument) also modifies the value outside the function (otherwise, the output value would not be observable after the function returns). Note that this is not possible in all languages with all types.

• lirtosiast: Does this mean that in languages that allow it, the seven characters for return can simply be replaced with an assignment?
• Martin Ender: @ThomasKwa Yes, provided the changed value will then be accessible in the context that called the function.
• Jasen: so you mean this is only legal in languages that have out arguments, pass by reference, or some similar semantic? for instance not in shell...
• Martin Ender: @Jasen I don't know about shell, but yes to your question.
• izzyg: This should be clarified to say that you have to actually modify the argument, not just overwrite it. See this for an incorrect use.
• Peter Taylor: @isaacg you need to link to a specific revision for that comment to make sense.
• izzyg: The revision that I referred to about is codegolf.stackexchange.com/revisions/128782/6

Answer 14

For stack-based languages, function's input may be pushed to the stack before calling

Stack-based languages may assume that the input for their function is automatically pushed to the stack.

• Martin Ender: +1, this is the de facto standard for CJam and GolfScript (and probably some other stack-based languages). You might want to make a corresponding answer for output.
• Sp3000: I think the emphasis here needs to be on "function", as opposed to snippet. There's a very fine line between the two...
• flawr: Are there any stack based langauges that let you define functions? As far as I am aware most of them do not and do only let you write full programs with IO via STDIO. (in my opinion functions should satisfy (perhaps among other criterions): pieces of code that can be reused in the same program, without typing them out again)
• Addison Crump: @flawr Vitsy has functions, in some aspects. There are a few stack-based languages that have scope, but I can't name them immediately.
• fede s.: @flawr for completion sake: lots (most?) of non-esoteric stack-based languages let you define functions. Factor, Forth, Postscript, Joy, Cat, and many others all have functions (both named and anonymous). dc (the RPN calculator) has registers that can work like functions.
• user62131: @flawr: Underload lets you reuse the functions in question (in a kind-of awkward way, as Underload has no method of naming things, but it's possible and fairly common).
• Adalynn: Would this apply to a language that has explicit but no implicit IO?

Answer 15

Programs may output to a file

• xnor: Why are people against this?
• Patrick Roberts: @xnor is /dev/null considered a valid output file? I might as well submit a blank program and claim it outputs the correct answer to /dev/null.
• DLosc: @PatrickRoberts No, /dev/null is not a valid output file, for the exact reason that the output cannot be examined later. "Output to a file" means that you can see the program's output by viewing a particular file.
• Jasen: using strace (or chroot, or mv!) you can see what was written to /dev/null
• MilkyWay90: @PatrickRoberts But it doesn't output to /dev/null

Answer 16

Functions may take input via the same methods as full programs

(This depends on how the poll goes, but could be any subset of STDIN, ARGV and file.)

• John Dvorak: This violates the single-responsibility principle, the separation of concerns principle, and I don't think it's really useful in code golf. Moreover, functional programming languages won't let you do that at all, and I want some more love for Haskell.
• Martin Ender: @JanDvorak I don't see the relevance of good coding practices for code golf. And some languages don't even have functions and they aren't necessarily at a disadvantage by allowing functions. The idea of being more liberal in the options for I/O is so that each language can pick what's really shortest.
• trichoplax is on Codidact now: The only time that I would object to a function taking input in the same way as a program is if the question was phrased "Write a function that takes the following arguments". This poll applies to questions that are not in such a specific format, so I see no reason to exclude functions that read from stdin.
• John Dvorak: @MartinBüttner noted. I still don't like this option, but feel free to override my preferences. The vote split is very close to the threshold ATM.

Answer 17

Programs may combine two or more input methods

For example, if the inputs are a string and an int, a function that takes a string as an argument and an int from STDIN would be valid.

The input format must still be consistent for a given program.

Answer 18

Input may be entirely ignored

If a challenge specifies that your program should take a certain selection of inputs, but not all of them are necessary for you to solve the task, you can simply not take the ones you don't need. (This is equivalent to taking them via a method that your language doesn't look at, such as via command-line argument in a language which has no access to the command line arguments.)

• Esolanging Fruit: Are you implying that if the challenge was "take three numbers as input a, b, and c, and return a+b", then print(input()+input()) would be a valid Python submission?
• user62131: @Challenger5: Right. Also that that would be a valid submission if the request was to return a+c. (Even without this, you could still do that by saying, for example, "a and c are input via standard input, and b from a file". This rule's basically meant to avoid the absurdity of having to do that.)

Answer 19

Where applicable, Turing machines supporting multiple halt states may also output via their halt state

This is essentially equivalent to programs outputting via their exit code.

• SuperJedi224: This has my upvote.
• xnor: Turing Machines as I've seen them defined leave output on their tape and have a single halt state. Is there some precedent for what you are suggesting?
• SuperJedi224: @xnor: That is usually the case, but I have seen dialects of turing machine code that do support multiple halt states. (For example, in the morphett.info dialect that I usually use, halt is the basic halt state, but any state of the form halt-<any string> is also treated as a halt state.)

Answer 20

Input for Turing machines may be written to the tape pre-execution

The read-write head should start on the leftmost cell of the portion of the tape containing the input.

• SuperJedi224: This has my upvote.
• mbomb007: I think this should only be allowed if that's the only method of input.
• SuperJedi224: For standard turing machines, it is.
• MD XF: What happened here, SuperJedi? You posted an answer, then commented "This has my upvote" an hour later?
• SuperJedi224: @MDXF At the time, several other answers on this question had comments like that added by their authors (including many of Martin's answers). At least one was instead commented "This has my downvote." Basically, it was intended as a way to vote on your own meta proposals because you can't actually upvote or downvote your own answers on this site. Interestingly, most of those comments on this topic seem to have since been removed.
• MD XF: @SuperJedi224 Ah, okay, thanks. I was suspecting sock action, but that makes sense.

Answer 21

Functions may return a list via acting as a generator

In other words, writing a function to return list elements lazily on request, using support that the language already has for doing that, is allowed (currently it's frequently seen but something of a gray area). This would be idiomatic in most of the languages which have it.

• user45941: I wasn't aware this was even a point of contention...
• user62131: I don't think it is, but we don't have it listed, and converting a generator to a list is fairly expensive byte-wise in Prolog (there's a builtin for it but it has a fairly long name). So I wanted to confirm that I don't have to do the conversion in my Prolog answers.
• mbomb007: Some things in Python 2 return a list, but in Python 3 it return a generator. Like range.
• Linnea Gräf: @mbomb007 I think Python 3s range is equivalent to Python 2s xrange and Python 2s range is something different...
• mbomb007: Basically. Python 2's range is like calling list() on the result.
• The Empty String Photographer: Maybe not in fastest code...

Answer 22

SQL submissions may take input from a named table

which is probably not good enough. But I don't know a better way.

• roblogic: Should be OK, as long as that input is treated as data, not executable code

Answer 23

Assembly programs may take input from registers

If there are no I/O devices available, an answer might consist of a subroutine that reads its input values from the machine registers.

• lirtosiast: There's potential for abuse. What if someone creates a machine with a million different special-purpose registers?
• cat: @lirtosiast Good for code colf then. I don't see how that's different from golfing languages with prefilled vars
• yyny: @lirtosiast I believe the programming language used has to be defined before the challenge starts.
• Peter Cordes: If you golf with x86-64, the standard calling conventions already pass args in registers. So you can just write a normal function that's callable from compiler-generated code.
• user62131: This needs clarification: can we use any register, or only the ones that would be specified to be used by the ABI?
• Peter Cordes: Update on calling conventions: you don't have to use a standard C calling convention. Your language is assembly / machine code, so presumably that's the caller's language as well. They can pass args and take return value(s) in any registers including FLAGS on machines with condition codes, as discussed in Tips for golfing in x86/x64 machine code

Answer 24

Programs may take input as the value of the last expression

Using something like TI-84 BASIC's Ans, which is a variable that stores the value of the last expression. This the shortest way for TI-BASIC to take input, but its validity was recently questioned. To be clear, this is how the process works with Ans:

• Within a program, type the input (number, list, string, etc.), followed by a newline to separate statements, followed by the program name, then press Enter. For example,

  1337
  prgmFACTOR
  

• Outside a program, type the input, press Enter, then type the program name and press Enter.

Edit: I came across a way of using Ans that is similar to command-line arguments, which are allowed: All as one expression, type the input, followed by a colon, followed by pgrmNAMEHERE. For example, 1337:pgrmFACTOR.

• Martin Ender: I think by definition that submission would not be a full program but a snippet that requires some sort of REPL environment which are disallowed by default.
• Conor O'Brien: I think that TI-BASIC is a special case, it being fundamentally different from other languages, and only being callable from what can be considered a REPL; so what then is to differ a full program from a snippet?
• flawr: @CᴏɴᴏʀO'Bʀɪᴇɴ I disagree, you can write full programs in the REPL but you can also write them in separate "files" that can be executed. They even support arguments when the program is called, but can also read from stdin or via prompt.
• lirtosiast: @flawr On the 84+ series they can't take arguments, and there's no STDIN, only a prompt.
• flawr: @lirtosiast According to the official TI 84+ guidebook both commands Input and Prompt are available. Both let the program fetch user input. Both behave alot like e.g. input() in Python or MATLAB.
• lirtosiast: @flawr I agree about their similarity; what I meant is that they display a ? that prompts the user to type something.
• flawr: @lirtosiast Is that an issue? I think many languages do have symbols to denote new inputs/new lines.
• lirtosiast: @flawr It's not an issue.
• Jakob: @flawr It's worth noting that you can't write full programs in the REPL, at least for the TI-83 and -84 calculators. For example, the Input/Prompt and looping commands can't be used from the REPL.
• Esolanging Fruit: Does this mean using _ for the Python REPL is valid?
• SuperJedi224: @JakobCornell There are actually assembly programs that let you bypass that

Answer 25

Functions may return a boolean value via the presence or absence of an error/exception

That is, crashing to return false, and not crashing to return true. This is the function equivalent of outputting via exit code, which is allowed for full programs (nearly all interpreters will return an exit code of 0 by default if the program doesn't crash, and nonzero if it does).

I realised this wasn't on the list while writing a post about how our I/O defaults applied to Brachylog; this method of outputting booleans is idiomatic in most declarative languages (i.e. it's something I'd be likely to write even if I weren't golfing), and it's still meaningful in other paradigms (much rarer, but it's nonetheless considered an acceptable programming technique in industry in languages like OCaml).

I'm not completely convinced either way about whether this should be accepted, although I'm currently mildly in favour (the main point in favour is that it's often the most natural way to do output even if you aren't golfing, and many languages have a try/catch or equivalent to convert it into a regular Boolean; the main point against is that it feels a bit cheap/abusable). However, we could definitely do with a ruling as to whether this is acceptable or not, as there are many cases in which it's going to be the shortest way to write the code, and thus it matters for how code-golf answers are written.

• AdmBorkBork: This is pretty much "output via exit code" for functions. +1 from me.
• xnor: Not sure about this either way, but it would be a significant change for Python golfing. Especially so if a recursive function that keeps nesting is considered a stack overflow error, which is a bit weird since it's common to assume there's unlimited stack depth.
• 12Me21: If anything, I think this should only be allowed in languages with some way of handling errors (usually try/catch). Otherwise, it would be impossible to actually use the code in a program.
• Οurous: @12Me21 I read "crash" as different to "error" or "exception", where "crash" is always non-recoverable.
• Dingus: Is there a contradiction here with our stance on function reusability, namely Do function submissions have to be reusable? and Should submissions be allowed to exit with an error?
• Dingus: More recent meta discussion that expands on my comment above: Functions throwing exceptions: an exception to the rules?

Answer 26

Functions in stack-based languages can leave the output on the stack

Mostly for completeness sake. This is the de-facto standard for CJam and GolfScript and probably other stack-based languages.

• mbomb007: This is also how Forth works.
• Adalynn: Would this apply to a language that has explicit but no implicit IO?
• Martin Ender: @Zacharý not sure what you mean?
• Adalynn: A language like Befunge or something similar. Or am I misunderstanding the concept of "function" in these kinds of languages?
• Martin Ender: @Zacharý Befunge doesn't have functions so this answer doesn't apply.
• MD XF: I disagree. I think they can leave it on the stack if implicit output is available. There is no way to check if it's left on the stack otherwise.
• Martin Ender: @MDXF Wouldn't that be equivalent to allowing functions in "normal" languages only to use their return value, if the language has implicit output for return values, otherwise there's no way to check if the return value actually has the correct output?
• kepe: Yeah, like BIT++

Answer 27

Assembly programs may write output to some specified memory location

If there are no I/O devices available, an answer might consist of a subroutine that writes its output values to some specified memory location (e.g. write a machine word to 0000ドル).

• qwr: I think this includes writing to a buffer for outputting multiple values.
• 12Me21: I feel like all these similar posts could be combined into something like "Programs may use registers/variables/memory locations for input/output if no input/output device is available". That would cover assembly programs as well as SQL and other higher level languages with the same problems.

Answer 28

Input and output of a single value may be represented as a singleton list

For example, if the value was 4, [4] would be an acceptable representation (using Python list syntax).

• Esolanging Fruit: This is useful in array languages like J.

Answer 29

Submissions may use list of strings instead of multi-line strings

Input or output may be the equivalent of the string separated by newlines. For example, the multiline string:

1
2
Fizz
4
Buzz

May be instead be taken as a list of strings like:

["1","2","Fizz","4","Buzz"]

• Jo King: I always see comments about this in multiline challenges, so I thought I'd throw this up there and see if it is well-recieved as a standard form of input
• lirtosiast: I would add an exception for ascii-art as linebreaks can be an important part of those challenges.
• Steve Bennett: @lirtosiast I don't understand this remark about exceptions for ascii-art. I have written several ascii-art challenges and I don't see why linebreaks need to be treated differently there? (Obviously any challenge can override the defaults specified here anyway...)

Answer 30

In languages without any method of input (e.g. ///) programs may get input through an insertion into the source code

• boboquack: @DLosc What about code in /// that requires changing at the end? E.g. a simple program that adds a full stop to the end of a string if there isn't already a full stop there /.;/;//x///;/.x;/(input goes here); where x is a special character (maybe a newline) that is guaranteed not to be in the input.
• bb216b3acfd8f72cbc8f899d4d6963: This is called hard coding.
• user85052: @boboquack Can we allow hard-coded input in languages that support input commands?
• boboquack: That's a good question, and from experience I would guess no, but you should search on meta/ask your own question here. @A_
• rydwolf: Does this include 2d programming languages and/or cellular automata?
• boboquack: @RedwolfPrograms It's been a long time since I've thought about this, but I would assume yes.
• rydwolf: @boboquack I'm going to post a separate set of answers, as it seems different enough the consensus may differ. Thanks for the quick reply!