promote int to real, double-based divide function about stanc3 HOT 8 CLOSED

to_real is not necessary since you can always multiply by 1.0. And as far as I know division is the only place where the compiler doesn't autoconvert ints to reals.

The int division problem came up on the forum a few months ago. Does anyone object to my proposal for integer division operator?

int a;
int b;
int c = a %/ b; // integer division, rounds down
real d = a / b; // real division, no rounding

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Does anyone object to my proposal for integer division operator?

Yes, we can't change int / int to return a real because it breaks backward compatibility.

Multiplying by 1.0 will work, but it feels like a trick, so I'd prefer something like to_real for readability. It's more efficient to cast, but I can't see it ever being done enough to make a difference relative to everything else going on in a Stan program.

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These are easy enough to address.

@thelseraphim or @mitzimorris: if you have free cycles, this should be straightforward to tackle.

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Can this be closed?

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It hasn't been implemented yet. The suggestion is for a function

real div(real x, real y);

so that we can do:

real x = div(1, 2);

and get 0.5 rather than the 0 we get with 1 / 2. This is a minor issue.

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If integer division is added as a new operator, I think we should try to follow at least one other language and not invent our own, although most languages I've seen have named functions for integer (floor) division.

I've tried to do a survey of integer division operators in use:

• // in Python (we can't have this as it is start of comment in Stan)
• %/% in R - I like this because many Stan users use R (although few would be actually familiar with this operator)
• \ used in Visual Basic as well as Access and other MS products. Also in ColdFusion.
• ÷ used in Julia - although I think you can't write this on most keyboards...
• div used in Pascal (this might cause some name conflicts)

Some languages define integer division also for floating point numbers. I think we shouldn't do this and only allow integer division of two ints.

I agree with @bob-carpenter that int / int needs to stay, but it should IMHO provide a warning with suggestion to either use to_real or integer division.

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int / int emits a warning right now and is going to round for at least a few releases. Users who want fractions have to use "the real division operator" *1.0/.
Changing the return type is breaking but often not silent; int division is mostly useful for calculating indices and there an unexpected real causes a type error.

The question is what to call the int division operator. @martinmodrak's survey has only one good option.

• Python's // is my personal favorite but of course it's not available. (thanks, C++)
• R's %/%: I'd prefer the shorter %/ but this is fine too.
• Visual Basic \: Nope, \ is already a Stan operator, it means matrix left-division.
• Julia ÷: The parser does not allow non-ascii characters outside of comments.
• Pascal div: A keyword that is an infix operator? Please no.

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So which is the good option? Wasn't @nhuurre's original proposal to overload ./ to scalar types? If we do that, it makes sense to cast to real since it's underlyingly a vector operation and vectors are all real. I don't think any of these (including just leaving things the way they are) make it clear to a reader of a program what's going on.

To undertand int / int is an int, they need to know that Stan works like C++ and Java, not like R. To understand what div or %/% or ./ means, they need to read our doc.

Also, we already use \ for left matrix division.

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