Questions about how Mathematica evaluates expressions, tracing evaluation (Trace*), handling expressions in unevaluated form (Hold*, Unevaluated), nonstandard evaluation, etc.

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10
votes
1answer
121 views

Aborting from inside a ScheduledTask

If you have an Abort[] inside a ScheduledTask expression, what does it abort? The scheduled task evaluation, or any main kernel ...
10
votes
1answer
458 views

Manipulate evaluation sequence

Manipulate doesn't always evaluate in the order I expect it to. What is the evaluation sequence for Manipulate and other ...
10
votes
2answers
260 views

Held keys in associations

Jacob Akkerboom commented: Note that PositionIndex does work correctly with held expressions, whereas this is a bit painful to implement using ...
10
votes
1answer
270 views

Why does Plot[Hold[x], {x,0,1}] work?

It appears that, counter to my expectation, all of these (and probably many others) seem to work fine: ...
10
votes
1answer
116 views

Use EvaluationCompletionAction to notify me only after long evaluation

Is it possible to use the option EvaluationCompletionAction to notify me (e.g. with a Beep[]) when an evaluation that took ...
10
votes
1answer
1k views

Can Mathematica do two evaluations at the same time?

When one evaluation is running, if I want do another evaluation, Mathematica will put the second one in the queue and run it after the first evaluation finish. However, the first evaluation may take ...
10
votes
2answers
303 views

Another difference between Set and Setdelayed. Evaluation shortcut?

A little while ago I wondered why f[x_] = f[x] gives an infinite iteration. I ended up discovering the following difference in evaluation between ...
10
votes
2answers
284 views

Return partial result when MemoryConstrained aborts NDSolve

I use NDSolve to solve a large set (~400) of coupled ODEs. Sometimes, the memory (~4GB) gets filled up, and my computer becomes impossible to work with, because it ...
9
votes
3answers
356 views

How to write a function-defining function which stores the function arguments in a stack?

I have a function-defining function SetAttributes[DefFn, HoldAll]; DefFn[f_[args___], body_] := f[args] := body; I am trying to modify this function to record ...
9
votes
3answers
294 views
9
votes
4answers
233 views

Leave expression tree formatting the same when a replacement rule is applied

If I type e1 := 3 x + 2 (1 + y) z // TreeForm, I get an expression tree. If I then type e1 /. x -> 3, I get another ...
9
votes
3answers
3k views

Keyboard shortcut to evaluate notebook

(If this is entirely trivial, although I couldn't find an answer as much as I searched the archives, I will delete the question.) How could I add a keyboard shortcut to evaluate the entire notebook ...
9
votes
2answers
436 views

Why do I have to put Evaluate[] here

I wanted to draw some contours. I succeeded with this: ...
9
votes
2answers
342 views

Using up-values and down-values

I have a problem I'm trying hard to find a solution for, although I don't know if a solution exists. The following rules produce the result e[2].f[] ...
9
votes
2answers
316 views

How to see which arguments are passed into a function

It is often that one wonders which arguments are evaluated before being passed into a function. I would like to know if it is possible to see which argumets are being passed into a function without ...
9
votes
2answers
711 views

How to make Mathematica re-evaluate a cell after some event?

I am doing a little project to demonstrate something: ...
9
votes
1answer
105 views

Different behaviour of ValueQ inside and outside of a loop

I need to store a rather big number of variables and I tried to do that by storing each variable as Z[n] with varying n. I guess this was not my best idea. The first problem I encountered is when I ...
9
votes
2answers
395 views

Defining a ForEach function

Consider the following function definition: ForEach[iterators__] := Table[#, iterators] &; Two questions about it: I. It works perfectly if you call it like ...
9
votes
1answer
155 views

Tracking Initialization

How to track Initialization step by step? Any tricky solution is ok. One condition, x should be ...
9
votes
2answers
252 views

Menu item “Interrupt Evaluation…” missing in V10

The menu item “Interrupt Evaluation…” seems to have disappeared in Mathematica 10 on OS X. Well, this is a bummer. I personally use Mathematica for very long calculations, and I was using this ...
9
votes
3answers
176 views

Implement settable properties

I am looking for advice on implementing the following. (It is possible that the answer is that it is simply not a good idea to do this.) I would like to have an object similar to ...
9
votes
2answers
355 views

Understanding Villegas-Gayley

Temporary message: I have written another question about the behaviour the code by Oleksandr R. . Sadly I clicked "discard" and that was lost. I am a bit demotivated now and in order to not let the ...
9
votes
1answer
139 views

Is it possible to use the global definition of a symbol as part of its new local definition set in a Block?

Consider the following expression: Block[{Print = CopyToClipboard}, doSomething[]] Now all calls to Print from ...
9
votes
1answer
124 views

Why does $Failed have attribute HoldAll?

Why does $Failed have attribute HoldAll? Are there typical situations where $Failed is ...
9
votes
2answers
174 views

A notebook created on demand from a main document showing executable input along with output cells

I would like to make Mathematica documents with hidden calculations, such that they can be visualized on demand. In general it can be done in several forms and I actually use some. I would like to ask ...
9
votes
1answer
347 views

Performance of Dispatch and lists of Rules

The issue outlined here seems to be no longer present in version 10, if you use a Dispatch table! This because a Dispatch table ...
9
votes
1answer
147 views

Prevent Plus from threading InterpolatingFunction

For an InterpolatingFunction $y:\ \mathbb{R}\to\mathbb{R}^2$, Plus is unaware of this, so when I do any modifications to it in ...
9
votes
2answers
151 views

stop notebook evaluation when Assert fails

I would like a notebook evaluation to entirely stop when an Assert fails, without quitting the kernel. Is this possible (e.g., by setting ...
9
votes
1answer
174 views

Comparing exact expressions for equality — is it really OK if I get wrong answer?

Bug introduced in 7.0 or earlier and fixed in 10.2.0 I found an unexpected behavior (that I think of as a bug) in evaluation of the equality operator applied to mathematical functions with exact ...
8
votes
4answers
230 views

How to inject unevaluated list of Unique into a RuleDelayed?

I am writing a function that constructs a rule which has Unique[] in the RHS. Here is a simplified segment of code ...
8
votes
5answers
272 views

Generate list of strings from a list of assigned variables

How do I generate a list of strings from a list of assigned variables? Eg. Convert ...
8
votes
2answers
305 views

Unexpected behavior of rule matching a pattern

I am a beginner exploring the world of Mathematica. I expected the following code T[6, 5, 4, 1, 2, 3] /. {T[a___, 1, b___] -> Length[List[b]]} should return ...
8
votes
2answers
222 views

Why does the name of a pattern affect the result of a transformation rule?

I was playing a little dirty, trying to get a template for defining a Curl[] operator. I wrote the following rule, which I knew was going to spit out a few ...
8
votes
4answers
2k views

Force function to make assumptions about its input variables in Mathematica

I'm trying to force a Mathematica function to make an assumption about its input variables. In my case, I'm trying to define a function to return the pdf of a Gaussian. But let's use a simple toy ...
8
votes
3answers
796 views

Why are functions called first-class objects in Mathematica?

I've heard lots of people say a function is a first-class object in Mathematica. "Everything is an expression, including function." "There is no distinction between functions and data." Having read ...
8
votes
3answers
239 views

Graphics in Notebook Different from Graphics Expression?

Consider the expression TreeForm[a+b^2+c^3+d] which displays as This is a nice Graphics object, as I can prove by typing ...
8
votes
2answers
1k views

Obtaining the name of a variable as a string [duplicate]

Suppose that I have an arbitrary variable myVariable. Is there some function fun -- either built-in or one that I can define -- ...
8
votes
2answers
204 views

Keywords to find documentation for this syntax

Mathematica allows this syntax, but I cannot find any documentation (or books) that discuss the various uses/advantages of it ...
8
votes
3answers
175 views

With/Function(/etc) replacement behaviour

Can anyone explain to me this behaviour? I've been having more than a couple of similar doubts these last weeks. For example f[_?NumericQ] := 8; Now, if I do ...
8
votes
2answers
174 views

Obtain input expression from history without evaluating

I typed and evaluated an expression, but deleted it from the notebook. For instance, suppose it was In[1]:= Round@SessionTime[] Out[1]= 2 I now want to retrieve ...
8
votes
1answer
140 views

How to catch the “Preemptive” evaluations from within the main loop?

When trying to make an optimized version of my plotRange fonction I faced a problem of catching the "Preemptive" evaluations ...
8
votes
2answers
580 views

Exporting held expressions through JSON

Trying to round-trip expressions through JSON, I'm getting unexpected errors for held expressions, and would be grateful for advice or clues. Consider, first, something that works well ...
8
votes
1answer
296 views

Why is building a table of function values so much slower than just plotting the function?

Here is a function that take some time to evaluate: n = 1000; coeffs = RandomVariate[NormalDistribution[], n]; f[x_] := Sum[coeffs[[k]] Sin[k x]/k, {k, 1, n}]; ...
8
votes
1answer
223 views

How do I write a ValueQ function that only returns True if there exists an OwnValue?

Reading the comments in this answer has motivated me to request a full solution to part of this problem. What I'd like is an efficient solution that returns True ...
8
votes
2answers
788 views

Mathematica rule substitution memory

First I present the specific programming question, and then I provide further background details. Given an expression in Mathematica, say f=3x+7y, there are two ...
8
votes
2answers
212 views

Half-delayed evaluation

In a certain wider context, I need to make some delayed definitions, such as x := a + b + c, but within these definitions I want ...
8
votes
1answer
384 views

How to force an evaluation to use only one core?

There are some evaluations which are automatically parallelized or multithread if there are cores available.( for example, Eigenvalues) But I don't want the ...
8
votes
1answer
187 views

The Flat Attribute, Unevaluated and the Evaluation Process

To make this question easy to follow let's give some symbols a couple attributes. Attributes[f] = {Flat}; Attributes[s] = {Orderless}; Now consider the ...
8
votes
1answer
715 views

How to properly DumpSave & Get Global`s symbols inside packages while not touching Global context?

For efficiency reasons I prefer to use DumpSave instead of Save. For ease of access I prefer to save symbols in ...
8
votes
1answer
157 views

Is it possible to write a function immune to symbol redefinitions in any Block surrounding its invocation?

Normally, semantics of symbols used in a function definition can be changed by their redefinitions in Blocks surrounding the function invocation: ...