## STD-PROPOSALS | |

**Subject:** Re: [std-proposals] new approximately equal operators

**From:** Andrew Tomazos (*andrewtomazos_at_[hidden]*)

**Date:** 2020-06-21 03:09:58

**Next message:**Bo Persson: "Re: [std-proposals] new approximately equal operators"**Previous message:**andrei_at_[hidden]: "[std-proposals] Re-purposing the function exception specification"**In reply to:**Vishal Oza: "[std-proposals] new approximately equal operators"**Next in thread:**Bo Persson: "Re: [std-proposals] new approximately equal operators"**Reply:**Bo Persson: "Re: [std-proposals] new approximately equal operators"

If you're interested in working in this space, I'd start by publishing a

survey of some popular existing C++ libraries that provide floating-point

approximately equal functionality - and carefully analyzing and comparing

how they do it and how they are used. Next step would be to propose this

functionality into the standard library (std::approx_equal or similar),

probably initially to the Numerics subgroup (SG6). Once the standard

library has the functionality I can imagine a proposal to provide operators

for it - but the latter is unlikely to be accepted because new C++

operators have to meet an insanely high threshold of interest for some

reason. (Amusing to compare other languages like Scala or Perl 6 - where

arbitrary new operators can be user-defined like functions)

On Tue, Jun 9, 2020 at 5:55 AM Vishal Oza via Std-Proposals <

std-proposals_at_[hidden]> wrote:

> I was wondering if there is any interest in adding approximately equal to

> operators into the language. The main purpose is to compare floating point

> calculations with rounding error, however they could be used for similarity

> of objects or fixed point rounded values as well.

>

> the operator I propose are: ~=, ~< , ~>, ~!

> these operators would translate at least to floating point as

> a ~= b ==> abs(a-b) < epsilon

> a ~< b ==> (a < b) || (abs(a-b) < epsilon)

> a ~> b ==> (a > b) || (abs(a-b) < epsilon)

> a ~! b ==> abs(a-b) >= epsilon

>

> There are two routes I can see with default implementation this language

> feature. either adding a default epsilon into the language that can be set

> as a global variable or adding some way of tracking errors. I would like

> more feedback on either way of default implementation.

>

> I would also like a way to specify the epsilon value or function for a

> single comparison. the best I can think of is

> a ~=b(c) ==> abs(a-b) < c

> a ~=b:c ==> abs(a-b) < c

> [c]a ~=b ==> abs(a-b) < c

>

> The precedence should be on the same level of all of the current

> comparison operators.

>

> This could be introduced in C++ 23 or later in there is any interate.

>

> Sincerely,

> Vishal Oza

> --

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>

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**Next message:**Bo Persson: "Re: [std-proposals] new approximately equal operators"**Previous message:**andrei_at_[hidden]: "[std-proposals] Re-purposing the function exception specification"**In reply to:**Vishal Oza: "[std-proposals] new approximately equal operators"**Next in thread:**Bo Persson: "Re: [std-proposals] new approximately equal operators"**Reply:**Bo Persson: "Re: [std-proposals] new approximately equal operators"

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