# Dimensional analysis

My earlier post about converting Decimal to Double was part of a larger effort by me to write a calculator app that supports dimensional analysis/unit conversions. I've explored some existing libraries on the topic, ranging from this one in Common Lisp (<http://www.cs.utexas.edu/users/novak/units95.html&gt;\) to this nice-looking C++ header-only implementation (<https://github.com/nholthaus/units&gt;\). Here's one done in Swift (<https://github.com/michalkonturek/MKUnits&gt;\), which has similarities to the C++ library.

My calculator allows expressions like this:

1-3/4" x 2 + (4.3 mm + 0.1") -> <XX> mm

And produces values in both inch and mm, but I want to be able to expand that to more units.

I also want to support proper dimensional analysis:

2 mm x 4" -> 203.2 mm^2

While my calculator generally only works with lengths, areas, and angles, I figure a general-purpose dimensional analysis library would be useful.

Dimensional analysis has come up before (<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160111/006596.html&gt; and <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160229/011666.html&gt;\). It seems support for constant expressions is useful for dimensional analysis, and Swift doesn't currently support that.

Given that limitation, what approach would the gurus recommend? The library should support:

• Generating a result with derived units based on the input
• Simplifying composite units to some canonical form
• Validating conversion from one (derived) unit to another
• Supporting leniency in conversions (e.g. conflating lb and lb•f).
• Compile-time errors when mixing incompatible types.

In my case, the latter requirement is not so important, since all the expressions will be input by the user.

The existing Swift library might be sufficient for my purposes, but I was interested in the experts' opinions and suggestions.

Thanks!

···

--
Rick Mann
rmann@latencyzero.com

Great idea to have dimensions library. I use Quantity in Mathematica all the time and it is great. Even if some of the checks are at runtime it is still useful, better to throw an assert rather than nothing. All the rest of your list, in addition to compile time check, I also use.

-- Howard.

···

On 29 Nov. 2016, at 9:37 am, Rick Mann via swift-users <swift-users@swift.org> wrote:

My earlier post about converting Decimal to Double was part of a larger effort by me to write a calculator app that supports dimensional analysis/unit conversions. I've explored some existing libraries on the topic, ranging from this one in Common Lisp (<http://www.cs.utexas.edu/users/novak/units95.html&gt;\) to this nice-looking C++ header-only implementation (<https://github.com/nholthaus/units&gt;\). Here's one done in Swift (<https://github.com/michalkonturek/MKUnits&gt;\), which has similarities to the C++ library.

My calculator allows expressions like this:

1-3/4" x 2 + (4.3 mm + 0.1") -> <XX> mm

And produces values in both inch and mm, but I want to be able to expand that to more units.

I also want to support proper dimensional analysis:

2 mm x 4" -> 203.2 mm^2

While my calculator generally only works with lengths, areas, and angles, I figure a general-purpose dimensional analysis library would be useful.

Dimensional analysis has come up before (<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160111/006596.html&gt; and <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160229/011666.html&gt;\). It seems support for constant expressions is useful for dimensional analysis, and Swift doesn't currently support that.

Given that limitation, what approach would the gurus recommend? The library should support:

• Generating a result with derived units based on the input
• Simplifying composite units to some canonical form
• Validating conversion from one (derived) unit to another
• Supporting leniency in conversions (e.g. conflating lb and lb•f).
• Compile-time errors when mixing incompatible types.

In my case, the latter requirement is not so important, since all the expressions will be input by the user.

The existing Swift library might be sufficient for my purposes, but I was interested in the experts' opinions and suggestions.

Thanks!

--
Rick Mann
rmann@latencyzero.com

_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users

Let me add my support for this as well. I am currently using a bridge version of:

And it works quite well. But I think a pure Swift implementation, especially with dimensional analysis, would be useful to a great number of people.

My earlier post about converting Decimal to Double was part of a
larger effort by me to write a calculator app that supports
dimensional analysis/unit conversions. I've explored some existing
libraries on the topic, ranging from this one in Common Lisp
(<http://www.cs.utexas.edu/users/novak/units95.html&gt;\) to this
(<https://github.com/nholthaus/units&gt;\). Here's one done in Swift
(<https://github.com/michalkonturek/MKUnits&gt;\), which has similarities
to the C++ library.

My calculator allows expressions like this:

1-3/4" x 2 + (4.3 mm + 0.1") -> <XX> mm

And produces values in both inch and mm, but I want to be able to expand that to more units.

I also want to support proper dimensional analysis:

2 mm x 4" -> 203.2 mm^2

While my calculator generally only works with lengths, areas, and angles, I figure a general-purpose
dimensional analysis library would be useful.

Dimensional analysis has come up before
(<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160111/006596.html&gt;
and
<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160229/011666.html&gt;\). It
seems support for constant expressions is useful for dimensional
analysis, and Swift doesn't currently support that.

Given that limitation, what approach would the gurus recommend? The library should support:

• Generating a result with derived units based on the input
• Simplifying composite units to some canonical form
• Validating conversion from one (derived) unit to another
• Supporting leniency in conversions (e.g. conflating lb and lb•f).

Is that conflating pounds with foot-pounds? If so that seems
undesirable, and also incompatible with the very next bullet. If not,
what are you suggesting?

• Compile-time errors when mixing incompatible types.

Because of the lack of numeric generic parameters in Swift, we don't
know how to create a library that satisfies the latter requirement.

···

on Mon Nov 28 2016, Rick Mann <swift-users-AT-swift.org> wrote:

--
-Dave

Another feature I would like would be a dimensions wrapper for an array. So
that I can mark all values in the array as having the same units, rather
than mark each individual element as having a particular unit.

It is conceivable that wrapping each element, both performance and memory
wise, is OK depending on implementation. However my experience with
Mathematica's Quantity is that wrapping each element is a significant
overhead. Therefore I would like a dimensioned array.

As an aside. It might be a lot easier to write an array wrapper when we get
Conditional Conformance To Protocols with the improved generics slated for
Swift 4.1. Therefore it might be worth waiting until 4.1. before attempting
a dimensioned array.

-- Howard.

···

On 1 December 2016 at 01:17, Maury Markowitz via swift-users < swift-users@swift.org> wrote:

Let me add my support for this as well. I am currently using a bridge
version of:

GitHub - unixpickle/Expressions: An object-oriented mathematical expression parser for Objective-C

And it works quite well. But I think a pure Swift implementation,
especially with dimensional analysis, would be useful to a great number of
people.
_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users

• Supporting leniency in conversions (e.g. conflating lb and lb•f).

Is that conflating pounds with foot-pounds? If so that seems
undesirable, and also incompatible with the very next bullet. If not,
what are you suggesting?

It’s pounds force isn’t it? It still shouldn’t be conflated with pounds mass which has different dimensions.

My earlier post about converting Decimal to Double was part of a
larger effort by me to write a calculator app that supports
dimensional analysis/unit conversions. I've explored some existing
libraries on the topic, ranging from this one in Common Lisp
(<http://www.cs.utexas.edu/users/novak/units95.html&gt;\) to this
(<https://github.com/nholthaus/units&gt;\). Here's one done in Swift
(<https://github.com/michalkonturek/MKUnits&gt;\), which has similarities
to the C++ library.

My calculator allows expressions like this:

1-3/4" x 2 + (4.3 mm + 0.1") -> <XX> mm

And produces values in both inch and mm, but I want to be able to expand that to more units.

I also want to support proper dimensional analysis:

2 mm x 4" -> 203.2 mm^2

While my calculator generally only works with lengths, areas, and angles, I figure a general-purpose
dimensional analysis library would be useful.

Dimensional analysis has come up before
(<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160111/006596.html&gt;
and
<https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160229/011666.html&gt;\). It
seems support for constant expressions is useful for dimensional
analysis, and Swift doesn't currently support that.

Given that limitation, what approach would the gurus recommend? The library should support:

• Generating a result with derived units based on the input
• Simplifying composite units to some canonical form
• Validating conversion from one (derived) unit to another
• Supporting leniency in conversions (e.g. conflating lb and lb•f).

Is that conflating pounds with foot-pounds? If so that seems
undesirable, and also incompatible with the very next bullet. If not,
what are you suggesting?

lb and lb-force. They are mass and force, respectively, but in a 1-gee gravity field, they result in the same value. Ideally, the library would support being lenient, if desired, but with the (usual) option of being strict.

• Compile-time errors when mixing incompatible types.

Because of the lack of numeric generic parameters in Swift, we don't
know how to create a library that satisfies the latter requirement.

Right, that's what I gathered from the older posts touching on the subject.

···

On Dec 1, 2016, at 11:36 , Dave Abrahams via swift-users <swift-users@swift.org> wrote:
on Mon Nov 28 2016, Rick Mann <swift-users-AT-swift.org> wrote:

--
Rick

--
Rick Mann
rmann@latencyzero.com