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digitalmars.D - Could we have mod in std.math?

reply Caligo <iteronvexor gmail.com> writes:
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1.
The % operator, just like in C/C++, calculates the remainder, but it
doesn't handle negative numbers properly.  It's not a mod operator, even
though sometimes it's called that.

  assert(-6 %  20 =3D=3D -6);
  assert( 6 % -20 =3D=3D  6);
  assert(-6 % -20 =3D=3D -6);

I use my own mod function whenever I need to handle negative numbers.  It
looks like this:

pure T mod(T)(T n, T d) if(isIntegral!(T)){
  T r =3D n % d;
  return sgn(r) =3D=3D -(sgn(d)) ? r + d : r;
}

  assert(mod(-6,  20) =3D=3D  14);
  assert(mod( 6, -20) =3D=3D -14);
  assert(mod(-6, -20) =3D=3D -6);

I'm hoping to see something like the above mod function in Phobos someday.
And perhapse a 'rem' or 'remainder' function that's a wrapper for the %
operator, just to stay consistent.

2.
With the above, the math.fmod then would have to be renamed to 'frem'
because, just like the % for integrals, it doesn't handle negative numbers
properly only calculates the remainder.

  assert(fmod(-6,  20) =3D=3D -6);
  assert(fmod( 6, -20) =3D=3D  6);
  assert(fmod(-6, -20) =3D=3D -6);

I'm not so sure why we have 're=ADmain=ADder` and `remquo` in std.math when
there is 'fmod`, though.

What do you guys think?

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1.<br>The % operator, just like in C/C++, calculates the remainder, but it =
doesn&#39;t handle negative numbers properly.=A0 It&#39;s not a mod operato=
r, even though sometimes it&#39;s called that.<br><br>=A0 assert(-6 %=A0 20=
 =3D=3D -6);<br>
=A0 assert( 6 % -20 =3D=3D=A0 6);<br>=A0 assert(-6 % -20 =3D=3D -6);<br><br=
I use my own mod function whenever I need to handle negative numbers.=A0 I=

=A0 T r =3D n % d;<br> =A0 return sgn(r) =3D=3D -(sgn(d)) ? r + d : r;<br>}<br><br>=A0 assert(mod(= -6,=A0 20) =3D=3D=A0 14);<br>=A0 assert(mod( 6, -20) =3D=3D -14);<br>=A0 as= sert(mod(-6, -20) =3D=3D -6);<br><br>I&#39;m hoping to see something like t= he above mod function in Phobos someday.=A0 And perhapse a &#39;rem&#39; or= &#39;remainder&#39; function that&#39;s a wrapper for the % operator, just= to stay consistent.<br> <br>2.<br>With the above, the math.fmod then would have to be renamed to &#= 39;frem&#39; because, just like the % for integrals, it doesn&#39;t handle = negative numbers properly only calculates the remainder.<br><br>=A0 assert(= fmod(-6,=A0 20) =3D=3D -6);<br> =A0 assert(fmod( 6, -20) =3D=3D=A0 6);<br>=A0 assert(fmod(-6, -20) =3D=3D -= 6);<br><br>I&#39;m not so sure why we have &#39;<span class=3D"ddoc_psymbol= ">re=ADmain=ADder` and `</span><a name=3D"remquo"></a><span class=3D"ddoc_p= symbol">remquo` in std.math when there is &#39;fmod`, though.<br> <br>What do you guys think?<br></span> --14dae9ccd4ecf67d0004b4940264--
Dec 20 2011
next sibling parent Don <nospam nospam.com> writes:
On 21.12.2011 07:08, Caligo wrote:
 1.
 The % operator, just like in C/C++, calculates the remainder, but it
 doesn't handle negative numbers properly.  It's not a mod operator, even
 though sometimes it's called that.

    assert(-6 %  20 == -6);
    assert( 6 % -20 ==  6);
    assert(-6 % -20 == -6);

 I use my own mod function whenever I need to handle negative numbers.
 It looks like this:

 pure T mod(T)(T n, T d) if(isIntegral!(T)){
    T r = n % d;
    return sgn(r) == -(sgn(d)) ? r + d : r;
 }

    assert(mod(-6,  20) ==  14);
    assert(mod( 6, -20) == -14);
    assert(mod(-6, -20) == -6);

 I'm hoping to see something like the above mod function in Phobos
 someday.  And perhapse a 'rem' or 'remainder' function that's a wrapper
 for the % operator, just to stay consistent.

 2.
 With the above, the math.fmod then would have to be renamed to 'frem'
 because, just like the % for integrals, it doesn't handle negative
 numbers properly only calculates the remainder.

    assert(fmod(-6,  20) == -6);
    assert(fmod( 6, -20) ==  6);
    assert(fmod(-6, -20) == -6);

 I'm not so sure why we have 're­main­der` and `remquo` in std.math when
 there is 'fmod`, though.

 What do you guys think?

Those names and semantics all come from C. We can't change them without an extremely good reason. Note that things are a bit more complicated than you describe -- there is also the IEEE754 remainder function, which is really wierd. A mod() function might not be a bad idea.
Dec 20 2011
prev sibling next sibling parent Stewart Gordon <smjg_1998 yahoo.com> writes:
On 21/12/2011 06:08, Caligo wrote:
<snip>
    assert(mod(-6,  20) ==  14);
    assert(mod( 6, -20) == -14);
    assert(mod(-6, -20) == -6);

 I'm hoping to see something like the above mod function in Phobos someday. 
And perhapse a
 'rem' or 'remainder' function that's a wrapper for the % operator, just to
stay consistent.

And a floor-divide along the same lines. See also http://www.digitalmars.com/d/archives/digitalmars/D/13125.html Stewart.
Dec 21 2011
prev sibling parent reply bearophile <bearophileHUGS lycos.com> writes:
Caligo:

 I'm hoping to see something like the above mod function in Phobos someday.

I'd like that, for both int/long/BigInts. And I'd like some functions for BigInts: - divmod: returns both % and div and %, but it's more efficient if you need both values; - gcd - pow: with a third optional argument (like in Python pow built-in) to quickly compute (x ^^ y) % z. Bye, bearophile
Dec 24 2011
parent reply Don <nospam nospam.com> writes:
On 24.12.2011 22:12, bearophile wrote:
 Caligo:

 I'm hoping to see something like the above mod function in Phobos someday.

I'd like that, for both int/long/BigInts. And I'd like some functions for BigInts: - divmod: returns both % and div and %, but it's more efficient if you need both values; - gcd

These should both exist for int and long as well.
 - pow: with a third optional argument (like in Python pow built-in) to quickly
compute (x ^^ y) % z.

 Bye,
 bearophile

Dec 25 2011
parent Walter Bright <newshound2 digitalmars.com> writes:
On 12/25/2011 10:57 PM, Don wrote:
 On 24.12.2011 22:12, bearophile wrote:
 Caligo:

 I'm hoping to see something like the above mod function in Phobos someday.

I'd like that, for both int/long/BigInts. And I'd like some functions for BigInts: - divmod: returns both % and div and %, but it's more efficient if you need both values; - gcd

These should both exist for int and long as well.

Consider the following program: int div(int a, int b) { auto x = a / b; auto y = a % b; return x + y; } Compile with: dmd -c test -O Disassemble with obj2asm: _D4test3divFiiZi comdat push EAX mov EAX,8[ESP] cdq idiv [ESP] add EAX,EDX pop ECX ret 4
Dec 25 2011