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

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8
votes
2answers
129 views

Automatically scroll along with currently evaluating cell?

If I'm evaluating an entire large notebook, I usually just press Alt-V Y to find the currently evaluating cell. Is there a way to keep the currently evaluating cell automatically in focus so that the ...
8
votes
2answers
597 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
209 views

Trouble with polynomial multiplication

Bug introduced in 10.1.0 and fixed in 10.4.0 ...
8
votes
1answer
340 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
2answers
797 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
1answer
226 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
1answer
145 views

Why one needs two Unevaluated to show 1+1 correctly in TreeForm?

The task sounds simple, give the tree plot of 1+1. However, after some trying, I can only make this possible by using two Unevaluated. ...
8
votes
2answers
233 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
407 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
188 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
919 views

Creating self destructing notebook

Inspired by this post I've been trying to implement the idea in Wolfram with no success yet. I've come pretty close though and need some help. So I've managed to do this: ...
8
votes
1answer
761 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: ...
7
votes
6answers
214 views

How to join held Lists?

Let's say I have a = Hold@{2+2} b = Hold@{4+4} How can I get Hold@{2+2,4+4} ie join the two held lists ? More ...
7
votes
4answers
266 views

Button's command is not evaluated in iteration

I wish I didn't feel like a newbie after several months, but... If I enter the following: Table[ Button[ "Number: " <> ToString @ i, Print @ i], {i, 1, 5}] ...
7
votes
3answers
405 views
7
votes
2answers
1k views

Nothing appears in simple plot

I'm trying to get the following simple code to work. (The code is just supposed to graph the norm of the second derivative of a vector function). ...
7
votes
3answers
256 views

Why does Mathematica not seem to know that $i^x$ cannot be equal to $0$?

I am experiencing a curious behaviour of Simplify when handling expressions involving powers of $\mathbf{i}$, such as $\mathbf{i}^x$ where $x$ is unknown. For ...
7
votes
2answers
257 views

Evaluation in lambda function

If I have a function, that returns a function, for example: f[a_] := a^2 * # & then f[3] == 3^2 # & Is there a way ...
7
votes
2answers
211 views

Is there a way to show the details of a numerical computation?

I have recently started using Mathematica as a calculator while teaching (definitely overkill, but I try to use as few tools as possible). I would like my students to be able to trace simple numerical ...
7
votes
2answers
8k views

Simple ways to evaluate a derivative at a point?

The contrast in behavior between, say, f[x_] = Sin[x^2]; f'[2] vs. u[x_, y_] = Cos[x + y^2]; has always bothered my ...
7
votes
2answers
191 views

“A number in base b” and pure functions: why doesn't b^^# & work?

Why do the following not work (in Mathematica 7)? 2^^ # & /@ {1000, 1101} and 2^^ # & @ 1101 This does work: ...
7
votes
4answers
537 views

Attaching persistent assumptions to symbol definition

Is it possible to attach assumptions to a symbol? This relates to this question. Most of my work involves physical equations, i.e. there are basic assumptions on variables that will always hold true ...
7
votes
2answers
147 views

Evaluation inside Inactivate

The documentation for Inactivate makes the function sound really powerful because it gives granular control over which kind of expressions specifically not to ...
7
votes
2answers
226 views

How to correct this, `f[a_] := FindRoot[eq == 0, {x, 0.5}]`, where `eq` contains a parameter $a$, without using `eq[a]`?

I want to solve an equation with a parameter $a$, and define the result as a function of $a$. It is something like this: ...
7
votes
3answers
142 views

How to stop a single function to be evaluated?

See this example: AA = {x} \[Function] Normalize[x] BB = {x} \[Function] Evaluate[AA[x]*5] I need BB to be Normalize[x]*5. Some context:I call evaluate because ...
7
votes
2answers
109 views

Cannot Block Plus

It is customary to use the following idiom to partially evaluate something inside a held expression: ...
7
votes
3answers
344 views

Unexpected behavior of Unevaluated

Maybe it is about time I read some proper text about the main loop. Maybe I should not care (so much) about Unevaluated (as Leonid suggests). But I think this is an interesting question anyway. This ...
7
votes
1answer
386 views

Debugging a $RecursionLimit error?

I sometimes encounter bugs that involve a recursion limit. It's often very hard to find the origin of the problem. Would there be a way to copy the stack somewhere and abort the evaluation in order ...
7
votes
2answers
1k views

Possible to evaluate only selected cells?

Is it possible to evaluate only the selected cells programmatically that I am interested in? Say in current cell to bottom of the notebook, instead of evaluate notebook or cells?
7
votes
1answer
277 views

Module variable scoping in Scheduled Tasks?

I ran across a scoping puzzle while experimenting with ScheduledTasks, and I'd be grateful for an explanation from the sages here. I found a workaround by intuitive horse sense, but was unable to ...
7
votes
1answer
506 views

Changing the plot variable inside Plot [duplicate]

Possible Duplicate: Difference in Plot when using Evaluate vs when not using Evaluate Assume one has vector of functions (e.g., vec={x,x^2}). The ...
7
votes
2answers
139 views

Inserting symbols' definitions during read time of the code

Let's say we've got a package: ...
7
votes
1answer
130 views

Unevaluated @ {args} versus Unevaluated /@ {args}

Working my way through Robby Villegas's lovely notes on withholding evaluation, I almost got Polish Notation on my first try. Here is my final solution, which seems to work well enough: ...
7
votes
1answer
293 views

How is LHS = RHS; … ; LHS (nontrivially) different from … ; RHS

I'm trying to understand how Mathematica thinks about the results of Set. Suppose we perform LHS = RHS. I'm focusing on when ...
7
votes
1answer
83 views

How to reset the $ModuleNumber?

According to the documentation, we can reset $ModuleNumber to a positive machine integer. There is even an example in the doc: ...
7
votes
1answer
221 views

Difficulty catching thrown errors in asynchronous tasks

In this Question, Leonid explained that module variables can get garbage collected from a chain of asynchronous tasks unless their Temporary attribute is cleared ...
7
votes
1answer
269 views

NMinimize with NIntegrate (crash in symbolic evaluation, memory leak)

This is a "common" problem from what I've seen, but with a different spin. I have a function I use often that finds a fit of an expression to another expression with some free parameters (e.g. for ...
7
votes
1answer
152 views

Why can't I abort main evaluation from ScheduledTasks (Service Link)?

FrontEndExecute[FrontEndToken["EvaluatorAbort"]] is equivalent of the menu item: Evaluation >> Abort. It works with ...
7
votes
1answer
209 views

How can I have interruptible computations inside a manipulate?

Here's a perfect example of my problem: Manipulate[Pause[n], {n, {1, 10}, ContinuousAction -> False}, SynchronousUpdating -> False] It is illustrated by ...
7
votes
1answer
139 views

How would one return from a place of a pure function's body using Return?

From previous answers, replies and discussions it is known, while not formally documented (even in the most recent Wolfram Language Documentation Center), that Return can be used in a two-argument ...
7
votes
1answer
101 views

Strange behavior when setting NumericQ of a symbol

It happens both in v9&v10. If you try this a^0 a^0 (1/a)^(3/2) a^0 (1/a)^(5/2) Mathematica returns 1 (1/a)^(3/2) (1/a)^(5/2) It's trivial. ...
7
votes
2answers
184 views

How to track Part of Symbol or How to symbolize Parts of Symbol without copying data

Let's create simple function: SetAttributes[h, HoldFirst]; h[data_] := Dynamic[Grid[data], TrackedSymbols :> {data}] test = RandomReal[1, {2, 2}]; h[test] ...
6
votes
2answers
828 views

Normal[Series[ ]] does not give a normal expression

Let us say I want to show how the Taylor's series for Sin behaves: ...
6
votes
3answers
2k views

Keep function range as a variable

Plot[2*x^2 - x + 2, {x, -1, 1}] plots a function of x from -1 to 1. As far as I can see, I cannot "save" this range in a variable: ...
6
votes
4answers
143 views

How to map Unevaluated over a list

I have a list {a1, a2, ...... , an}, in which all as have been initialized to a number. I want to get: ...
6
votes
2answers
256 views

How do I “evaluate in place” programmatically? Need to evaluate Defer[] result in non-interactive setting

I need to evaluate the result returned from an external package, which contains Defer in it, but do this in a script without user interaction. I am not able to ...
6
votes
2answers
247 views

Remove versus ClearAll

What is the difference between Remove and ClearAll? I thought that "not being recognized by Mathematica", which is the effect ...
6
votes
2answers
364 views

Why is CityData[All, “Coordinates”] failing?

When I evaluate CityData[All, "Coordinates"] Mathematica returns {Missing["NotAvailable"], Missing["NotAvailable"]} ...
6
votes
1answer
178 views

Why does this pattern with Plus not work for numbers?

I have a very simple question about redefining basic built-in functions, such as Plus. I'm pretty sure someone must have asked a similar question around here, but I ...