Re: Compile a Module

*To*: mathgroup at smc.vnet.net*Subject*: [mg83197] Re: Compile a Module*From*: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>*Date*: Wed, 14 Nov 2007 04:54:07 -0500 (EST)*Organization*: The Open University, Milton Keynes, UK*References*: <fhbnuc$r4s$1@smc.vnet.net>

JOHN ERB wrote: > Can the following module be compiled? > It needs to be used 10^4 times in my program. > It is used to calculate the distance parallel and perpendicular > to the midpoint of 2D line segment with reference to a point of interest (calc). > > AlongAway[{tip_,end_,calc_}]:=Module[{mid,r,theta,along,away}, > mid=(tip+end)/2; > r=Sqrt[Dot[calc-mid,calc-mid]]; > theta=VectorAngle[calc-mid,end-mid]; > along=r Cos[theta]; > away=r Sin[theta]; > {along,away}] > > AlongAway[{{-1, 0}, {1, 0}, {2, 1}}] > (* answer is {2,1} *) > > (* the code below does not seem to work *) > cAlongAway=Compile[{{tip,_Real},{end,_Real},{calc,_Real}},AlongAway] John, I have made the following modifications to your code to get it compilable and to be sure it runs the compiled version when fed with floating-point-number arguments. The main argument of the function is declared as a tensor of real numbers of rank two (i.e. a real matrix). The arguments tip, end , and calc, are set up within the Module construct. Finally, we tell Mathematica that, in our case, the built-in function *VectorAngle* will always return a floating-point number (*VectorAngle can return infinite, arbitrary, or machine-size precision numbers depending on its arguments). In[1]:= cAlongAway = Compile[{{m, _Real, 2}}, Module[{mid, r, theta, along, away, tip = m[[1]], end = m[[2]], calc = m[[3]]}, mid = (tip + end)/2; r = Sqrt[Dot[calc - mid, calc - mid]]; theta = VectorAngle[calc - mid, end - mid]; along = r Cos[theta]; away = r Sin[theta]; {along, away}], {{VectorAngle[_, _], _Real}}]; cAlongAway[{{-1., 0.}, {1., 0.}, {2., 1.}}] Out[2]= {2., 1.} HTH, -- Jean-Marc