180

I did a very simple (in fact over-simple) snowflake simulator with CellularAutomaton years before. It's based on the hexagonal grid: and range-1 rules: Initial code First we'll need some functions to display our snowflakes: Clear[vertexFunc] vertexFunc = Compile[{{para, _Real, 1}}, Module[{center, ratio}, center = para[[1 ;; 2]]; ratio = para[[...


74

========== update =========== Remember guys how we can cut out a snowflake from a sheet of paper carving 12th folded part? Like the image below. So I decided to write an app to imitate the process. It also can be used to make random snowflakes (similar to to @bill s' but with reflection to imitate real cutting paper process and reflective symmetry of ...


66

The only reason I am attempting to answer this is to perhaps get a Reversal badge. There you go... We will go slowly and this answer is the basis for what comes next. Let's start with two dimensions. You'll see why. We create a rectangular region: Needs["NDSolve`FEM`"] mesh = ToElementMesh[FullRegion[2], {{0, 5}, {0, 1}}, "MeshOrder" -> 1, "...


55

After correcting the syntax errors in the original code, the actual question can be addressed: How to display the four variables x1[t]...y2[t] as an animation in a way that conveys their meaning? The basic idea is to use ListAnimate on a list of frames that I define below: Clear[phi1, phi2, t]; sol = First[ NDSolve[{2*phi1''[t] + phi2''[t]*Cos[phi1[t] - ...


44

To answer your question: I don't think it's a bad or good idea to use If. It depends on how you do it. To demonstrate I'll use If combined very powerfully with Mathematica 10's ability to tell if a point is inside a specified region or not. step[position_, region_] := Module[{randomStep}, randomStep = RandomChoice[{{-1, 0}, {1, 0}, {0, -1}, {0, 1}}]; If[...


40

Update June 2015 Here is an updated version of the program. I've made it compatible with newer Mathematica versions (AstronomicalData returns Quantity structures in newer versions, which wrangled calculations). It should now work on versions 8 through 10. Let me know if it doesn't. I added animation and simplified the presentation (no tooltips in the ...


38

The power of Mathematica's syntax allows us to create dice in several different ways. Here's one way that I like: dice[n_Integer] := dice[n, Black] Format[dice[n_Integer, c_]] := With[{ dots = {1 -> {5}, 2 -> {3, 7}, 3 -> {3, 5, 7}, 4 -> {1, 3, 7, 9}, 5 -> {1, 3, 5, 7, 9}, 6 -> {1, 2, 3, 7, 8, 9}} /. l : {...


38

Usage Just use this function with any polyhedron in in form: GraphicsComplex[pts_, Polygon[vertices_, ___]]. When I find time and motivation maybe I will add more DownValues so it can be more general. At the moment you can play with solids given by PolyhedronData[... "Faces"]: polyhedronRandomWalk[ PolyhedronData["DuerersSolid", "Faces"] ] It should ...


38

DynamicModule[{t = 0, d = 5, a = .08, base, distortion, pts, r, f, n = 10}, r[y_] := .08 y^4; f[x_] := -2 Pi Dynamic[t] + d x; (*f does not evaluate to a number but FE will take care of that later*) base = Array[List, n {3, 1}, {{0, Pi}, {0, 1}} ]; distortion = Array[ Function[{x, y}, r[y] {Cos @ f @ x, Sin @ f @ x}], n {3, 1}, {{0, Pi}, {0, 1}} ...


37

If it is at all an option to represent the grid as a 2D list instead of a list of infected coordinates, I would model this is a cellular automaton. What you've essentially got is an outer totalistic cellular automaton with a von Neumann neighbourhood. The rule in Game-of-Life notation is B234/S01234, i.e. a cell comes to life if it has two or more live ...


36

Edit V10! This is simple example what we can now do in real time! R = RegionUnion @@ Table[Disk[{Cos[i], Sin[i]}, .4], {i, 0, 2 Pi, Pi/6.}]; R2 = RegionBoundary@DiscretizeRegion@R; go[] := (While[r > .105, x += v; r = RegionDistance[R2, x]; Pause[.01]]; bounce[];) bounce[] := With[{normal = Normalize[x - RegionNearest[R2, x]]}, If[break, Abort[]]; ...


33

Yes you can. Below is a fairly general, Mathematica-compiled, fast and robust version. Examples 1. Michaelis-Menten kinetics Michaelis-Menten kinetics for enzyme-directed substrate conversion. The enzyme (e) converts the susbtrate (s) through an enzyme-substrate complex (c) to the product (p). For comparison, I've included the deterministic ODE system ...


32

We have unicode support so we can use the following strings: {"⚀", "⚁", "⚂", "⚃", "⚄", "⚅"}: dice = FromCharacterCode /@ Range[9856, 9856 + 5]; Grid[Partition[RandomInteger[{1, 6}, {50, 2}], 5] /. { i : {__Integer} :> Style[ Row[dice[[i]], Spacer[1]], {Large, Total[i] /. {7 -> Red, _ -> Black}}]} , Frame -> All]


31

This was a fun question to answer, even considering that I know nothing about general relativity. It's all a matter of translating the equations presented in this paper by Oliver James, Eugenie von Tunzelmann, Paul Franklin, and Kip Thorne into notebook expressions. Embedding Diagrams The paper gives some really cool figures to show the curvature of 4-...


29

Here is a simple method that begins with an $n$-sided polygon (defined by the $n$ points in tab), then rotates the polygon and superimposes it six times to achieve the six-fold symmetry. The makeFlake function is: makeFlake[n_] := Module[{tab, rot}, tab = RandomReal[{-1/2, 1/2}, {n, 2}]; rot = RotationMatrix[Pi/3]; Graphics[{Hue[RandomReal[]], ...


28

Update: 2/7/2019 I have just released a new version of the package: MathematicaStan v2.0 I just have released a beta version of MathematicaStan, a package to interact with CmdStan. https://github.com/vincent-picaud/MathematicaStan Usage example: (* Defines the working directory and loads CmdStan.m *) SetDirectory["~/GitHub/MathematicaStan/Examples/...


27

This is my port of the Processing code that you referenced. It doesn't try to optimize, so I didn't try it either, for example I didn't use Nearest to find collisions even though that would be fast since it uses a quadtree. I found that I didn't need those optimizations to recreate the animation on the previously linked to website. About your questions: 1) ...


26

After importing a free dice 3D model {pd, vd} = Import["c:\\dice.stl", #] & /@ {"PolygonData", "VertexData"}; g2 = Translate[GraphicsComplex[vd, Polygon /@ pd], {-10, -37.5, -10}]; rv = {{0, 0, -1}, {0, -1, 0}, {0, 0, 1}, {0, 1, 0}, {-1, 0, 0}, {1, 0, 0}}; dice[x_List, n_Integer] := Rasterize@(Graphics3D[{EdgeForm[None], Blue, Rotate[g2,{{0, 0, 1},...


26

BernoulliDistribution is a perfect fit for this. RandomVariate[BernoulliDistribution[1 - 0.1], {50}] {1, 1, 1, 0, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1} Also, as kguler states, you can use RandomChoice, but the benefit of BernoulliDistribution is that you ...


25

Not so much snowflakes as random artworks with the same symmetry as snowflakes, but I wanted to join in the festive fun! These are generated with a "randomart" package I wrote a while ago (code at the bottom of the answer). It uses a kind of non-linear iterated function system to generate random images. Here's a grid of random images with snowflake symmetry:...


24

The programming style you are using is not very fitting for Mathematica. Here's a better way (shorter, much faster): n = 1000000; (* number of points to use *) octantVolume = N[ Total@UnitStep[1 - Norm /@ RandomReal[1, {n, 3}]]/n ] The reason why you get the error you mention is that for some x, y, the expression 1 - x^2 - y^2 is negative, thus its square ...


24

For the sake of completeness let me advertise someone else's code which implements MCMC in mathematica. Josh Burkart has implemented Mathematica Markov Chain Monte Carlo which is available on github. His read-me reads: Mathematica Markov Chain Monte Carlo Mathematica package containing a general-purpose Markov chain Monte Carlo routine Josh Burkart ...


23

You are trying to implement Euler-Maruyama simulation method for a 2-stage short-term interest rate model which is given by the following system of SDEs: $$\begin{eqnarray} \mathrm{d} \theta_t &=& -\lambda_\theta \left( \theta_t - \bar\theta\right) \mathrm{d}t + \sigma_\theta \mathrm{d}W_{\theta,t} \\ \mathrm{d} \pi_t &=& -\lambda_\pi\...


23

Thanks to J.M. drop = SetAlphaChannel[#, ColorNegate@#] &@ Binarize@Rasterize@ ParametricPlot[{r Cos[t] (1 - Sin[t]), -3 + r (5/2 (Sin[t] - 1) + 3)}, {t, 0, 2 Pi}, {r, 0, 1}, BoundaryStyle -> None, Axes -> False, Frame -> False] Something to start with: circle = Table[ Translate[ Point[{##, 0} & @@@ CirclePoints[...


23

data generates n balls, here: 10 Note that it might be wise to make the box larger, if different WhenEvent[]s happen at the same time (both box and ball collision), one can overwrite the other. data = Table[{RandomReal[{-0.85, 0.85}, {3}], RandomReal[{-1, 1}, {3}], RGBColor[RandomReal[{-1, 1}], RandomReal[{-1, 1}], RandomReal[{-1, 1}]]}, {i, 10}]; I'...


23

You can use ListConvolve to simulate a single diffusion time step and build a simulation out of that. I'll show a simple example: Let's say we start with simple initial conditions like in your example (initialconditions = Normal@SparseArray[{{3, 3} -> 1}, {5, 5}]) // MatrixForm and a diffusion kernel kernel = { {1/120, 1/60, 1/120}, {1/60, 9/10, 1/...


21

Well I guess one more couldn't hurt. Using an iterated matrix-replacement scheme and some fancy opacity: powzerz = 2; width = 550; primitive = Scale[Cuboid[], 0.99999]; matrix0 = {{{1}}}; matrixT = CrossMatrix[{1, 1, 1}]; rules = {0 -> (0 #1 &), 1 -> (#1 &)}; iterate[matrix0_, matrixT_, rules_, power_] := Nest[Function[prev, ...


21

The printed version of the 2002 edition was printed 3 times and sold out 3 times; Springer and Google recently started selling it (book only) as a PDF eBook (no software) on the Springer and Google sites for $79. I know other authors (e.g. here) have gone to some trouble to make their books available here on stack exchange ... We are delighted to be able ...


20

This answer is going to be a bit of a sprawl. Please read on. I am going to present several methods of simulation, hopefully in increasing order of performance. Method 1 We can carry out the filling of seats, at least as I understand the puzzle, quite literally like this: fillseats[seats_List] := ReplacePart[seats, {{1}, {2}} + RandomChoice @ ...


20

My goal was quite ambitious. I wanted to create a way to let any rigid body bounce elastically against any other surface in MMA V9. To do this I use "masks" for the object and the environment. These masks are black and white images. White indicates that this is where the object/surface is, black is empty space. I can calculate the overlap between the object ...


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