### Re: D and math, can you isolate this ?

```
On Tuesday, 20 September 2016 at 12:35:18 UTC, Basile B. wrote:
I've recently started an easing/interpolation family of
function in my D user library. It's based on something I know
well since I've already used them in 2012 in a VST plugin
called GrainPlot (RIP).

However for one of the function, I can't manage to get the
inverse.

A function that's fully implemented:
https://github.com/BBasile/iz/blob/master/import/iz/math.d#L598
- f(x,c) = x*x*x - x*x*c + x*c;
- c(f(0.5)) = 4 * (y - 0.125));

Another:
https://github.com/BBasile/iz/blob/master/import/iz/math.d#L749
- f(x,c) = pow(x, c);
- c(f(0.5)) = log(y) / log(0.5));

The problem is here:
https://github.com/BBasile/iz/blob/master/import/iz/math.d#L849
- f(x,c) = 1.0 - pow(1.0 - pow(x, 2.0/c), c * 0.5);
- c(f0.5)) = ?

Which means that I ask you if you can isolate c for

y = 1.0 - pow(1.0 - pow(0.5, 2.0/c), c * 0.5);

y is always f(0.5,c)

So if we rearrange and take the logs of both sides and divide by
c we get

2*log(1-y)/c = log(1-2^(-2/c))

and then that we have one occurrence of c on each side do an
iterative back substitution to find the intersection given that
you know for y=0.5 ,c = 2.
We used this method for finding voltages and currents in circuits
with semiconductors.

```

### Re: What exactly does the compiler switch -betterC do?

```
On Monday, 20 June 2016 at 06:35:32 UTC, Jacob Carlborg wrote:

On 2016-06-19 21:53, Gary Willoughby wrote:
When compiling, what exactly does the -betterC flag do? The
command help
says "omit generating some runtime information and helper
functions" but

what does this really mean? Is there any specifics somewhere?

It is intended to allow you to link an application without
druntime. [...]

What is the equavilent in gdc and ldc?

```

### Re: Append const to array

```On Tuesday, 20 September 2016 at 22:38:33 UTC, Jonathan M Davis
wrote:
On Tuesday, September 20, 2016 22:23:08 Yuxuan Shui via
Digitalmars-d-learn wrote:

struct A {
ulong[] x;
}
struct B {
ulong x;
}
void main() {
B[] b;
const(B) xx = B(1);
b ~= xx; // Works

A[] c;
const(A) yy = A([1]);
c ~= yy; // Does not
}

What gives?

const(A) means that the ulong[] inside is const(ulong[]). When
yy is copied
to be appended to c, it goes from const(A) to A, which means
that
const(ulong[]) would need to be sliced and and set to ulong[],
which would
violate const, because it would mean that the last element in c
could mutate
then elements of its x, which would then mutate the elements in
yy.

- Jonathan M Davis

That makes sense, thanks.

```

### Re: Append const to array

```On Tuesday, September 20, 2016 22:23:08 Yuxuan Shui via Digitalmars-d-learn
wrote:
> struct A {
>   ulong[] x;
> }
> struct B {
>   ulong x;
> }
> void main() {
>   B[] b;
>   const(B) xx = B(1);
>   b ~= xx; // Works
>
>   A[] c;
>   const(A) yy = A([1]);
>   c ~= yy; // Does not
> }
>
> What gives?

const(A) means that the ulong[] inside is const(ulong[]). When yy is copied
to be appended to c, it goes from const(A) to A, which means that
const(ulong[]) would need to be sliced and and set to ulong[], which would
violate const, because it would mean that the last element in c could mutate
then elements of its x, which would then mutate the elements in yy.

- Jonathan M Davis

```

### Append const to array

```

struct A {
ulong[] x;
}
struct B {
ulong x;
}
void main() {
B[] b;
const(B) xx = B(1);
b ~= xx; // Works

A[] c;
const(A) yy = A([1]);
c ~= yy; // Does not
}

What gives?

```

### Re: Using Libraries

```
On Tuesday, 20 September 2016 at 15:38:55 UTC, Darren wrote:
On Tuesday, 20 September 2016 at 15:07:53 UTC, rikki cattermole
wrote:

Ok lets start at the very beginning...

I think I need to start before that, haha.

I might need more of a step-by-step guide.  I'm a complete
beginner to programming, not just D.  I worked through
Programming in D, where I was just compiling with dmd, then
when I decided to learn OpenGL I seem to be using dub for
everything.

There have been a few libraries I've wanted to use but couldn't
because they didn't have a pre-compiled binary, which is all
I've been able to get working through sheer trial and error.
Some sites say to use things like CMake and cygwin, but I'm
uncomfortable using things I have no idea about.

Dub is like a package manager for D (like what npm is to
node.js). All dub libraries are hosted at code.dlang.org. When
you see a library at code.dlang.org you want to use, you could
either type "dub install packagename" whilst in the dub project
ROOT or specify dependencies in the dub.json file.
You can then run "dub run" which will take care of fetching and
building dependencies/libraries from code.dlang.org (including

For example, there is a web framework called vibe.d. If I want to
use vide.d, I can specify dependencies as;

dependencies: {
"vide-d":"^0.7.29"
}

In my app.d file (which is available for any dub project created
using "dub init projectname") I can import vibe.d using;

import vide.d;
void main() {
...
}

I can now compile and run the program with "dub run" or "dub
build" to only build and link without running.

```

### setting fields of object using traits

```I'm trying to set fields of object from JSON with traits library.
How i can to it properly?

import std.stdio;
import std.json;
import std.traits;
import std.meta:  Alias;

class Obj{
void fromJSON(this T)(JSONValue j){
foreach(field; FieldNameTuple!T){
alias member = Alias!(__traits(getMember, T, field));
static if (__traits(hasMember, member, "fromJSON")){
member.fromJSON(j[field]);
} else {
member = j[field];
}
}
}
}

class A : Obj{
int a,b;
C c;
this(){
c = new C();
}

}

class C : Obj{
int a;
this(){
a = 0;
};
}

int main(string[] argv)
{
string s = "{\"a\": 1, \"b\": 2, \"c\": {\"a\": 3} }";
JSONValue j = parseJSON(s);
A a = new A();
a.fromJSON(j);
writeln(a.b);
return 0;
}

main.d(14): Error: need 'this' for 'a' of type 'int'
main.d(14): Error: need 'this' for 'b' of type 'int'
main.d(12): Error: template main.Obj.fromJSON cannot deduce
function from argument types !(c)(JSONValue), candidates are:

main.d(8):main.Obj.fromJSON(this T)(JSONValue j)
main.d(41): Error: template instance main.Obj.fromJSON!(A) error
instantiating

```

### Re: thisExePath purity

```On Tuesday, 20 September 2016 at 13:35:27 UTC, Steven
Schveighoffer wrote:
Yes, but if your code does instantiate it, it is called, even
if you don't ever call the function that calls it.
Yes, it's not ideal but better then just global variable and
static block - it's called in any case, even if variable is not
used at all.

Ideal solution will be something like attribute for static block
leading to make it optional, so module will not be included if no
usage of other symbols found. But I don't know way how to make it
so template is used.

Note that if you don't import the module that contains the
static ctor, it should be trimmed by the linker.
Let's imagine linker can trim even imported module with static
ctor, if we have something like:

immutable string executablePath;

@local shared static this()
{
import std.file : thisExePath;
executablePath = thisExePath();
}

and there is no references to executablePath. Here it would be
useful, I think. Attribute @local (or @module? the name does not
matter) mean this block used only to init other symbols in this
module so it can be skipped if no references.

I would absolutely caution you from putting static this()
inside any template. Unfortunately, due to the way D generates
these static constructors, any module that uses staticMemoize,
or *imports a module that uses it*, will be marked as having a
static constructor, and will potentially create cycles.
this? https://isocpp.org/wiki/faq/ctors#static-init-order

```

### Re: thisExePath purity

```
On Tuesday, 20 September 2016 at 09:14:39 UTC, Marc Schütz wrote:

Have a look at `std.concurrency.initOnce`:
https://dlang.org/phobos/std_concurrency.html#.initOnce

But you will still need to use assumePure() for calling
`thisExePath`, and it might do other things that are impure...

Yes, it's near but in this case I try to fix purity, so any
variants of lazy initialization is not applicable here.

```

### Re: thisExePath purity

```On Tuesday, 20 September 2016 at 04:26:05 UTC, Jonathan M Davis
wrote:
On Tuesday, September 20, 2016 04:17:21 crimaniak via
Digitalmars-d-learn wrote:

static shared immutable ReturnType!T value;

I would point out that immutable is implicitly shared, so
there's no reason to put shared on an immutable variable.
However, you _do_ want to put shared on a static constructor
that initializes an immutable variable so that it's only run
really should enforce that, but there's a longstanding bug that
allows you to reinitialize an immutable variable by not putting
shared on the static constructor and starting multiple threads).

Ok, I got it. Thanks.

```

### Re: D and math, can you isolate this ?

```On Tue, Sep 20, 2016 at 09:22:19AM -0700, H. S. Teoh via Digitalmars-d-learn
wrote:
> On Tue, Sep 20, 2016 at 12:35:18PM +, Basile B. via Digitalmars-d-learn
> wrote:
> [...]
[...]
> > Which means that I ask you if you can isolate c for
> >
> > y = 1.0 - pow(1.0 - pow(0.5, 2.0/c), c * 0.5);
> >
> > y is always f(0.5,c)
[...]
> That probably means the inverse cannot be expressed in terms of
> elementary functions. Probably the only thing you can do is to use
> some kind of numerical approximation, like some form of Newton's
> method or some such, to find the value of c.
[...]

It may be analytically very hard to solve this equation, but it's
probably not so hard to solve numerically. Based on the graph of the
equation produced by Wolfram Alpha, it seems that y must always lie
between 0 and 1, and that it has a horizontal asymptote at y=1.  At
around c=6 or thereabouts, y becomes very close to 1.  The value of c
for y=0.5 is approximately 2, so that seems like a good initial guess
for an iterative method.

So if y<0 or y>1, return NaN. If y=1, return +inf. Otherwise, use an
iterative method with a starting value of c=2. Because of the horizontal
asymptote at y=1, though, values of c much greater than 6 will probably
be quite inaccurate, so hopefully your application doesn't depend on the
exact value in that case!

T

--
Freedom of speech: the whole world has no right *not* to hear my spouting off!

```

### Re: D and math, can you isolate this ?

```
On Tuesday, 20 September 2016 at 16:22:19 UTC, H. S. Teoh wrote:
On Tue, Sep 20, 2016 at 12:35:18PM +, Basile B. via
Digitalmars-d-learn wrote: [...]

The problem is here:
https://github.com/BBasile/iz/blob/master/import/iz/math.d#L849
- f(x,c) = 1.0 - pow(1.0 - pow(x, 2.0/c), c * 0.5);
- c(f0.5)) = ?

Which means that I ask you if you can isolate c for

y = 1.0 - pow(1.0 - pow(0.5, 2.0/c), c * 0.5);

y is always f(0.5,c)

I couldn't manage to solve it.  Nested exponentials are very
nasty to invert. :-(  At first, I thought it might be solvable
in terms of the Lambert W function (aka ProductLog) but I
couldn't manage to get the equation into the right form.  Then
I checked on Wolfram Alpha and it says "no result found in
terms of standard mathematical functions".

That probably means the inverse cannot be expressed in terms of
elementary functions. Probably the only thing you can do is to
use some kind of numerical approximation, like some form of
Newton's method or some such, to find the value of c.

T

Thanks for trying, you're not the first to tell me about the
Newton's method...

```

### Re: D and math, can you isolate this ?

```On Tue, Sep 20, 2016 at 12:35:18PM +, Basile B. via Digitalmars-d-learn
wrote:
[...]
> The problem is here:
> https://github.com/BBasile/iz/blob/master/import/iz/math.d#L849
> - f(x,c) = 1.0 - pow(1.0 - pow(x, 2.0/c), c * 0.5);
> - c(f0.5)) = ?
>
> Which means that I ask you if you can isolate c for
>
> y = 1.0 - pow(1.0 - pow(0.5, 2.0/c), c * 0.5);
>
> y is always f(0.5,c)

I couldn't manage to solve it.  Nested exponentials are very nasty to
invert. :-(  At first, I thought it might be solvable in terms of the
Lambert W function (aka ProductLog) but I couldn't manage to get the
equation into the right form.  Then I checked on Wolfram Alpha and it
says "no result found in terms of standard mathematical functions".

That probably means the inverse cannot be expressed in terms of
elementary functions. Probably the only thing you can do is to use some
kind of numerical approximation, like some form of Newton's method or
some such, to find the value of c.

T

--
Questions are the beginning of intelligence, but the fear of God is the
beginning of wisdom.

```

### Re: Using Libraries

```On Tuesday, 20 September 2016 at 15:07:53 UTC, rikki cattermole
wrote:

Ok lets start at the very beginning...

I think I need to start before that, haha.

I might need more of a step-by-step guide.  I'm a complete
beginner to programming, not just D.  I worked through
Programming in D, where I was just compiling with dmd, then when
I decided to learn OpenGL I seem to be using dub for everything.

There have been a few libraries I've wanted to use but couldn't
because they didn't have a pre-compiled binary, which is all I've
been able to get working through sheer trial and error.  Some
sites say to use things like CMake and cygwin, but I'm
uncomfortable using things I have no idea about.

```

### Re: Using Libraries

```
On 21/09/2016 3:01 AM, Darren wrote:

Hey, all

I keep hitting roadblocks and that's mainly due to not knowing how to
and including the necessary dependencies in the dub.json file and having
that build/run my project.  I'm sure I'm making a mess of that, too, but
it works and now I need to learn how to include static libraries (and
probably understand github and other dub features).

Right now, for example, I want to use the gl3n package:
https://github.com/Dav1dde/gl3n

What do I need in order to build libraries, and have dub include them
when I import modules?   Can I keep all of the libraries in one place so
I'm not copy-pasting them (like in lib and bin folders that I keep seeing)?

As you can tell, I'm still very new to all of this and I have no idea
where to start.  Thank you for your time!

Ok lets start at the very beginning with straight dmd.

You have a total of two things you can pass in, an import directory and
source files.
Source files are compiled in and import directories are basically a way
to tell the compiler that certain symbols can exist but it won't create
them.

Now from this you abstract away into dependencies such as packages /
subpackages.
This is where dub comes in, it will fetch (known) projects with dub
definition files (dub.json/sdl), place them into a folder under your
profile directory and allow you to build against them and automatically
provide them as import directories as required.

So how do you do that? Simple.

"dependencies": {
"mypackage": ">=0.0.0"
}

Inside of a dub.json file (sdl is a little different check
code.dlang.org for more help on the subject).

```

### Using Libraries

```
Hey, all

I keep hitting roadblocks and that's mainly due to not knowing
how to include libraries.  So far I've been getting by with
the dub.json file and having that build/run my project.  I'm sure
I'm making a mess of that, too, but it works and now I need to
learn how to include static libraries (and probably understand
github and other dub features).

Right now, for example, I want to use the gl3n package:
https://github.com/Dav1dde/gl3n

What do I need in order to build libraries, and have dub include
them when I import modules?   Can I keep all of the libraries in
one place so I'm not copy-pasting them (like in lib and bin
folders that I keep seeing)?

As you can tell, I'm still very new to all of this and I have no
idea where to start.  Thank you for your time!

```

### Re: D and math, can you isolate this ?

```
On Tuesday, 20 September 2016 at 12:35:18 UTC, Basile B. wrote:
I've recently started an easing/interpolation family of
function in my D user library. It's based on something I know
well since I've already used them in 2012 in a VST plugin
called GrainPlot (RIP).

However for one of the function, I can't manage to get the
inverse.

[...]
The problem is here:
https://github.com/BBasile/iz/blob/master/import/iz/math.d#L849
- f(x,c) = 1.0 - pow(1.0 - pow(x, 2.0/c), c * 0.5);
- c(f0.5)) = ?

Which means that I ask you if you can isolate c for

y = 1.0 - pow(1.0 - pow(0.5, 2.0/c), c * 0.5);

y is always f(0.5,c)

If you don't understand, these function have a control point, for
"parabol" and "pow" it's easy to get the c Coefficient that
manages the slope. But for the ellipse (aka the super ellipse)
it's a math nightmare )

For example is use the three functions in the same order
(parabol, pow, ellipse):

http://sendvid.com/ygti5jmr

for the ellipse you can see that the mouse position is not in
sync with the control point at the middle...it's the problem.

I need to isolate c when "y = 1.0 - pow(1.0 - pow(0.5, 2.0/c), c
* 0.5)".

I know it's hard...otherwise I wouldn't ask ;]

```

### Re: thisExePath purity

```
On 9/20/16 12:17 AM, crimaniak wrote:

Hi and thanks all!

On Tuesday, 20 September 2016 at 00:43:10 UTC, Jonathan M Davis wrote:

immutable string executablePath;

shared static this()
{
import std.file : thisExePath();
executablePath = thisExePath();
}

This code is good for my needs but I start to think about how to call
thisExePath only if it is really used and come to this solution:

import std.traits: ReturnType, Parameters;

string staticMemoize(alias T, Parms = Parameters!T)() pure
{
struct Holder(alias T)
{
static shared immutable ReturnType!T value;
shared static this(){ value = T(Parms); }
}

return Holder!T.value;
}

unittest
{
import std.file : thisExePath;
assert(staticMemoize!thisExePath == thisExePath);
}

Something like this. Need to refine about input parameters, but I hope,
idea is clear.
Unlike the function memoize from phobos staticMemoize really pure. And
unlike proposed solution with ordinary variable staticMemoize is lazy,
because no call - no instantiation.

Yes, but if your code does instantiate it, it is called, even if you
don't ever call the function that calls it.

Note that if you don't import the module that contains the static ctor,
it should be trimmed by the linker.

I would absolutely caution you from putting static this() inside any
template. Unfortunately, due to the way D generates these static
constructors, any module that uses staticMemoize, or *imports a module
that uses it*, will be marked as having a static constructor, and will
potentially create cycles.

-Steve

```

### D and math, can you isolate this ?

```I've recently started an easing/interpolation family of function
in my D user library. It's based on something I know well since
I've already used them in 2012 in a VST plugin called GrainPlot
(RIP).

However for one of the function, I can't manage to get the
inverse.

A function that's fully implemented:
https://github.com/BBasile/iz/blob/master/import/iz/math.d#L598
- f(x,c) = x*x*x - x*x*c + x*c;
- c(f(0.5)) = 4 * (y - 0.125));

Another:
https://github.com/BBasile/iz/blob/master/import/iz/math.d#L749
- f(x,c) = pow(x, c);
- c(f(0.5)) = log(y) / log(0.5));

The problem is here:
https://github.com/BBasile/iz/blob/master/import/iz/math.d#L849
- f(x,c) = 1.0 - pow(1.0 - pow(x, 2.0/c), c * 0.5);
- c(f0.5)) = ?

Which means that I ask you if you can isolate c for

y = 1.0 - pow(1.0 - pow(0.5, 2.0/c), c * 0.5);

y is always f(0.5,c)

```

### Re: thisExePath purity

```
On Tuesday, 20 September 2016 at 04:17:21 UTC, crimaniak wrote:

Hi and thanks all!

On Tuesday, 20 September 2016 at 00:43:10 UTC, Jonathan M Davis
wrote:

immutable string executablePath;

shared static this()
{
import std.file : thisExePath();
executablePath = thisExePath();
}

This code is good for my needs but I start to think about how
to call thisExePath only if it is really used and come to this
solution:

import std.traits: ReturnType, Parameters;

string staticMemoize(alias T, Parms = Parameters!T)() pure
{
struct Holder(alias T)
{
static shared immutable ReturnType!T value;
shared static this(){ value = T(Parms); }
}

return Holder!T.value;
}

unittest
{
import std.file : thisExePath;
assert(staticMemoize!thisExePath == thisExePath);
}

Something like this. Need to refine about input parameters, but
I hope, idea is clear.
Unlike the function memoize from phobos staticMemoize really
pure. And unlike proposed solution with ordinary variable
staticMemoize is lazy, because no call - no instantiation.

Have a look at `std.concurrency.initOnce`:
https://dlang.org/phobos/std_concurrency.html#.initOnce

But you will still need to use assumePure() for calling
`thisExePath`, and it might do other things that are impure...

```

```
Don't forget the Planet D aggregator :)
http://planet.dsource.org/

Here's my contribution:
https://theartofmachinery.com/tags/dlang/

```

```
On Monday, 19 September 2016 at 19:36:22 UTC, Karabuta wrote:

On Monday, 19 September 2016 at 19:29:25 UTC, A D dev wrote:

On Monday, 19 September 2016 at 17:42:51 UTC, A D dev wrote:

Hi list,

To be more clear:

- what blogs that include posts on D, would you recommend to a
D beginner?

Thanks.

I have one here on Vibe.d for beginners
https://laberba.github.io/2016/hello-world-app-with-the-vibe.d-web-framework/

I will be writing more for-beginners blogs in the coming few
weeks.

You blog looks gorgeous. Absolutely beautiful!

```