Hot answers tagged

74

Prelude The items in this post are not generally regressions; they are simply changes and enhancements that may break code or cause problems in moving from one version to another. The work-arounds are offered as specific solutions to instances of incompatibility, not as recommendations of general practice. For example, Plot Themes are a powerful tool one ...


46

I am a developer at Wolfram Research and I have been working on linter technology that I think would be nice to share publicly. There are 2 paclets on the public paclet server that you can download and use for finding problems with WL code. The two paclets you will need are CodeParser and CodeInspector: In[1]:= PacletInstall["CodeParser"] Out[1]= ...


45

For me the operator forms of Map and Apply will probably provide the most important benefits in terms of code readability. Often I need to apply a sequence of transformations to some data, and I am fond of infix notation for this purpose. For example I find a ~Position~ 0 ~SortBy~ Last more readable than the "conventional" SortBy[Position[a, 0], Last] ...


38

I would have liked to have more experience with the operator forms before this question was asked as I am short on examples, and I'm sure my opinion will evolve over time. Nevertheless I think I have enough familiarity with similar syntax to provide some useful comments. Taliesin Beynon provided some background for this functionality in Chat: Operator ...


34

Clearly the @ notation is inspired by the usual mathematical notation for function composition. f@g[x] looks very similar to the mathematical notation $(f\circ g)(x)$. But it is important to understand that @ does not denote function composition. In mathematical notation $f\circ g$ is also a function. In Mathematica f@x is simply a different way to ...


29

So recently I've learned from John Fultz that RawBoxes are kind of verbatim indicator for MakeBoxes which is not well stressed out in documentation. This or I've missed the point but it doesn't matter, here we have handy way to do this: x = 5; ToExpression @ MakeBoxes[RawBoxes["x"] = 123]; x 123


29

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 ...


29

Maybe I miss the point here, but FullForm[x ↗ y] gives UpperRightArrow[x,y]. This is described in the documentation to UpperRightArrow and since this symbol is not protected and has not built-in meaning, you can just define it the way you like: UpperRightArrow[x_, y_] := FooBar[x, y] and this instantly gives you Update: As answer to Jacobs comment I ...


28

Good News Everyone! Two-parameter syntax for Fold and FoldList has been (silently) implemented! Taliesin Beynon informs me that this was implemented in 2011, so check your older versions as well. As Naitree notes this is now documented in 10.0.2: Fold[f, a] FoldList[f, a] f[f[f[1, 2], 3], 4] {1, f[1, 2], f[f[1, 2], 3], f[f[f[1, 2], 3], 4]} And the held ...


25

You can use String "keys" for indexed variables, as I did for A combination of Set::setraw and Set::shape errors. The strings can have spaces or any other characters you want to use: var["Degree of the First Polynomial"] = (* stuff *); You also have a wide range of characters, many of which can be used in Symbol names. Go to menu Palettes > Special ...


24

I find the value of the new operator forms becomes critical when working with datasets. Consider titanic = ExampleData[{"Dataset", "Titanic"}]; titanic[Count[#], "survived"] & /@ {True, False, _Missing} {500, 809, 0} Derive a data set for analyzing the survival of very young passengers. cutoff = 8; youngest = titanic[All, {"age", "survived"}][Select[#...


23

Try this: Map[If[#==1,Unevaluated@Sequence[],#]&,{1,2,3}] Note the output. The 1 is gone. That's because Unevaluated@Sequence[] puts the empty sequence there, that is, "nothing". ##&[] is a shorthand that can be used in most places for same - ## is the sequence of arguments, & makes it a function to apply to something, [] is that something - ...


22

I mentioned this in a comment, but I believe this might really be the correct answer to your specific inquiry about underscore. You can escape underscore (Esc+_+Esc or Ecs+ls+Esc), which will give you a \[LetterSpace], which looks like underscore but is slightly lighter. This is just treated like a regular old letter and you can therefore use it in variable ...


22

This syntax was deprecated in the version 6.0 era. According to the legacy documentation, For example, in version 5.2, the following strings are interpreted differently string1 = "first line second line" string2 = "\<first line second line\>"


21

I can see the source of your confusion: If you use Head[f[x]] and Head[5] you get f and Integer respectively. Then, you read the documentation Apply[f,expr] or f@@expr replaces the head of expr by f. and you expect Cos@@5 to replace the Integer head by Cos. The way I explain it to myself is by saying Mathematica has two (types of) heads ;-) One type is ...


21

I know this has been answered already on this site, but I cannot seem to find it. Map and Apply do subtly different things. For example, Map[f, {a,b,c}] (* {f[a], f[b], f[c]} *) If you have a list that is more deeply nested, without using the third argument which is for level specification, you get Map[f, {{a,b}, {c}}] (* {f[{a,b}], f[{c}]} *) or, if ...


20

You can use any built in operator modified with subscripts, superscripts, etc, and retain its precedence, for your own purposes. For example, say you want a general Apply operator like @@ that could work at any level. One could use create the operator @@ with a number subscripted for the level of Apply seems appropriate MakeExpression[RowBox[{fun_, ...


20

I can't answer how the association is made for the built-in operators, but I can show how to add your own. If your symbol is already an operator you can do this simply as halirutan showed. This question may be a duplicate of How can one define an infix operator with an arbitrary unicode character? but since it admits a simpler interpretation I shall not ...


20

... why introduce Composition as a new feature? Composition is used to create a new anonymous function that can be used in all the standard ways such as Map and Apply etc. To achieve the same thing without it one needs a Function. Much like operator forms the use of Composition allows one to eliminate extraneous Function constructs which can make code ...


20

One way would be to redirect all messages issued by ToExpression to a string-stream. Here is an example of that approach, with minimal error-checking: Needs["Developer`"] interpret[str_String] := Module[{s = StreamToString[], r, m} , Block[{$Messages = {s}}, r = ToExpression[str, InputForm, HoldComplete]] ; m = StringFromStream[s] ; Close[s] ; &...


20

I've looked into this myself and I don't think the problem is with Mathematica. The problem is how to represent the choice of the host. Here's an attempt I tried: So the basic idea here is: I pick a number between from 1 to 3 and so does the car. The host picks randomly between the numbers 1 and 2 and adds that number (mod 3) to mine to pick a different door ...


19

This is in fact tricky. But j is not what it looks. TensorRank[i] gives 2 and its dimensions are {3, 1}. j is different: TensorRank[j] gives 1 and its dimensions are {3} instead of {3, 1}. A fix. j = {{1, 2, 3}} and you get i.j {{1, 2, 3}, {2, 4, 6}, {3, 6, 9}} j.i gives {{14}}. The reason it apparently works with j.i is that in this case ...


19

Mathematica does not have the concept of row or column vectors like you may be used to. The concept isn't really necessary either and is just a convention to visualize the dot product (although I know there are people that vehemently object to this statement). In dot products like $M\cdot\vec{x}$ and $\vec{x}^{^\top}\cdot M$ Mathematica uses $\vec{x}$ as ...


19

Also PlotRange[plot] PlotRange /. AbsoluteOptions[plot] Last @@ AbsoluteOptions[plot, PlotRange] PlotRange /. plot[[2]] all give (* {{0.,10.},{-0.999999,1.}} *) Note: Regarding usage of PlotRange as a function, it is undocumented, and the earliest reference I could find on this site is this answer dated Oct 11, 2012: The same range on each plot in a ...


19

Description In software engineering, it is a good practice to comment your code. I would advise you to utilise comments to partition your code in a following way. Additionally, if you have ever worked with any other programming languages, you could employ indentation. Alternatively, you could modularize your application as to develop it in smaller, more ...


19

I appear to be in the minority but I never write big blocks of Mathematica code, it's just too difficult to read. The way I look at it you have to consider how a reader will understand your app. So I make the main block very small, like this: Manipulate[ Row[{ vectorPlotAndTrajectory[y0, b], Show[fy, position[y0, b]] }] , {b, -7.99, 8} , {{y0, 2}...


19

Quick-n-dirty. I dispense with open/close bracket, trivial to put in if it matters: fn = ToExpression/@ImportString[StringReplace[#, ";" -> "\n"], "Table"] &; mymat = "2 4; 3 4 5 ; 5 6" // fn mymat2 = "a b;2 c; d 5" // fn {{2, 4}, {3, 4, 5}, {5, 6}} {{a, b}, {2, c}, {d, 5}}


18

I believe it does work, just not how you expect. :-) From the documentation for PutAppend: Note that there are no quotation marks around filename in the first line. It is not made particularly clear but you can use this syntax with >>>: Range[10] >>> file.txt Which outputs to a file named file.txt directly. This is a special and ...


18

Intermingling Operator Forms & Linguistic Connections This answer attempts to draw out linguistic connections in understanding why operator forms seem so useful, a process that can perhaps point to further utility. Operator Forms are a type of modularization with the standard re-use and combinatory advantages but IMO the biggest benefit is cognitive - ...


17

It is not a bug. Iconize works with expressions. Comments / line breaks / indentation and other box/input features are stripped by the FrontEnd before sending it to the Kernel. Iconize[ Hold@Range[1, 10 ] ] (*evaluate and convert to StandardForm*) Hold[Range[1, 10]] (*no line breaks, indentation or @ *)


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