# Tag Info

31

Let you have a function and an initial point f[x_] := Cos[x] x0 = 0.2; Then you can calculate a sequence seq = NestList[f, x0, 10] (* {0.2, 0.980067, 0.556967, 0.848862, 0.660838, 0.789478, \ 0.704216, 0.76212, 0.723374, 0.749577, 0.731977} *) and vizualize it with a so-called Cobweb plot p = Join @@ ({{#, #}, {##}} & @@@ Partition[seq, 2, 1]); ...

29

First let me note that I didn't write PositionIndex, so I can't speak to its internals without doing a bit of digging (which at the moment I do not have time to do). I agree performance could be improved in the case where there are many collisions. Let's quantify how bad the situation is, especially since complexity was mentioned! We'll use the ...

24

Summary We can look at the code of DeleteDuplicatesBy and it turns out it uses GroupBy. The test cases proposed by Mr.Wizard are all handled by some part of the code of DeleteDuplicatesBy. Other parts of this code also seem to have some issues. Most of the members of the *By family of functions seem to have side effects. How DeleteDuplicatesBy works It ...

24

The following is based on the fact that the determinant of a matrix is equal to zero when two rows are the same. Thus, if you plug any of the points in, you get a true statement. SeedRandom[3]; pts = RandomReal[{-1, 1}, {5, 2}]; row[{x_, y_}] := {1, x, y, x*y, x^2, y^2}; eq = Det[Prepend[row /@ pts, row[{x, y}]]] == 0 (* Out: ...

22

test = {5, 6, 9, 3, 2, 6, 7, 8, 1, 1, 4, 7} MaxFilter[test, 1] (* {6, 9, 9, 9, 6, 7, 8, 8, 8, 4, 7, 7} *) You can also use Max /@ Transpose[{Rest[Append[#, 0]], #, Most[Prepend[#, 0]]}] &[yourList] which is competitive with the MM MaxFilter, but will allow you to change the 'slide' (e.g.pad with zeroes, or other arbitrary 'start').

21

Diagnosis Spelunking the definition of Commonest, which is written in top-level Mathematica code, I see that the two parameter form is handled by this internal function: Commonest; (* preload *) ? StatisticsDescriptiveDumpoCommonestSetLength oCommonestSetLength[list_, n_] := Catch[Block[{res, reslen, ord}, res = Tally[list]; reslen = Length[res]; ...

19

One way is to use an extra argument that acts as a switch. Clear[f]; f[0] = 1; f[1] = 1; f[n_, True] := f[n - 1] + f[n - 2] Example: f7 = f[7, True] (* Out[329]= f[5] + f[6] *) To proceed another step, can do a replacement. f7 /. f[aa_] :> f[aa, True] (* Out[330]= f[3] + 2 f[4] + f[5] *) Can use Nest to repeat this n times. Nest[# /. f[aa_] ...

19

Max[StringLength@Names["System*"]] 38 Select[ Names["System*"], 38 == StringLength[#] &] {"MultivariateHypergeometricDistribution"} As far as I can say there is no limit for lengths of symbol names, besides that of the memory limitation.

17

I see no mention of the new-in-10 PositionIndex in the other answers, which takes a list (or association) of values and returns a 'reverse lookup' that maps from values in the list to the positions where they occur: In[1]:= index = PositionIndex[{a, b, c, a, c, a}] Out[1]= <|a -> {1, 4, 6}, b -> {2}, c -> {3, 5}|> It doesn't take a level ...

16

Looking at the Trace of one which does work: x = Sin[Pi/5] (* Sqrt[5/8 - Sqrt[5]/8] *) Trace[ArcSin[x], TraceInternal -> True] It appears that Mathematica computes the ArcSin numerically and then recognises the result, 0.628319 as possibly equal to Pi/5. To check it computes Sin[Pi/5], and subtracts it from the original argument to see if it gets ...

16

Maximize[{y, x^2 + y^2 == (2 x^2 + 2 y^2 - x)^2}, {x, y}] {(3 Sqrt[3])/8, {x -> 3/8, y -> (3 Sqrt[3])/8}}

16

Attempting to analyze the performance of this function in the manner that Taliesin Beynon did for PositionIndex I shall use the same tools. The old method that will be compared in all cases below: myDeDupeBy[x_, f_] := GatherBy[x, f][[All, 1]] Speed A BenchmarkPlot of DeleteDuplicatesBy versus myDeDupeBy: Needs["GeneralUtilities"] BenchmarkPlot[ ...

16

This is competitive with Mr Wizards code and seems faster in some cases: fn2[x_Integer, n_Integer] := Module[{y = Range @ x}, (y[[# ;; x ;; #]] = 0) & /@ Prime[Range @ Min[n, PrimePi @ x]]; SparseArray[y]["NonzeroValues"]] AbsoluteTiming[fn[10000, 1223];] (* {0.004000, Null} *) AbsoluteTiming[fn2[10000, 1223];] (* {0.010001, Null} *) ...

15

I'm guessing you're coming from a programming language where every expression must evaluate to a value, and if it didn't evaluate to something (like 5[Cos+Sin]), it's a syntax error. To me, Mathematica started to make a lot more sense, once I stopped thinking about functions and values, and started to think of every expression as evaluating to an "expression ...

15

Multiple versions of Mathematica can co-exist on a computer without problems. The best approach is to have both version 10 and version 9 installed simultaneously until you are confident that all your critical code work with 10. When using multiple versions it is useful to go to Preferences -> System and check "Create and maintain version specific front end ...

15

I believe this is correct, and very fast: fn[x_Integer, n_Integer] := Complement[Range @ x, Join @@ Range[#, x, #]] & @ Prime @ Range @ n Test: fn[10000, 1223] {1, 9929, 9931, 9941, 9949, 9967, 9973} It seems I am a bit late to return to this problem and Simon Woods already provided a memory optimized approach. His sieve is comparatively ...

15

Let me first answer your second question, since I can only guess about the main question: I also observed that the syntax colouring (version 10, windows 7) suggests that Trace can be used with only two arguments. It's really just the coloring that goes wrong and has nothing to do with functionality. You can see that it is not even related to ...

14

It depends on what you want to do with result, but you could try like this: Function[{x}, {x, #}] /@ {1, 2} {{1, #1}, {2, #1}}

14

The term pure function used in Mathematica is not being used in the same sense as the cited Wikipedia article. In Mathematica it refers to an anonymous function. In the Wikipedia article it is a term extracted by analogy from the increasingly popular term "purely functional" which refers (mainly) to deterministic programming free of side-effects. The ...

13

Let's start by taking a look at the compiled form of one of our queries: DatasetCompileQuery[Query @ First @ spans] (* DatasetWithOverrides@*Checked[Slice[205 ;; 313], Identity] *) We can see that the operation is not implemented directly in terms of part. Indeed, there are three components: DatasetWithOverrides, GeneralUtilities`Checked and ...

13

The separation-of-variables solution you quoted has two indices appearing in it: n and j (the subscripts of the coefficient $A_{nj}$). Here, n is azimuthal mode order, i.e. it counts the number of nodes along the direction in which the polar-angle $\theta$ varies (divided by 2). The index j is needed because the wave is supposed to satisfy the boundary ...

13

For full ranges There is the function DayRange that can be used for this purpose, but not in the same simple way like CharacterRange. For the days: DayName /@ DayRange[Today, Today ~DatePlus~ {{1, "Week"}, {-1, "Day"}}] {Wednesday, Thursday, Friday, Saturday, Sunday, Monday, Tuesday} For the months: DateValue[#, "MonthName"] & /@ DayRange[Today, ...

13

Your question is not well specified for several reasons: Pure functions accept a flexible number of arguments #1 + #2 &[a, b, c, d] a + b It is common for some arguments to not be used: #1 + #3 & @@ {a, b, c, d} a + c SlotSequence includes all arguments after the given position: +##3 & @@ {a, b, c, d} c + d Without ...

13

I don't know why no one mentioned this, but all you have to do is to use a special form of OptionsPattern: pfunc[x0_, plotopts : OptionsPattern[{Plot, pfunc}]] := your-code where inside OptionsPattern go all sub-functions you need, in a list. Now everything is fine and dandy. This has been explained already in this answer of Mr. Wizard, so this answer ...

13

Some functions have new return types in MM10. For instance, I'm running into trouble with date functions (like DatePlus) since they return DateObjects in MM10 but returned date lists in MM9. Other functions (like DateDifference) now return Quantity objects instead of a number.

13

Normally I like to use On and Off for this kind of tracing as it is easy to set up without modifying any symbols. However, it does not immediately work in this case: On[Roots] Solve[x^3 - 2 x + 12 == 0, x]; Off[] This does not produce any trace messages. Something must be using Quiet to suppress them. We can check this hypothesis: On[Quiet] Solve[x^3 ...

12

Yes, there is. Group your extra arguments in a list, and address them by their positions in the function under Fold. For your particular example: FoldList[#1 (1 + First@#2) - Last@#2 &, 1000, Transpose@{{.01, .02, .03}, {100, 200, 300}}] (* {1000, 910., 728.2, 450.046} *)

12

Here is a different approach based on the awesome new Region functions: f[x_] := x^3 g[x_] := x^5 - 2 x^3 - 3 x We solve for the intersections: sol = x /. NSolve[f[x] == g[x], x, Reals] {-1.94712297, 0, 1.94712297} Edit Here are the regions of interest: r1 = ImplicitRegion[g[x] > y && f[x] < y, {{x, -1.94712297, 0}, y}]; r2 = ...

11

Using Function with a named parameter, as halmir showed, is the standard way to do this, however anything that prevents a literal Slot[1] from appearing in the body of the Function will work. Inactive as chuy showed is one possibility, but I find this cleaner: {#, Slot @@ {1}} & /@ {1, 2} {{1, #1}, {2, #1}} If the body will not be evaluated you ...

11

Maybe something like (using Inactive): Activate[{#, Inactive[Slot][1]}] & /@ {1, 2} (* {{1, #1}, {2, #1}} *)

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