# Tag Info

177

What the @#%^&*?! do all those funny signs mean? Questions frequently arise about the meaning of the basic operators, and I hope it will prove useful to have a sort of index for them. It would be nice to have them organized by sign instead of topic, but they do not have a natural order. One can use the find/search feature of a browser to locate an ...

127

I. General I will first try to briefly answer the questions, and then illustrate this with a small but practical application. 1.Speed of insertion / deletion Associations are based on so called Hash Array Mapped Trie persistent data structure. One can think of this as a nested hash table, but it is more than that, because it has the following properties: ...

96

Avoiding procedural loops People coming from other languages often translate directly from what they are used to into Mathematica. And that usually means lots of nested For loops and things like that. So "say no to loops" and get programming the Mathematica way! See also this excellent answer for some guidance on how Mathematica differs from more ...

87

Basic syntax issues Mathematica is case-sensitive. sin is not the same as Sin. Symbol names cannot contain underscore. _ is a reserved character used for pattern matching. Avoid using subscripted symbols in your code. While it can be done, it causes a lot of confusion and is harder to use than just sym[j] or whatever your symbol might be. The reason is ...

86

Understand that semicolon (;) is not a delimiter Although it may look to newcomers that semicolons are used in Mathematica as statement terminators as in C or Java, or perhaps as statement separators as in Pascal and its derivatives, in fact, semicolons are the infix form of the function CompoundExpression, just as plus-signs (+) are the infix form of the ...

77

Using the result of functions that return replacement rules Most new Mathematica users will at some point encounter the seemingly odd formatting of the output given by functions such as Solve or Root. Let's start with the follwing simple example: Solve[x^2 == 4, x] {{x -> -2}, {x -> 2}} You might find this output strange for two reasons. We'll have ...

72

Understand the difference between Set (or =) and SetDelayed (or :=) A common misconception is that = is always used to define variables (such as x = 1) and := is used to define functions (such as f[x_] := x^2). However, there really is no explicit distinction in Mathematica as to what constitutes a "variable" and what constitutes a "function" — they're both ...

65

Understand what Set (=) really does Because WRI's tutorials and documentation encourage the use of =, the infix operator version of Set, in a manner that mimics assignment in other programming languages, newcomers to Mathematica are likely to presume that Set is the equivalent of whatever kind of assignment operator they have previously encountered. It is ...

62

Learn how to use the Documentation Center effectively Mathematica comes with the most comprehensive documentation I have ever seen in a software product. This documentation contains reference pages for every Mathematica function tutorials for various topics, which show you step by step how to achieve something guide pages to give you an overview of ...

56

Assuming commands will have side effects when they don't Consider: In[97]:= list = {1, 2, 3} Out[97]= {1, 2, 3} In[98]:= Append[list, 4] Out[98]= {1, 2, 3, 4} In[99]:= list Out[99]= {1, 2, 3} When I was first learning Mathematica, I assumed that Append[list, 4] would take the list list and append the element 4 to it, overwriting the previous list. But ...

56

User-defined functions, numerical approximation, and NumericQ Frequently there are questions, to which the answer is to use x_?NumericQ, about defining functions that call or sometimes are passed to FindRoot, NIntegrate, NMaximize, NMinimize, FindMaximum, FindMinimum, NDSolve, ParametricNDSolve, FindFit, LinearModelFit, NonlinearModelFit, and so on. ...

55

Attempting to make an assignment to the argument of a function Quite frequently new users attempt something like this: foo[bar_, new_] := AppendTo[bar, new] x = {1}; foo[x, 2] To be met with: AppendTo::rvalue: {1} is not a variable with a value, so its value cannot be changed. >> Or: f[x_, y_] := (x = x + y; x) a = 1; b = 2; f[a, b] ...

51

As an Eterprise CDF user, I can say I have really tried, and my current opinion is that creating a standalone GUI program with the Wolfram Language is not an easy/commercial/deliverable task at the moment. Here are my points: All the interface controls are very limited. You will have a lot of difficulty to do basic things like make Tab jump between fields, ...

50

Lingering Definitions: when calculations go bad One aspect of Mathematica that sometimes confuses new users, and has confused me often enough, is the Lingering Definition Problem. Mathematica diligently accumulates all definitions (functions, variables, etc.) during a session, and they remain in effect in the memory until explicitly cleared/removed. Here's ...

49

Understand the difference between exact and approximate (Real) numbers Unlike many other computational software, Mathematica allows you to deal with exact integers and rational numbers (heads Integer and Rational), as well as normal floating-point (Real) numbers. While you can use both exact and floating-point numbers in a calculation, using exact ...

46

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 bullet points are offered as specific solutions to instances of incompatibility, and not as recommendations of general practice. For example, Plot Themes are a powerful tool ...

38

Multiple front-end undo is not available in versions less than 10 As the title already claims, in versions less than 10, there is no overall option to undo certain steps in Mathematica files. Nevertheless, inside the boxes one can undo as long as one stays inside. Personal recommendations: 1. Never delete some code except if what you were doing was ...

37

Compose and Composition There is, but it is deprecated (in favor of Composition): Compose: MapThread[Compose, {{a, b, c}, {1, 2, 3}}] (* {a[1], b[2], c[3]} *) I still use Compose myself, but I would not take the responsibility to recommend this as a common practice. You can also use Composition[#1][#2] &, although this is hardly better than your ...

37

The displayed form may substantially differ from the internal form As soon as you discover replacement rules, you are bound to find that they mysteriously fail to replace subexpressions, or replace subexpressions you didn't expect to be replaced. For example, consider the definition foo = (a+b)(c+d)(e-f)/Sqrt[2] which will cause Mathematica output an ...

36

These three functions are similar (speaking commonly), and in some applications any of them could be used, yet they have very different special applications. Rudimentarily: Map wraps (sub)expressions in a given Head, and returns the modified input Apply replaces Heads in (sub)expressions, and returns the modified input Scan "visits" (sub)expressions, ...

34

I helped design Association, and I designed and implemented Dataset, so I wanted to comment on question 3: Dataset is designed explicitly for hierarchical data. It supports any 'shape' of data, inferring the shape when the Dataset is first created. It also tracks the shape of the data as transformations are applied to the dataset, using a type-inference ...

33

Use Consistent Naming Conventions This is basic, and good practice in any programming language, but Mathematica's slow-to-fail nature makes it in a sense a less forgiving language than others, so those of us who have in the past gotten away with bad habits may run into trouble. Suppose I have a function loseMemoriesLikeTearsInRain[] which I later try to ...

32

Updated Both Hold and Inactive block evaluation; the key difference is that Inactive is meant to be wrapped around heads rather than a whole expression. Inactivate does this. Inactivate[1 + 2 + 3 * 4 ^ 5 ] // FullForm Inactive[Plus][1, 2, Inactive[Times][3, Inactive[Power][4, 5]]] It is of course possible to use Inactive directly, and it will behave ...

32

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 ...

31

Mathematica's own programming model: functions and expressions There are many books about Mathematica programming, still one sees many people falling to understand Mathematica's programming model and usually misunderstand it as functional programming. This is, because one can pass a function as an argument, like plotZeroPi[f_] := Plot[f[x], {x,0,Pi}]; ...

29

Don't leave the Suggestions Bar enabled The predictive interface (Suggestions Bar) is the source of many bugs reported on this site and surely many more that have yet to be reported. I strongly suggest that all new users turn off the Suggestions Bar to avoid unexpected problems such as massive memory usage, peculiar evaluation leaks, and broken ...

28

# is a placeholder for an expression. If you want to define a function, $y(x)=x^2$, you just could do: f = #^2 & The & "pumps in" the expression into the # sign. That is important for pairing & and # when you have nested functions. f[2] (* 4 *) If you have a function operating on two variables, you could do: f = #1 + #2 & So ...

27

The default $HistoryLength causes Mathematica to crash! By default$HistoryLength = Infinity, which is absurd. That ensures Mathematica will crash after making output with graphics or images for a few hours. Besides, who would do something like In[2634]:=Expand[Out[93]].... You can ensure a reasonable default setting by including (\$HistoryLength=2), or ...

26

Mathematica can be much more than a scratchpad My impression is that Mathematica is predominately used as a super graphical calculator, or as a programming language and sometimes as a mathematical word processor. Although it is in part all of these things, there is a more powerful usage paradigm for Mathematica. Mathematica stackexchange itself tends to be ...

26

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 ...

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