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digitalmars.D - Fixing the imaginary/complex mess

reply Don <nospam nospam.com> writes:
D currently allows some conversions between complex/imaginary types and 
real types, which are highly dubious.

Given creal z, ireal y, these casts are legal:
real x = cast(real)z;
x = cast(real)y; // always sets x==0, regardless of the value of y.
But I believe that should not be legal.

For the first case, it should be written as:   real x = z.re;
(which is shorter and clearer), and the second case is probably a bug, 
and should be x = y.im; (unless the intention really was to set x=0!).

By the same logic, we could have sqrt(-1)==0, since the real part is 0.

The most important effect of disallowing these casts would be to fix a 
host of bugs and wierd behaviour. All the A op= B operations involve a 
cast to A. If those nonsensical casts become illegal, the nonsensical 
op= operations become illegal automatically.

Eg, ireal y;
y *= y; // mathematically nonsense, y*y is real, so can't be stored in a 
pure imaginary type!

There are a few segfault/ICE bugs (eg 718, 2839) which involve 
int/=complex, an operation which never makes any sense anyway.

I think we're just making problems for ourselves by allowing these 
useless operations. I think they should be killed.
Does anyone object? (If not, I'll create a patch to do it; it's not very 
difficult).
Apr 30 2009
next sibling parent dsimcha <dsimcha yahoo.com> writes:
== Quote from Don (nospam nospam.com)'s article

 D currently allows some conversions between complex/imaginary types and
 real types, which are highly dubious.
 Given creal z, ireal y, these casts are legal:
 real x = cast(real)z;
 x = cast(real)y; // always sets x==0, regardless of the value of y.
 But I believe that should not be legal.
 For the first case, it should be written as:   real x = z.re;
 (which is shorter and clearer), and the second case is probably a bug,
 and should be x = y.im; (unless the intention really was to set x=0!).
 By the same logic, we could have sqrt(-1)==0, since the real part is 0.
 The most important effect of disallowing these casts would be to fix a
 host of bugs and wierd behaviour. All the A op= B operations involve a
 cast to A. If those nonsensical casts become illegal, the nonsensical
 op= operations become illegal automatically.
 Eg, ireal y;
 y *= y; // mathematically nonsense, y*y is real, so can't be stored in a
 pure imaginary type!
 There are a few segfault/ICE bugs (eg 718, 2839) which involve
 int/=complex, an operation which never makes any sense anyway.
 I think we're just making problems for ourselves by allowing these
 useless operations. I think they should be killed.
 Does anyone object? (If not, I'll create a patch to do it; it's not very
 difficult).


I'm totally ok with you creating such a patch and applying it, as I don't use
complex numbers anyhow, but if we're going to look at fixing complex numbers,
shouldn't we discuss whether they even still belong in the core language?  IMHO
they don't because D's user defined types have advanced pretty significantly
since
the decision was made to put them in, and they're a *very* niche feature.  The
only people who need complex and imaginary numbers are scientific computing
people, and not all or even most scientific computing people need them.

Show of hands, how many people here actually use D's complex numbers on a
regular
enough basis that you would miss them if they were removed from the core
language
completely?
Apr 30 2009
prev sibling next sibling parent reply Andrei Alexandrescu <SeeWebsiteForEmail erdani.org> writes:
Don wrote:

 D currently allows some conversions between complex/imaginary types and 
 real types, which are highly dubious.
 
 Given creal z, ireal y, these casts are legal:
 real x = cast(real)z;
 x = cast(real)y; // always sets x==0, regardless of the value of y.
 But I believe that should not be legal.
 
 For the first case, it should be written as:   real x = z.re;
 (which is shorter and clearer), and the second case is probably a bug, 
 and should be x = y.im; (unless the intention really was to set x=0!).
 
 By the same logic, we could have sqrt(-1)==0, since the real part is 0.
 
 The most important effect of disallowing these casts would be to fix a 
 host of bugs and wierd behaviour. All the A op= B operations involve a 
 cast to A. If those nonsensical casts become illegal, the nonsensical 
 op= operations become illegal automatically.
 
 Eg, ireal y;
 y *= y; // mathematically nonsense, y*y is real, so can't be stored in a 
 pure imaginary type!
 
 There are a few segfault/ICE bugs (eg 718, 2839) which involve 
 int/=complex, an operation which never makes any sense anyway.
 
 I think we're just making problems for ourselves by allowing these 
 useless operations. I think they should be killed.
 Does anyone object? (If not, I'll create a patch to do it; it's not very 
 difficult).


Since you're looking into this already, I suggest we move for the kill 
and eliminate built-in complex.

Andrei
Apr 30 2009
next sibling parent reply Robert Fraser <fraserofthenight gmail.com> writes:
Andrei Alexandrescu wrote:

 Don wrote:

 D currently allows some conversions between complex/imaginary types 
 and real types, which are highly dubious.

 Given creal z, ireal y, these casts are legal:
 real x = cast(real)z;
 x = cast(real)y; // always sets x==0, regardless of the value of y.
 But I believe that should not be legal.

 For the first case, it should be written as:   real x = z.re;
 (which is shorter and clearer), and the second case is probably a bug, 
 and should be x = y.im; (unless the intention really was to set x=0!).

 By the same logic, we could have sqrt(-1)==0, since the real part is 0.

 The most important effect of disallowing these casts would be to fix a 
 host of bugs and wierd behaviour. All the A op= B operations involve a 
 cast to A. If those nonsensical casts become illegal, the nonsensical 
 op= operations become illegal automatically.

 Eg, ireal y;
 y *= y; // mathematically nonsense, y*y is real, so can't be stored in 
 a pure imaginary type!

 There are a few segfault/ICE bugs (eg 718, 2839) which involve 
 int/=complex, an operation which never makes any sense anyway.

 I think we're just making problems for ourselves by allowing these 
 useless operations. I think they should be killed.
 Does anyone object? (If not, I'll create a patch to do it; it's not 
 very difficult).


 
 Since you're looking into this already, I suggest we move for the kill 
 and eliminate built-in complex.
 
 Andrei


Ehhh.... why not leave complex and just kill imaginary? That'd solve 
most of the problems.
Apr 30 2009
next sibling parent reply bearophile <bearophileHUGS lycos.com> writes:
Robert Fraser:

 Ehhh.... why not leave complex and just kill imaginary? That'd solve 
 most of the problems.


You can't remove only half of a feature of the language, so I don't like this
idea.
I accept to remove them fully if their semantics can be fully replaced by the
std lib. Otherwise just let Don fix the broken things.

Bye,
bearophile
Apr 30 2009
parent reply Robert Fraser <fraserofthenight gmail.com> writes:
bearophile wrote:

 Robert Fraser:

 Ehhh.... why not leave complex and just kill imaginary? That'd solve 
 most of the problems.


 
 You can't remove only half of a feature of the language, so I don't like this
idea.
 I accept to remove them fully if their semantics can be fully replaced by the
std lib. Otherwise just let Don fix the broken things.
 
 Bye,
 bearophile


It's not necessarily "removing half a feature". Imaginary numbers aren't 
closed under many operations. The idea is that imaginary literals are 
turned into complex. So "ireal x = 10i;" becomes "creal x = 0 + 10i;". 
Since complex numbers are closed under all the builtin operators, it 
eliminates many of the problems (casts from complex to real).

Putting it in a library eliminates the ability to use complex/imaginary 
literals (unless it's a runtime type).
Apr 30 2009
parent Andrei Alexandrescu <SeeWebsiteForEmail erdani.org> writes:
Robert Fraser wrote:

 Putting it in a library eliminates the ability to use complex/imaginary 
 literals (unless it's a runtime type).


Yah, but then I'm thinking: how many numeric literals are in a program 
at all? I think the vision of the mad scientist that uses one imaginary 
literal every five lines of code is quite unreal :o).

Andrei
Apr 30 2009
prev sibling parent "Joel C. Salomon" <joelcsalomon gmail.com> writes:
Robert Fraser wrote:

 Ehhh.... why not leave complex and just kill imaginary? That'd solve
 most of the problems.


The reasons for a separate imaginary type are listed in the D docs at
<http://digitalmars.com/d/2.0/cppcomplex.html>, referencing William
Kahan’s “Branch Cuts for Complex Elementary Functions, or Much Ado About
Nothing’s Sign Bit”.

It just doesn’t have to be a built-in type anymore.

—Joel Salomon
Apr 30 2009
prev sibling parent reply Don <nospam nospam.com> writes:
Andrei Alexandrescu wrote:

 Don wrote:

 D currently allows some conversions between complex/imaginary types 
 and real types, which are highly dubious.

 Given creal z, ireal y, these casts are legal:
 real x = cast(real)z;
 x = cast(real)y; // always sets x==0, regardless of the value of y.
 But I believe that should not be legal.

 For the first case, it should be written as:   real x = z.re;
 (which is shorter and clearer), and the second case is probably a bug, 
 and should be x = y.im; (unless the intention really was to set x=0!).

 By the same logic, we could have sqrt(-1)==0, since the real part is 0.

 The most important effect of disallowing these casts would be to fix a 
 host of bugs and wierd behaviour. All the A op= B operations involve a 
 cast to A. If those nonsensical casts become illegal, the nonsensical 
 op= operations become illegal automatically.

 Eg, ireal y;
 y *= y; // mathematically nonsense, y*y is real, so can't be stored in 
 a pure imaginary type!

 There are a few segfault/ICE bugs (eg 718, 2839) which involve 
 int/=complex, an operation which never makes any sense anyway.

 I think we're just making problems for ourselves by allowing these 
 useless operations. I think they should be killed.
 Does anyone object? (If not, I'll create a patch to do it; it's not 
 very difficult).


 
 Since you're looking into this already, I suggest we move for the kill 
 and eliminate built-in complex.


I'd like to actually eliminate these operations from D1. Otherwise, I 
can't see the related D1 bugs ever getting fixed. Any code which relies 
on these operations is broken; I think there's an excellent chance that 
there isn't any code using it.
Also, these operations would not be provided by a library class, so I 
think it's a useful intermediate step.
We can't remove the complex types until we have a library solution.
Apr 30 2009
parent reply Brad Roberts <braddr puremagic.com> writes:
Don wrote:

 I'd like to actually eliminate these operations from D1. Otherwise, I
 can't see the related D1 bugs ever getting fixed. Any code which relies
 on these operations is broken; I think there's an excellent chance that
 there isn't any code using it.
 Also, these operations would not be provided by a library class, so I
 think it's a useful intermediate step.
 We can't remove the complex types until we have a library solution.


I think it's perfectly acceptable to not fix these in D1 and only address them
in D2.  There's lots of things in D1 that are only fixed in D2.

Later,
Brad
Apr 30 2009
parent Don <nospam nospam.com> writes:
Brad Roberts wrote:

 Don wrote:

 I'd like to actually eliminate these operations from D1. Otherwise, I
 can't see the related D1 bugs ever getting fixed. Any code which relies
 on these operations is broken; I think there's an excellent chance that
 there isn't any code using it.
 Also, these operations would not be provided by a library class, so I
 think it's a useful intermediate step.
 We can't remove the complex types until we have a library solution.


 
 I think it's perfectly acceptable to not fix these in D1 and only address them
 in D2.  There's lots of things in D1 that are only fixed in D2.


I think so many people are actually using D1, that ICE bugs really 
should get fixed.

 
 Later,
 Brad
Apr 30 2009
prev sibling next sibling parent bearophile <bearophileHUGS lycos.com> writes:
Don:

 I think we're just making problems for ourselves by allowing these useless
operations. I think they should be killed. Does anyone object? (If not, I'll
create a patch to do it; it's not very difficult).<


I agree to disallow/fix those things.
And thanks for your work!

-------------

dsimcha:

Show of hands, how many people here actually use D's complex numbers on a
regular enough basis that you would miss them if they were removed from the
core language completely?<


D is in a flux state still, so I think very few engineer/scientists are using
it (I am not using complex numbers). So probably the answers your will receive
will not be enough to judge future true usages of the D language.
A possible solution is to remove complex types now from D2 (moving them in
Phobos and Tango), and later if enough people ask for them, to put them back
into the built-ins.
Are there problems in adding them back later if they are asked for and regarded
as useful?

-------------

Andrei Alexandrescu:

Since you're looking into this already, I suggest we move for the kill and
eliminate built-in complex.<


Is the current semantics fully doable with std lib complex numbers?

Bye,
bearophile
Apr 30 2009
prev sibling next sibling parent reply Georg Wrede <georg.wrede iki.fi> writes:
Don wrote:

 D currently allows some conversions between complex/imaginary types and 
 real types, which are highly dubious.
 
 Given creal z, ireal y, these casts are legal:
 real x = cast(real)z;
 x = cast(real)y; // always sets x==0, regardless of the value of y.
 But I believe that should not be legal.
 
 For the first case, it should be written as:   real x = z.re;
 (which is shorter and clearer), and the second case is probably a bug, 
 and should be x = y.im; (unless the intention really was to set x=0!).
 
 By the same logic, we could have sqrt(-1)==0, since the real part is 0.
 
 The most important effect of disallowing these casts would be to fix a 
 host of bugs and wierd behaviour. All the A op= B operations involve a 
 cast to A. If those nonsensical casts become illegal, the nonsensical 
 op= operations become illegal automatically.
 
 Eg, ireal y;
 y *= y; // mathematically nonsense, y*y is real, so can't be stored in a 
 pure imaginary type!
 
 There are a few segfault/ICE bugs (eg 718, 2839) which involve 
 int/=complex, an operation which never makes any sense anyway.
 
 I think we're just making problems for ourselves by allowing these 
 useless operations. I think they should be killed.
 Does anyone object? (If not, I'll create a patch to do it; it's not very 
 difficult).


It might be a good idea to write in the docs why these operations are 
missing. Even better would of course be if the user got a message 
stating that this operation isn't implemented because it doesn't make 
mathematical sense.

If the latter isn't done, then many people will just think there's 
something wrong with the compiler/language. Bad press we don't need.
Apr 30 2009
parent reply Don <nospam nospam.com> writes:
Georg Wrede wrote:

 Don wrote:

 D currently allows some conversions between complex/imaginary types 
 and real types, which are highly dubious.

 Given creal z, ireal y, these casts are legal:
 real x = cast(real)z;
 x = cast(real)y; // always sets x==0, regardless of the value of y.
 But I believe that should not be legal.

 For the first case, it should be written as:   real x = z.re;
 (which is shorter and clearer), and the second case is probably a bug, 
 and should be x = y.im; (unless the intention really was to set x=0!).

 By the same logic, we could have sqrt(-1)==0, since the real part is 0.

 The most important effect of disallowing these casts would be to fix a 
 host of bugs and wierd behaviour. All the A op= B operations involve a 
 cast to A. If those nonsensical casts become illegal, the nonsensical 
 op= operations become illegal automatically.

 Eg, ireal y;
 y *= y; // mathematically nonsense, y*y is real, so can't be stored in 
 a pure imaginary type!

 There are a few segfault/ICE bugs (eg 718, 2839) which involve 
 int/=complex, an operation which never makes any sense anyway.

 I think we're just making problems for ourselves by allowing these 
 useless operations. I think they should be killed.
 Does anyone object? (If not, I'll create a patch to do it; it's not 
 very difficult).


 
 It might be a good idea to write in the docs why these operations are 
 missing. Even better would of course be if the user got a message 
 stating that this operation isn't implemented because it doesn't make 
 mathematical sense.


I'll make sure the error messages are sensible.


 
 If the latter isn't done, then many people will just think there's 
 something wrong with the compiler/language. Bad press we don't need.


I don't think anyone expects to be able to divide an integer by an 
imaginary, and then assign it to an integer. I was astonished that the 
compiler accepted it.
Apr 30 2009
next sibling parent Georg Wrede <georg.wrede iki.fi> writes:
Don wrote:

 Georg Wrede wrote:

 Don wrote:

 D currently allows some conversions between complex/imaginary types 
 and real types, which are highly dubious.

 Given creal z, ireal y, these casts are legal:
 real x = cast(real)z;
 x = cast(real)y; // always sets x==0, regardless of the value of y.
 But I believe that should not be legal.

 For the first case, it should be written as:   real x = z.re;
 (which is shorter and clearer), and the second case is probably a 
 bug, and should be x = y.im; (unless the intention really was to set 
 x=0!).

 By the same logic, we could have sqrt(-1)==0, since the real part is 0.

 The most important effect of disallowing these casts would be to fix 
 a host of bugs and wierd behaviour. All the A op= B operations 
 involve a cast to A. If those nonsensical casts become illegal, the 
 nonsensical op= operations become illegal automatically.

 Eg, ireal y;
 y *= y; // mathematically nonsense, y*y is real, so can't be stored 
 in a pure imaginary type!

 There are a few segfault/ICE bugs (eg 718, 2839) which involve 
 int/=complex, an operation which never makes any sense anyway.

 I think we're just making problems for ourselves by allowing these 
 useless operations. I think they should be killed.
 Does anyone object? (If not, I'll create a patch to do it; it's not 
 very difficult).


 It might be a good idea to write in the docs why these operations are 
 missing. Even better would of course be if the user got a message 
 stating that this operation isn't implemented because it doesn't make 
 mathematical sense.


 
 I'll make sure the error messages are sensible.
 

 If the latter isn't done, then many people will just think there's 
 something wrong with the compiler/language. Bad press we don't need.


 
 I don't think anyone expects to be able to divide an integer by an 
 imaginary, and then assign it to an integer. I was astonished that the 
 compiler accepted it.


Just goes to show you the average math knowledge. I never would have 
thought of omitting it either. :-)
Apr 30 2009
prev sibling parent reply Walter Bright <newshound1 digitalmars.com> writes:
Don wrote:

 I don't think anyone expects to be able to divide an integer by an 
 imaginary, and then assign it to an integer. I was astonished that the 
 compiler accepted it.


There actually is a reason - completeness. Mathematically, there is a 
definite answer to it, so why not fill in all the entries for all the 
combinations?
May 03 2009
next sibling parent Bill Baxter <wbaxter gmail.com> writes:
On Sun, May 3, 2009 at 2:35 AM, Walter Bright
<newshound1 digitalmars.com> wrote:

 Don wrote:

 I don't think anyone expects to be able to divide an integer by an
 imaginary, and then assign it to an integer. I was astonished that the
 compiler accepted it.


 There actually is a reason - completeness. Mathematically, there is a
 definite answer to it, so why not fill in all the entries for all the
 combinations?


But the result of int divided by imaginary is pure imaginary.  So I
think Don's point was that you shouldn't be able to assign that back
to an int.

--bb
May 03 2009
prev sibling parent reply Don <nospam nospam.com> writes:
Walter Bright wrote:

 Don wrote:

 I don't think anyone expects to be able to divide an integer by an 
 imaginary, and then assign it to an integer. I was astonished that the 
 compiler accepted it.


 
 There actually is a reason - completeness. Mathematically, there is a 
 definite answer to it, so why not fill in all the entries for all the 
 combinations?


In the case complex = int/complex, there's no problem. It's the case int 
= int/complex that doesn't make sense.
And that's fundamentally because cast(real)(2 + 3i) doesn't have a 
definite answer.
There's no mathematical operation to convert a complex number to a real.
Normally, to get a real result, you need to do something like 
multiplying by the complex conjugate. You CAN take the real part of a 
complex number, and you can also take the modulus (which makes a lot of 
sense if you're using polar represantation).

Of course, cast() is not a mathematical operation, it's a D operation, 
so we can define it however we like. But defining it
cast(real)z = z.re;
is an arbitrary decision, which introduces lots of complexity to the 
language, and the many compiler bugs are a consequence.
It's confusing for newbies (there's been questions on D.learn asking, "I 
can get the real part of a complex number with cast(real), but how do I 
get the imaginary part? cast(ireal) gives me an ireal, not a real!").

Since D supports the very nice .re syntax, there's really no reason to 
define cast(real) at all. z.re is better in 100% of use cases. There are 
*NO* use cases for cast(real); any use of cast(real) is a bug. We should 
kill it.
May 04 2009
next sibling parent reply Frits van Bommel <fvbommel REMwOVExCAPSs.nl> writes:
Don wrote:

 In the case complex = int/complex, there's no problem. It's the case int 
 = int/complex that doesn't make sense.
 And that's fundamentally because cast(real)(2 + 3i) doesn't have a 
 definite answer.


Devil's advocate: one could argue it's the same as cast(int) of a float -- it 
returns the closest representable value, for some definition of "closest".
May 04 2009
next sibling parent Don <nospam nospam.com> writes:
Frits van Bommel wrote:

 Don wrote:

 In the case complex = int/complex, there's no problem. It's the case 
 int = int/complex that doesn't make sense.
 And that's fundamentally because cast(real)(2 + 3i) doesn't have a 
 definite answer.


 
 Devil's advocate: one could argue it's the same as cast(int) of a float 
 -- it returns the closest representable value, for some definition of 
 "closest".


Sure, you're at liberty to argue anything. But mathematicians don't do 
that. Nor should we. It's misleading, and just not helpful.
BTW, another equally reasonable and equally unhelpful way you could 
define cast(real)(x + iy) is as sqrt(x*x - y*y).

The most justifiable way to do it would be to say
cast(real)(x+iy) is:
  if (y==0) return x; else throw(CastError);
But again, it's just not helpful. Make it a compile-time error.
May 04 2009
prev sibling next sibling parent Derek Parnell <derek psych.ward> writes:
On Mon, 04 May 2009 10:56:41 +0200, Frits van Bommel wrote:


 Don wrote:

 In the case complex = int/complex, there's no problem. It's the case int 
 = int/complex that doesn't make sense.
 And that's fundamentally because cast(real)(2 + 3i) doesn't have a 
 definite answer.


 
 Devil's advocate: one could argue it's the same as cast(int) of a float -- it 
 returns the closest representable value, for some definition of "closest".


Hmmm. not quite because we can not do ...

   int i = floatnum.integer;

but we can do 

   real r = complexnum.re;

-- 
Derek Parnell
Melbourne, Australia
skype: derek.j.parnell
May 04 2009
prev sibling parent Jason House <jason.james.house gmail.com> writes:
Frits van Bommel Wrote:


 Don wrote:

 In the case complex = int/complex, there's no problem. It's the case int 
 = int/complex that doesn't make sense.
 And that's fundamentally because cast(real)(2 + 3i) doesn't have a 
 definite answer.


 
 Devil's advocate: one could argue it's the same as cast(int) of a float -- it 
 returns the closest representable value, for some definition of "closest".


I consider this to be a much different case. Assuming the float to an int
results on approximately the same number... And everyone knows what to expect.
It's also possible to convert back to float and still have approximately the
same number. 

For complex to int, the same things are not true. Conversion to float and then
back to real can give a much different number... And not just for overflow, but
normal uses of complex numbers. It's also unclear to mathematically inclined
folk like me and Don what the conversion is supposed to do. A magnitude makes
as much, if not more, sense.

Adding to all this, there are easier, shorter, and clearer ways to do all of
these conversions, and you _have_ to agree with Don ;)
May 04 2009
prev sibling next sibling parent bearophile <bearophileHUGS lycos.com> writes:
Don Wrote:

 Since D supports the very nice .re syntax, there's really no reason to 
 define cast(real) at all. z.re is better in 100% of use cases. There are 
 *NO* use cases for cast(real); any use of cast(real) is a bug. We should 
 kill it.


I am with you. Making the language tidy is the way to go :-)
Thank you for your work, and keep improving things when you can.

Bye,
bearophile
May 04 2009
prev sibling parent reply Georg Wrede <georg.wrede iki.fi> writes:
Don wrote:

 Walter Bright wrote:

 Don wrote:

 I don't think anyone expects to be able to divide an integer by an 
 imaginary, and then assign it to an integer. I was astonished that 
 the compiler accepted it.


 There actually is a reason - completeness. Mathematically, there is a 
 definite answer to it, so why not fill in all the entries for all the 
 combinations?


 
 In the case complex = int/complex, there's no problem. It's the case int 
 = int/complex that doesn't make sense.
 And that's fundamentally because cast(real)(2 + 3i) doesn't have a 
 definite answer.
 There's no mathematical operation to convert a complex number to a real.
 Normally, to get a real result, you need to do something like 
 multiplying by the complex conjugate. You CAN take the real part of a 
 complex number, and you can also take the modulus (which makes a lot of 
 sense if you're using polar represantation).
 
 Of course, cast() is not a mathematical operation, it's a D operation, 
 so we can define it however we like. But defining it
 cast(real)z = z.re;
 is an arbitrary decision, which introduces lots of complexity to the 
 language, and the many compiler bugs are a consequence.
 It's confusing for newbies (there's been questions on D.learn asking, "I 
 can get the real part of a complex number with cast(real), but how do I 
 get the imaginary part? cast(ireal) gives me an ireal, not a real!").
 
 Since D supports the very nice .re syntax, there's really no reason to 
 define cast(real) at all. z.re is better in 100% of use cases. There are 
 *NO* use cases for cast(real); any use of cast(real) is a bug. We should 
 kill it.


Walter, could the error message then include "Please use z.re or z.im to 
get the parts, instead of a cast." or something. Otherwise we have to 
explain to all users separately what's going on, /and/ that the omission 
of implementing the cast in D isn't an omission.
May 04 2009
parent reply Andrei Alexandrescu <SeeWebsiteForEmail erdani.org> writes:
Georg Wrede wrote:

 Walter, could the error message then include "Please use z.re or z.im to 
 get the parts, instead of a cast." or something. Otherwise we have to 
 explain to all users separately what's going on, /and/ that the omission 
 of implementing the cast in D isn't an omission.


If we cut the limb, why first apply an ointment to it?

Andrei
May 04 2009
parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Mon, 04 May 2009 09:44:18 -0400, Andrei Alexandrescu  
<SeeWebsiteForEmail erdani.org> wrote:


 Georg Wrede wrote:

 Walter, could the error message then include "Please use z.re or z.im  
 to get the parts, instead of a cast." or something. Otherwise we have  
 to explain to all users separately what's going on, /and/ that the  
 omission of implementing the cast in D isn't an omission.


 If we cut the limb, why first apply an ointment to it?


I think the point is relevant to D1, where you can't really remove the  
limb, but you can apply make-up to make it look less deformed ;)

-Steve
May 04 2009
prev sibling parent reply Andrei Alexandrescu <SeeWebsiteForEmail erdani.org> writes:
On 04/30/2009 02:20 PM, Don wrote:

 D currently allows some conversions between complex/imaginary types and
 real types, which are highly dubious.

 Given creal z, ireal y, these casts are legal:
 real x = cast(real)z;
 x = cast(real)y; // always sets x==0, regardless of the value of y.
 But I believe that should not be legal.

 For the first case, it should be written as: real x = z.re;
 (which is shorter and clearer), and the second case is probably a bug,
 and should be x = y.im; (unless the intention really was to set x=0!).

 By the same logic, we could have sqrt(-1)==0, since the real part is 0.

 The most important effect of disallowing these casts would be to fix a
 host of bugs and wierd behaviour. All the A op= B operations involve a
 cast to A. If those nonsensical casts become illegal, the nonsensical
 op= operations become illegal automatically.

 Eg, ireal y;
 y *= y; // mathematically nonsense, y*y is real, so can't be stored in a
 pure imaginary type!

 There are a few segfault/ICE bugs (eg 718, 2839) which involve
 int/=complex, an operation which never makes any sense anyway.

 I think we're just making problems for ourselves by allowing these
 useless operations. I think they should be killed.
 Does anyone object? (If not, I'll create a patch to do it; it's not very
 difficult).


Don, instead of this fix, could be at best phase everything built-in 
about complex out?

Thanks,

Andrei
Jun 12 2011
parent reply bearophile <bearophileHUGS lycos.com> writes:
Andrei:


 Don, instead of this fix, could be at best phase everything built-in 
 about complex out?


Two usages of complex numbers in the RosettaCode site.

Acommon place where you find complex numbers is to plot  Mandelbrot/Julia sets:
http://rosettacode.org/wiki/Mandelbrot_set#D

import std.stdio, std.math;

void main() {
    enum maxIter = 1000;
    foreach (y; -39 .. 39) {
        foreach (x; -39 .. 39) {
            auto c = y/40.0 - 0.5 + x/40.0i,
                 z = 0.0 + 0.0i,
                 i = 0;
            for (; i < maxIter && abs(z) < 4; i++)
                z = z ^^ 2 + c;

        }
        writeln();
    }
}



Version using std.complex:

import std.stdio, std.complex;

void main() {
    enum maxIter = 1000;
    foreach (y; -39 .. 39) {
        foreach (x; -39 .. 39) {
            auto c = Complex!double(y/40.0 - 0.5, x/40.0),
                 z = Complex!double(0, 0),
                 i = 0;
            for (; i < maxIter && z.abs() < 4; i++)
                z = z ^^ 2 + c;

        }
        writeln();
    }
}


I think it's worth adding to std.complex module few examples of usage, plus a
complex() function so you are allowed to write (the imaginary part defaults to
zero if the real part is set to zero):
auto z = complex(5);

Instead of:
auto z = Complex!double(5, 0);


complex(5) probably has to create a Complex!double and not a Complex!int (that
is not even supported, std.complex.Complex support floating point values only).

---------------------------

Another common usage of D complex numbers is to represent generic 2D points.
http://rosettacode.org/wiki/Constrained_random_points_on_a_circle#D

Original code (to plot some points in a circle):

import std.stdio, std.random, std.math;
 
void main() {
    char[31][31] table = ' ';
 
    foreach (i; 0 .. 100) {
        int x, y;
        do {
            x = uniform(-15, 16);
            y = uniform(-15, 16);
        } while(abs(12.5 - abs(x + y * 1i)) > 2.5);
        table[x + 15][y + 15] = '*';
    }
 
    foreach (row; table)
        writeln(row);
}


Version using std.complex:

import std.stdio, std.random, std.complex, std.math;

void main() {
    char[31][31] table = ' ';

    foreach (i; 0 .. 100) {
        int x, y;
        do {
            x = uniform(-15, 16);
            y = uniform(-15, 16);
        } while(abs(12.5 - Complex!double(x, y).abs) > 2.5);
        table[x + 15][y + 15] = '*';
    }

    foreach (row; table)
        writeln(row);
}

---------------------------

Regarding the printing of Complex values, this program:

import std.stdio, std.complex;
void main() {
    writeln(Complex!double(0, -1));
}


Prints:
0-1i


While Python prints the same complex number as:


 0-1j






-1j

Bye,
bearophile
Jun 12 2011
parent reply Robert Clipsham <robert octarineparrot.com> writes:
On 12/06/2011 23:37, bearophile wrote:

 Andrei:


 Don, instead of this fix, could be at best phase everything
 built-in about complex out?


 Two usages of complex numbers in the RosettaCode site.

 Acommon place where you find complex numbers is to plot
 Mandelbrot/Julia sets: http://rosettacode.org/wiki/Mandelbrot_set#D

 import std.stdio, std.math;

 void main() { enum maxIter = 1000; foreach (y; -39 .. 39) { foreach
 (x; -39 .. 39) { auto c = y/40.0 - 0.5 + x/40.0i, z = 0.0 + 0.0i, i =
 0; for (; i<  maxIter&&  abs(z)<  4; i++) z = z ^^ 2 + c; write(i ==




 Version using std.complex:

 import std.stdio, std.complex;

 void main() { enum maxIter = 1000; foreach (y; -39 .. 39) { foreach
 (x; -39 .. 39) { auto c = Complex!double(y/40.0 - 0.5, x/40.0), z =
 Complex!double(0, 0), i = 0; for (; i<  maxIter&&  z.abs()<  4; i++)



 I think it's worth adding to std.complex module few examples of
 usage, plus a complex() function so you are allowed to write (the
 imaginary part defaults to zero if the real part is set to zero):
 auto z = complex(5);


I seemed to think the plan for complex numbers was to do what happened 
with associative arrays, that is, keep the language syntax, but have the 
feature implemented in the library. Is this not the case?

-- 
Robert
http://octarineparrot.com/
Jun 13 2011
next sibling parent "Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:
On Mon, 13 Jun 2011 12:36:27 +0100, Robert Clipsham wrote:


 On 12/06/2011 23:37, bearophile wrote:

 Andrei:


 Don, instead of this fix, could be at best phase everything built-in
 about complex out?


 Two usages of complex numbers in the RosettaCode site.

 Acommon place where you find complex numbers is to plot
 Mandelbrot/Julia sets: http://rosettacode.org/wiki/Mandelbrot_set#D

 import std.stdio, std.math;

 void main() { enum maxIter = 1000; foreach (y; -39 .. 39) { foreach (x;
 -39 .. 39) { auto c = y/40.0 - 0.5 + x/40.0i, z = 0.0 + 0.0i, i = 0;
 for (; i<  maxIter&&  abs(z)<  4; i++) z = z ^^ 2 + c; write(i ==




 Version using std.complex:

 import std.stdio, std.complex;

 void main() { enum maxIter = 1000; foreach (y; -39 .. 39) { foreach (x;
 -39 .. 39) { auto c = Complex!double(y/40.0 - 0.5, x/40.0), z =
 Complex!double(0, 0), i = 0; for (; i<  maxIter&&  z.abs()<  4; i++) z



 I think it's worth adding to std.complex module few examples of usage,
 plus a complex() function so you are allowed to write (the imaginary
 part defaults to zero if the real part is set to zero): auto z =
 complex(5);


 
 I seemed to think the plan for complex numbers was to do what happened
 with associative arrays, that is, keep the language syntax, but have the
 feature implemented in the library. Is this not the case?


That was my understanding as well, which is why I never added a complex() 
helper function.

It is, however, my humble opinion that we should just get rid of the 
complex literals.  (Otherwise, std.complex would have to be moved into 
druntime.)

-Lars
Jun 13 2011
prev sibling parent reply Andrei Alexandrescu <SeeWebsiteForEmail erdani.org> writes:
On 6/13/11 6:36 AM, Robert Clipsham wrote:

 I seemed to think the plan for complex numbers was to do what happened
 with associative arrays, that is, keep the language syntax, but have the
 feature implemented in the library. Is this not the case?


No, the current vision is to completely replace complex with library 
artifacts. Walter wants to keep the "i" postfix as a hack, but I think 
that's completely unnecessary.

A complex() convenience function should be useful. If anyone has the 
time, please create a pull request. It should never automatically create 
Complex with an integral type, as bearophile mentioned. For the rare 
cases where complex is needed with integrals, it should be fine to use 
Complex!int etc. directly.


Andrei
Jun 13 2011
next sibling parent reply "Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:
On Mon, 13 Jun 2011 08:36:32 -0500, Andrei Alexandrescu wrote:


 On 6/13/11 6:36 AM, Robert Clipsham wrote:

 I seemed to think the plan for complex numbers was to do what happened
 with associative arrays, that is, keep the language syntax, but have
 the feature implemented in the library. Is this not the case?


 
 No, the current vision is to completely replace complex with library
 artifacts. Walter wants to keep the "i" postfix as a hack, but I think
 that's completely unnecessary.
 
 A complex() convenience function should be useful. If anyone has the
 time, please create a pull request. It should never automatically create
 Complex with an integral type, as bearophile mentioned. For the rare
 cases where complex is needed with integrals, it should be fine to use
 Complex!int etc. directly.


Sure, I'll add complex().

Regarding Complex!int:  I have explicitly restricted Complex to floating-
point types.  Here is a message I sent to the Phobos mailing list, 
explaining why:

    http://lists.puremagic.com/pipermail/phobos/2010-April/000286.html

See also Don's reply to that message, it really drives the point home:

    http://lists.puremagic.com/pipermail/phobos/2010-April/000287.html

-Lars
Jun 13 2011
parent "Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:
On Mon, 13 Jun 2011 14:51:52 +0000, Lars T. Kyllingstad wrote:


 On Mon, 13 Jun 2011 08:36:32 -0500, Andrei Alexandrescu wrote:
 

 A complex() convenience function should be useful. If anyone has the
 time, please create a pull request. [...]


 
 Sure, I'll add complex().


As promised:
https://github.com/D-Programming-Language/phobos/pull/103

-Lars
Jun 13 2011
prev sibling parent bearophile < bearophileHUGS lycos.com> writes:
Andrei:


 Walter wants to keep the "i" postfix as a hack, but I think 
 that's completely unnecessary.


Before deciding I'd like to know the rationale to keep it. In principle I like
handy complex number literals. In practice I don't use them often enough...

Another note: I'd like std.math.abs to perform what std.complex.Complex!T.abs
does. So I will be able to use it functionally (currently std.math.abs is able
to digest built-in complex numbers too):

alias Complex!double C;
C[] a = [C(1,2), C(2,4)];
auto m = map!abs(a);

Bye,
bearophile
Jun 13 2011