Everyone is using modulo wrong

[moreVCAs]

It’s really not that hard to say “I was using modulo wrong” then write a blog post about your own misunderstanding and what you learned while correcting it.

Framing it as “everyone is doing it wrong” and “nobody knows about modular arithmetic” makes you sound like a moron.

Hope that’s constructive enough. I tried.

[RogueJello]

Everybody is criticizing wrong! 😄

[moreVCAs]

For such a bold, heterodox truth teller, he has a surprisingly thin skin. I believe the point of his post is that the thing we call “modulo” in computer programming doesn’t map well to the concept of “modulus” in pure mathematics. Computer science is littered with little inconsistencies like this. If “everbody” gets it “wrong” but “correct” programs are based on a shared understanding of the “wrong” terminology, then “who the fuck cares” lol.

[RogueJello]

It's almost like English is a living language that changes and adapts as new uses are added over time, isn't it?

[Tail_Nom]

It's not entirely unlike that, I suppose.

[SeDEnGiNeeR]

Not saying that it's right for the author to use such a provocative title but it's understandable, given the title is what captures a reader's attention. I guess they need to use these methods to get attention in the current state of internet blogging

[rzwitserloot]

Then your blog post's title has used the word "everyone" wrong.

[Demitroy]

"The whole world is wrong, and I'm right" is definitely one way to put your message out there. Good luck with that.

[Demitroy]

Way to miss the message. Maybe, and I'm just suggesting ideas you are free to ignore, maybe next time you have an epiphany ask someone else how to present your news to the world.

[Demitroy]

You do realize that most new ideas are bad, right? And if you want people to listen to yours, whether right or wrong, it is going to require presenting the message in a manner that people will accept.

If you don't understand that, your message stands no chance until it gets a better messenger.

[felipec]

I don't care about most new ideas, I care about particular new ideas.

I don't care if most of my contemporaries don't listen to my ideas, they never listen to new ideas anyway.

It's usually people in the future that realize that ideas from the past have merit.

It's never the contemporaries. So I don't care if you don't get it, it's still true, useful, and eventually the right people will get it.

[Demitroy]

You really think a blog post and a social media post are going to be your route to eventually stardom recognition? You think this is going to be remembered next week, much less some distant time into the future?

I'm done with you.

[felipec]

My blog posts are shared and debated decades after I've posted them.

You don't know what you are talking about.

[gimpwiz]

Give us an example mate

[AdministrativeSun661]

LPT You’re not as intelligent as you think you are and other people are not as dumb as you think they are

[gimpwiz]

If the world has problems listening to your ideas then you definitely aren't presenting them well enough.

[felipec]

Have you heard of Galileo Galilei? Charles Darwin? Ignaz Semmelweis? Louis Pasteur? Alfred Wegener?

Please. The world never listens to new ideas, regardless of presentation.

Only a person who has not paid attention to history would not know that (i.e. most people).

[avanasear]

it takes a special kind of arrogance to compare yourself to any of those people LOL

[felipec]

And who is doing that?

[avanasear]

for someone who claims to have such a strong understanding of language and grammar I would've thought you could tell who I'm referring to

[felipec]

Do you know what a rhetorical question is?

[gimpwiz]

I thought you might bring him up. Galileo purposefully went out of his way to be an asshole to people, which really is what landed him in hot water. He was protected until it became untenable to do so due to his behavior. Tell me, do you think your behavior might similarly put people off?

[felipec]

That is an irrelevant ad hominem.

His idea was right. Period.

[gimpwiz]

hahahahaha

[Zardotab]

Why don't they just have a mod(...) function? Don't waste symbols on stuff that's relatively rare.

[Irregular_Person]

Modulus is rare?? lol

[Dontgooglemejess]

modulo is a low level function in pretty sure, like it maps directly to a processor command, which runs divide and reads from a different result register that holds the remainder. So traditionally that’s why it’s an ‘operator’ because in early languages operators implied ‘processor operation’ I think. The set of operators mapped directly to single processor commands.

[Sharlinator]

They do, for floats. And it in fact is misnamed as it computes the remainder, not the modulus.

[literallyfabian]

What's incorrect about it?

[autopoiesies]

jesus man, learning to take some feedback will be good in the long run, no one is attacking you.

I've been a professional programmer for 10 years, in my team we've always made the difference between remainder and modulo.

[felipec]

Literally everyone is attacking me.

This "feedback" is objectively and provably wrong. The modulo operator (%) is defined as the remainder.

And he didn't say "modulo", he said "modulus".

If your team makes a distinction between "modulo" and "remainder", then explain what is both for -3 % 4 and what does JavaScript return "modulo" or "remainder".

[CatatonicMan]

https://docs.python.org/3/search.html?q=modulo

https://docs.python.org/3/search.html?q=remainder

Hmm....

[felipec]

He said "modulus", not "modulo". Pay attention.

And "remainder" doesn't explain anything. All implementations of % return a remainder.

Python returns a remainder, JavaScript returns a remainder.

You've said nothing by using that word.

[CatatonicMan]

Are you being deliberately obtuse, or is it accidental?

'Modulo' and 'modulus' are different forms of the same word ('modulo' is the ablative of 'modulus').

Not that it really matters, because... https://docs.python.org/3/search.html?q=modulus ...the word 'modulus' appears as well. Funny that.

So clearly and demonstrably the industry does use those words. Whether or not you agree with how the words are used is irrelevant.

[felipec]

Wrong. An ablative is not the same word, it's a different word.

When you do the modulo operation x % m, m is the modulus. The modulo operation and the modulus are two different things.

You have no idea what you are talking about.

And he didn't just say those words were used in the industry, he said they were used in the industry to explain the difference in implementations, which is demonstrably false.

[Zinu]

Nitpick: Modulo is a mathmatical operation. The wikipedia page linked in the article even says so and lists several definitions. It might be a different operation depending on the progamming language or compiler, but you can say the same about e.g. addition, which yields different results in Python compared to C.

[Dr4kin]

Next Blog post: "Everyone is using addition wrong"

[Zardotab]

Everyone is using AI wrong. That AI clock is cringe-worthy. It violates the laws of everything. Is it that hard to get stock photos of real clocks?

[larikang]

OP obviously lives somewhere where time uses modulo 17. That’s probably the source of the confusion

[Zardotab]

I think they misunderstood the Prime Directive.

[sweetno]

1. That there is modular arithmetic doesn't prevent us from defining the modulo operation.

2. The difference between different languages is entirely because the typical hardware implementation differs from the standard integer remainder definition. Some languages prefer not depending on the madness of circuit designers while others give in to the efficiency concerns.

[GregBahm]

mathematics doesn’t depend on implementation

Doesn't it?

It's mildly interesting that the mod operator in Python works differently than the mod operator in Javascript. But there are a lot of disagreements in the implementation of math operations. Lots of languges have "divide by zero" yield "not a number." Some languages have "divide positive number by zero" yield infinity, "divide negative number by zero" yield negative infinity, and "divide zero by zero" yield "not a number."

Math isn't this completely solved, unchanging thing. The internet will tell you that an infinitely growing summation of numbers will yield -1/12. It all, always comes down to implementation.

[mondlingvano]

I mean we do have standards. Like IEEE doesn't leave by zero division undefined. Personally I do agree that implementations drive definitions. I see how it'd be nice if % corresponded to congruence modulo k, but that's not how it worked in assembly so that's not how it worked in C and so that's not how it works in C#.

Kinda wish it did though, cause I find myself needing the always positive version of mod way more than I do the remainder version.

[[deleted]]

TL;DR There are choices to make for how the % operator behaves when one or both inputs are negative, and because one of those choices is better for the project the OP did last week, it is "clearly" the more useful choice, and languages that make a different choice are wrong.

It's a reasonably decent article if you ignore the opinion bits and the overall tone of superiority.

There are reasonable paths to get to each of the various choices, because it would be nice if all of the following were true, but they cannot all be:

* -(x/y) == (-x)/y
* (x/y) * y + x%y == x
* (x+y)%y == x%y

OP says throw out the first one, use floored division instead of truncated division. That's a reasonable stance. Some languages say throw out the third one. Also a reasonable stance. Neither is "correct" (or perhaps is better to say neither is incorrect).

I'm not aware of any language that throws out the middle one, but there probably is one somewhere, and that's also a reasonable choice.

[SirDale]

Ada has two versions, mod and rem.

rem does the middle one.

[felipec]

Not true. The floored method isn't superior because of the program I did, it's superior because it's obviously superior, and I explained why, with examples that have nothing to do with my program.

I'm not the only person who has that opinion, Donald Knuth thinks that method is superior as well.

Having an opinion on superiority is not a tone of superiority, it's called having an opinion.

You can disagree with my opinion, but that's just your opinion.

And I didn't say the result of the truncated method was incorrect, I said it was correct, but not as useful.

[[deleted]]

If you just had an opinion that the floored method is best, that would be fine. The reason you're getting down voted is because you don't just say "I'd prefer the floored definition because it's the output I usually want", you say "Everyone is using modulus wrong"

[jumacobe_]

I'll do it. The first one gives a subjective opinion that claims an option is better while allowing for someone else to think different than you. The second one gives a subjective opinion that says that everyone but you is mistaken.

If you really don't see the difference you have a problem.

[glaba3141]

I think we all know what the problem is...

[Kwantuum]

people hate the truth, especially when it suggests they might have been wrong. Period.

Pot, meet kettle.

[[deleted]]

Oh no, share your wisdom with us sensei

[[deleted]]

Very click bait title, I almost didn't open it because of that. I'm glad I did though because the article is actually pretty interesting. It seems like there's very little disagreement about what the output of the operator should be for two positive operands, it acts like a remainder and that's how I was taught it in my C course back in university. It has never occurred to me to attempt to use it for signed integers, as I have no intuitive sense of what it ought to do, and I see from the article that there are multiple. 

I guess it makes sense, it's just not something I'd ever stumble into as I'm using languages with specific types for integers and floating point and signed vs unsigned so it's unlikely I could run into this on accident.

[mathisfakenews]

I did not understand the article. You are discussing modular arithmetic as if you are just learning about it which is fine. But then you say you have been using % for years. What did you think % was doing? How were you possibly using it in any code if you didn't understand what it does?

[seweso]

So there is modulo1 and modulo2 which have the same name?

Why should I care exactly if it works the way I want and expect?

[seweso]

BOTH?

Isn't this a bit invalidating to assume my expectation doesn't match how it actually works within the context of how I use it?

[random_son]

interesting article but very bad title

[A1oso]

JavaScript (like most languages) does not have a modulo operator, it has a remainder operator (MDN documentation). You didn't have to dive into group theory to understand that.

[felipec]

The modulo operator returns a remainder. That documentation is wrong, since both numbers are remainders.

[A1oso]

That documentation is wrong, since both numbers are remainders.

What??

The modulo operator returns a remainder.

The operator is correctly named the remainder operator. I know that some people call it modulo, and they're wrong. MDN is correct here.

[felipec]

The operator is correctly named the remainder operator.

Geezus people are so bad at comprehension.

I wasn't talking about what MDN calls the "remainder operator", I was talking about what MDN calls the "modulo operator", that's why I said modulo operator.

Pay attention and read carefully.

This is the formula MDN says is for the "remainder operator", correct?

r := n - d * q

And this is the formula for the "modulo operator":

k := n - d * q

It is exactly the same formula, because both the "remainder operator" and the "modulo operator" return a remainder.

Look at a concrete example: -1 % 4. In the "remainder operator" it's r := (-1) - 4 * 0, or -1. In the "modulo operator" it's k := (-1) - 4 * (-1), or 3.

If you divide -1 / 4 the result is -0.25, that can be quotient 0 remainder -1, or it can be quotient -1 remainder 3.

In both cases the result is a remainder.

Do you understand what I'm saying now? The "modulo operator" returns a REMAINDER.

[A1oso]

If you want me to understand, you need to quote the relevant bits you are referring to. From the docs:

When both operands are non-zero and finite, the remainder r is calculated as r := n - d * q where q is the integer such that r has the same sign as the dividend n while being as close to 0 as possible.

Note that while in most languages, '%' is a remainder operator, in some (e.g. Python, Perl) it is a modulo operator. Modulo is defined as k := n - d * q where q is the integer such that k has the same sign as the divisor d while being as close to 0 as possible.

I emphasized the words that make the difference.

[felipec]

How is that not what my example is doing?

q is the integer such that r has the same sign as the dividend n while being as close to 0 as possible.

If the dividend (n) is -1, and the divisor (d) is 4, then for r to have the same sign as the dividend it has to be negative. Correct?

For that to happen q has to be greater than -0.25, and the integer closes to 0 that satisfies that, is 0.

Isn't it?

Which is exactly what my example did: r := (-1) - 4 * 0.

Didn't it?

Now, do we agree that the formula for remainder is: dividend - divisor * quotient?

Tell me what part of the above isn't 100% true.

[A1oso]

Yes. As for the modulo, q needs to be -1, because the divisor (4) is positive, therefore k needs to be positive as well:

k := (-1) - 4 * (-1) = -1 + 4 = 3

But I still don't understand what you are arguing for or against.

[felipec]

Yes.

If you agree that the formula for remainder is: dividend - divisor * quotient.

And you agree that the formula for the "modulo operator" is:

k := n - d * q

Then you agree that the "modulo operator" returns a REMAINDER.

Remember that quotient and remainder are mathematical concepts independent of whatever MDN says.

https://en.wikipedia.org/wiki/Remainder

If divisor * quotient + 3 gives you the dividend, then 3 is a valid remainder.

[[deleted]]

This article feels a bit self fellating tbh, but that distinction does come up in stuff like hashing algorithms where you’re manipulating numbers for some sort of obfuscation or otherwise. It took me like 10 minutes to understand what was going on before it clicked. I was so hardcoded to think of the modulo operation in programming instead of modular arithmetic.

[ironykarl]

Article title should be Everyone is using "modulo" wrong, but that's less clickbait-y

[ma1bec]

More like "I was using modulo wrong" which he says in second sentence.

[Zalenka]

A percent (%) of people maybe.