*From*: Dave Benson <daveb evolution com>*To*: Owen Taylor <otaylor redhat com>*Cc*: drlion-dated-1091219837 02c327 teepee ath cx, Jan Kratochvil <lace jankratochvil net>, gtk-devel-list gnome org, Sven Neumann <sven gimp org>*Subject*: Re: MATH_MOD: Include in GLib?*Date*: Mon, 26 Jul 2004 08:23:54 -0700

On Mon, Jul 26, 2004 at 11:17:48AM -0400, Owen Taylor wrote: > On Mon, 2004-07-26 at 06:41, Sven Neumann wrote: > > Hi, > > > > Jan Kratochvil <lace jankratochvil net> writes: > > > > > On Mon, 26 Jul 2004 10:43:21 +0200, Sven Neumann wrote: > > > > Daniel Brockman <drlion deepwood net> writes: > > > ... > > > > > #define MOD(x, m) ((x) >= 0 ? (x) % (m) : (m) + (x) % (m)) > > > ... > > > > If at all it would have to be G_MOD(x). But I doubt that > > > > the semantics of such a macro are obvious enough and that it would be > > > > of general usefulness. > > > > > > I consider this macro as a generally used workaround of a bug in C standard. > > > I intuitively expect the result of "x%m" will be 0..(m-1), not the C result > > > of -(m-1)..(m-1). It has similiar position as G_N_ELEMENTS(). > > > > The result of "x%m" will be 0..(m-1) provided that you respect the > > fact that the modulo operator is undefined on negative operands. The > > behaviour is machine-dependent and you should simply avoid to use > > modulo with signed variables. > > Are you sure about this? In my understanding, the C standard defines > division as truncation towards zero. And then defines: > > a % b == a - b * (a / b) it's a c99-ism. quoting http://home.tiscalinet.ch/t_wolf/tw/c/c9x_changes.html The integer division and modulus operators are defined to perform truncation towards zero. (In C89, it was implementation-defined whether truncation was done towards zero or -infinity. This is (obviously) important only if one or both operands are negative. Consider: -22 / 7 = -3 -22 % 7 = -1 truncation towards zero -22 / 7 = -4 -22 % 7 = 6 truncation towards -infinity Both satisfy the required equation (a/b)*b + a%b == a. The second has the advantage that the modulus is always positive -- but they decided on the other (more Fortran-like, less Pascal-like) variant...)

**References**:**MATH_MOD: Include in GLib?***From:*Daniel Brockman

**Re: MATH_MOD: Include in GLib?***From:*Sven Neumann

**Re: MATH_MOD: Include in GLib?***From:*Jan Kratochvil

**Re: MATH_MOD: Include in GLib?***From:*Sven Neumann

**Re: MATH_MOD: Include in GLib?***From:*Owen Taylor

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