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digitalmars.D.learn - log for complex

reply J-S Caux <js gmail.com> writes:
Simple question: how do I get the log of a complex number?

If I try the simple
logtest = log(complex(1.0, 2.0))

I get the compiler error

Error: function core.stdc.math.log(double x) is not callable 
using argument types (Complex!double)

Some basic functions are described in 
https://dlang.org/phobos/std_complex.html, but not the log...
Mar 06 2018
parent reply Simen =?UTF-8?B?S2rDpnLDpXM=?= <simen.kjaras gmail.com> writes:
On Wednesday, 7 March 2018 at 07:42:37 UTC, J-S Caux wrote:

 Simple question: how do I get the log of a complex number?

 If I try the simple
 logtest = log(complex(1.0, 2.0))

 I get the compiler error

 Error: function core.stdc.math.log(double x) is not callable 
 using argument types (Complex!double)

 Some basic functions are described in 
 https://dlang.org/phobos/std_complex.html, but not the log...


That seems like an oversight. For now, this should work:

auto log(T)(Complex!T x) {
     import std.math : log;
     return Complex!T(log (abs(x)), arg(x));
}

--
   Simen
Mar 07 2018
parent reply J-S Caux <js gmail.com> writes:
On Wednesday, 7 March 2018 at 08:04:36 UTC, Simen Kjærås wrote:

 On Wednesday, 7 March 2018 at 07:42:37 UTC, J-S Caux wrote:

 Simple question: how do I get the log of a complex number?

 If I try the simple
 logtest = log(complex(1.0, 2.0))

 I get the compiler error

 Error: function core.stdc.math.log(double x) is not callable 
 using argument types (Complex!double)

 Some basic functions are described in 
 https://dlang.org/phobos/std_complex.html, but not the log...


 That seems like an oversight. For now, this should work:

 auto log(T)(Complex!T x) {
     import std.math : log;
     return Complex!T(log (abs(x)), arg(x));
 }

 --
   Simen


Yes indeed I can do this, but isn't this inefficient as compared 
to what it should be? The internals of arg are presumable already 
doing part of/all the necessary work.
Mar 07 2018
parent reply Simen =?UTF-8?B?S2rDpnLDpXM=?= <simen.kjaras gmail.com> writes:
On Wednesday, 7 March 2018 at 10:10:49 UTC, J-S Caux wrote:

 On Wednesday, 7 March 2018 at 08:04:36 UTC, Simen Kjærås wrote:

 auto log(T)(Complex!T x) {
     import std.math : log;
     return Complex!T(log(abs(x)), arg(x));
 }


 Yes indeed I can do this, but isn't this inefficient as 
 compared to what it should be? The internals of arg are 
 presumable already doing part of/all the necessary work.


arg is just atan2 in disguise, and doesn't do any logarithms. I 
mean, in a way it does, but it's really just a fpatan 
instruction. I don't expect there to be many optimization 
opportunities here.

--
   Simen
Mar 07 2018
parent reply J-S Caux <js gmail.com> writes:
On Wednesday, 7 March 2018 at 10:28:23 UTC, Simen Kjærås wrote:

 On Wednesday, 7 March 2018 at 10:10:49 UTC, J-S Caux wrote:

 On Wednesday, 7 March 2018 at 08:04:36 UTC, Simen Kjærås wrote:

 auto log(T)(Complex!T x) {
     import std.math : log;
     return Complex!T(log(abs(x)), arg(x));
 }


 Yes indeed I can do this, but isn't this inefficient as 
 compared to what it should be? The internals of arg are 
 presumable already doing part of/all the necessary work.


 arg is just atan2 in disguise, and doesn't do any logarithms. I 
 mean, in a way it does, but it's really just a fpatan 
 instruction. I don't expect there to be many optimization 
 opportunities here.

 --
   Simen


OK thanks Simen. Should your suggestion for repairing this 
omission be filed as a bugreport? I can of course implement it 
locally for myself, but the language should really provide this 
function directly.
Mar 07 2018
parent "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Wed, Mar 07, 2018 at 10:47:40AM +0000, J-S Caux via Digitalmars-d-learn
wrote:

 On Wednesday, 7 March 2018 at 10:28:23 UTC, Simen Kjærås wrote:

 On Wednesday, 7 March 2018 at 10:10:49 UTC, J-S Caux wrote:

 On Wednesday, 7 March 2018 at 08:04:36 UTC, Simen Kjærås wrote:

 auto log(T)(Complex!T x) {
     import std.math : log;
     return Complex!T(log(abs(x)), arg(x));
 }


 
 Yes indeed I can do this, but isn't this inefficient as compared
 to what it should be? The internals of arg are presumable already
 doing part of/all the necessary work.


 
 arg is just atan2 in disguise, and doesn't do any logarithms. I
 mean, in a way it does, but it's really just a fpatan instruction. I
 don't expect there to be many optimization opportunities here.




[...]

 OK thanks Simen. Should your suggestion for repairing this omission be
 filed as a bugreport? I can of course implement it locally for myself,
 but the language should really provide this function directly.


Yes, please file a bug.


T

-- 
The trouble with TCP jokes is that it's like hearing the same joke over and
over.
Mar 07 2018