Re: Compiled Functions and Mathgroup Archive

*To*: mathgroup at smc.vnet.net*Subject*: [mg8935] Re: Compiled Functions and Mathgroup Archive*From*: Paul Abbott <paul at physics.uwa.edu.au>*Date*: Sat, 4 Oct 1997 22:08:12 -0400*Organization*: University of Western Australia*Sender*: owner-wri-mathgroup at wolfram.com

Peter Jay Salzman wrote: > 1 > - > I'm writing a numerical Schroedinger equation solver and stumbled on the > 'Compiled' function. The MMA book mentioned that sometimes functions run > faster when you don't compile them. It can be a lot faster to use MathLink to link to external compiled code. See, e.g., our second year quantum mechanics courseware at http://www.pd.uwa.edu.au/Physics/Courses/Second_Year/Quantum.html In addition you can use the InterCall Mathematica package to easily link in compiled code without needing to write MathLink template files. On MathSource <http://www.wolfram.com/mathsource/> you will find the following two listings: 0204-657: Schroedinger's Equation (March 1993) Author: Terry Robb Schroedinger's equation is numerically solved by calling an external subroutine from within Mathematica. By importing the Fortran subroutine (named SCHROED, which implements a Crank-Nicholson scheme) using InterCall, it is possible to do various experiments -- for example shooting a wave-packet at a potential barrier and watching an animation of the packet being mostly reflected but also partially tunnelling through the wall. The potential function V[x,t], required by the Fortran SCHROED routine, can be written as a Mathematica function which gives a lot of flexibility for interactive experimentation. This notebook demonstrates one such experiment. 0011: schroed.ma (772 Kb) 0204-679: Animating Schroedinger's Equation in Two Dimensions (April 1993) Author: Terry Robb An efficient method for solving parabolic partial differential equations is implemented in Mathematica using InterCall and an external C routine. As an application, the two-dimensional time-dependent Schroedinger's equation is solved for various initial conditions and potential functions. Four different numerical experiments are given: scattering of a particle from a cylindrical potential barrier; a double slit experiment; interaction of wave-packets; and stirring a wave-packet with a potential "stick". The resulting animations make excellant demonstrations of the properties of Schroedinger's equation. The technique used can also be applied to other similar parabolic partial differential equations. 0011: Schroed2D.ma (1.7 Mb) > I know that people must have asked this question before, but when should > I use Compile to generate faster code? My program obviously involes a > tremendous number of repetitive arithmetic operations, so it seems like I > should compile things like > > wavefunction[r_] = (alpha/Pi)^.75 E^(-.5 alpha r^2) > > or > > norm[wave] = Sqrt[ (4 Pi (dr)^3 Sum[ (Abs[ wave[[i+1]] ]*i)^2, > {i, 0, arsize-1} ] > > where arsize is the gridsize and will be on the order of thousands. I assume that you can use matrix (or vector) method instead of element-by-element computation? Using version 3.0 you can get very considerable speed-ups by compiling matrix operations. > 2 > - > Also, is there an archive of Mathgroup questions so I can look up > questions like this in the future? It's only useful if it's searchable. See http://smc.vnet.net/MathGroup.html and http://www.wolfram.com/cgi-bin/mathgroup/ However it is not searchable. I have Cc:d this to suggestions at wolfram.com so that they can consider making the archive searchable (which I agree would be _very_ useful). > 3 > - > How can I have MMA print something like: > The gridsize is arsize > but replace arsize with the contents of the variable arsize You can, of course, use Print. A better way is to use the built-in message command. After defining a string such as In[1]:= GridSize::size = "The gridsize is `1`."; then, for In[2]:= arsize = 0.2; get your code to call message, e.g., In[2]:= Message[GridSize::size,arsize] GridSize::"size": "The gridsize is 0.2." Have a look at Messages.m in SystemFiles Kernel TextResources English to complete see a list of the built-in messages. > 4 > - > I print a bunch of parameters for the current 'run', and would like to > prompt myself "If this is OK, press enter to continue but if it's not OK, > press any other key to end the program". You can use Input to do this: In[3]:= ?Input "Input[ ] interactively reads in one Mathematica expression. Input["prompt"] requests input, using the specified string as a prompt." Cheers, Paul ____________________________________________________________________ Paul Abbott Phone: +61-8-9380-2734 Department of Physics Fax: +61-8-9380-1014 The University of Western Australia Nedlands WA 6907 mailto:paul at physics.uwa.edu.au AUSTRALIA http://www.pd.uwa.edu.au/~paul God IS a weakly left-handed dice player ____________________________________________________________________

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