Re: expint: help with C++

[Daniel J Sebald]

On 08/17/2017 08:11 AM, Michele wrote:
> Dear all,
>
> I was thinking on move the Lentz algorithm of expint on a separate .cc
> file which will be called by expint.m. The problem is that I need it to
> work with the complex and I'm having some troubles when compiling. I
> attach the file and the log of the error.
>
> I would be glad if someone could help me.
>
> Best regards,

First error:

  CXX      libinterp/operators/libinterp_liboctinterp_la-op-b-sbm.lo
../octave/libinterp/corefcn/__expint_lentz_cmplx__.cc: In function 
'octave_value_list F__expint_lentz_cmplx__(const octave_value_list&, int)':
../octave/libinterp/corefcn/__expint_lentz_cmplx__.cc:48:36: error: no 
matching function for call to 'abs(octave_value)'
           while((std::abs(Delta - 1) > eps) & (m < maxit))
                                    ^
../octave/libinterp/corefcn/__expint_lentz_cmplx__.cc:48:36: note: 
candidates are:
In file included from /usr/include/c++/4.8/cstdlib:72:0,
                 from /usr/include/c++/4.8/ext/string_conversions.h:41,
                 from /usr/include/c++/4.8/bits/basic_string.h:2815,
                 from /usr/include/c++/4.8/string:52,
                 from ../octave/libinterp/corefcn/defun-int.h:28,
                 from ../octave/libinterp/corefcn/defun.h:32,
                 from 
../octave/libinterp/corefcn/__expint_lentz_cmplx__.cc:23:
/usr/include/stdlib.h:775:12: note: int abs(int)
 extern int abs (int __x) __THROW __attribute__ ((__const__)) __wur;
            ^
/usr/include/stdlib.h:775:12: note:   no known conversion for argument 1 
from 'octave_value' to 'int'
In file included from /usr/include/c++/4.8/ext/string_conversions.h:41:0,
                 from /usr/include/c++/4.8/bits/basic_string.h:2815,
                 from /usr/include/c++/4.8/string:52,
                 from ../octave/libinterp/corefcn/defun-int.h:28,
                 from ../octave/libinterp/corefcn/defun.h:32,
                 from 
../octave/libinterp/corefcn/__expint_lentz_cmplx__.cc:23:
/usr/include/c++/4.8/cstdlib:166:3: note: long int std::abs(long int)
ETC.

The compiler thinks that std::abs() function takes an int because it 
doesn't understand what the the argument Delta - 1 is.  The standard 
library has other input types for std::abs(), but the compiler is just 
picking the 'int' input variant to complain about because it has to 
choose one.  So, you must supply std::abs() with a variable type that 
the compiler knows.  It looks like you are trying to use the variant 
that has an input of std::complex

http://en.cppreference.com/w/cpp/numeric/complex/abs

However, even though

          std::complex<float> Delta = 0;

is std::complex, the compiler probably doesn't understand what "Delta - 
1" is because there probably is not automatic promotion from 1 to 
std::complex...I'm not sure.  So, perhaps you need

  Delta - complex <float> (1.0, 0.0)

or something similar (maybe complex <int> (1,0) will work, don't know, 
I'll let you experiment).  You might want to define something like

std::complex<float> cmplx1 = complex <float> (1.0, 0.0);

to reduce code bulkiness, up to you.

Note there may be another issue once you solve that, i.e.,:

(std::abs(Delta - 1) > eps)

will not work because eps is declared std::complex, i.e.,

          static const std::complex<float> eps = 
std::numeric_limits<float>::epsilon();

But complex numbers are not an ordered field.  The abs(complex<T>) 
template returns class T, so you will be getting a float back.  Rather 
than cast ::epsilon() to a complex, you can define it as a float or 
simply use ::epsilon() directly.

It's hard to describe things without having a compiler/code at hand, but 
that's the general idea.  The other errors in your report seem to fall 
along that same line.

Dan