Questions about how Mathematica evaluates expressions, tracing evaluation (Trace*), handling expressions in unevaluated form (Hold*, Unevaluated), nonstandard evaluation, etc.
9
votes
4answers
169 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
1answer
67 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
239 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
206 views
How can I set a keyboard shortcut to run a command?
I want to use this to add a keyboard shortcut to an undo function I wrote. The basic problem is the following:
I have a command, let's say x=1, and I want to run ...
9
votes
2answers
106 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 ...
8
votes
3answers
217 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 ...
8
votes
3answers
143 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
3answers
153 views
How can I test properties of a symbol from the string name without the symbol completely evaluating
Suppose I have a few symbols, one of which has a value:
{abc1, abc2 = 5, abc3};
I can use Names to get the list of names, as ...
8
votes
4answers
135 views
How to pass a list of arguments into HoldAll
I have a list of arguments (which in reality is lengthy):
arguments = {a, b, c}
arguments2 = {a_, b_, c_}
f[Sequence@@arguments2] := a + b + c
Note: It seems ...
8
votes
2answers
178 views
8
votes
1answer
93 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
219 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
2answers
308 views
How to make Mathematica re-evaluate a cell after some event?
I am doing a little project to demonstrate something:
...
8
votes
1answer
110 views
Get the name of a symbol passed to a function
I'm trying to get the name of a symbol passed to a function with this:
f[x_] := {SymbolName[x], x}
SetAttributes[f, HoldFirst]
x = 5;
f[x]
But ...
8
votes
2answers
452 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
266 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
97 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 ...
7
votes
3answers
926 views
General::ivar is not a valid variable when plotting - what actually causes this and how to avoid it?
I was just evaluating a couple of expressions and started to get errors like this:
...
7
votes
2answers
189 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 ...
7
votes
4answers
339 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 ...
7
votes
4answers
156 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
2answers
142 views
Bags and non-standard evaluation
What can internally be happening here? Is the evaluator just messing with us and going non-standard because its a Bag? Or am I just not seing how something like this could be done in MMA?
...
7
votes
2answers
153 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
2answers
377 views
How do you force a decimal output? [duplicate]
I have some very small values such as 2.601519253*10^-8. I'd like to output these values to CSV for another program to work with. I've tried N[value, 50], but Mathematica still insists on producing ...
7
votes
1answer
176 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 ...
7
votes
1answer
69 views
Performance of Dispatch and lists of Rules
First of all, I have seen the question "Why is this parallel evaluation with Dispatch[] so slow?", but that seems to be a different matter.
My question arose when I tried to answer this question by ...
6
votes
2answers
147 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
...
6
votes
4answers
224 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 ...
6
votes
2answers
243 views
How do I make NotebookEvaluate work inside Button?
The following creates a button to select a notebook to run. When the button is pressed it seems that Mathematica finds the notebook but cannot evaluate it. The following error occurs
Could not ...
6
votes
1answer
187 views
Why does Manipulate execute the expression twice?
When I execute Foobar[] in the code below and move a slider, the variable grad is printed twice. Why is this? How can I make the ...
6
votes
2answers
358 views
Expression evaluation inside of FindRoot inside a Compiled Function
I'm trying to get some performance increase out of my own implicit differential equation solver using Compile[]. The uncompiled function is of the following form:
...
6
votes
1answer
173 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 ...
6
votes
1answer
152 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 ...
6
votes
1answer
133 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 ...
6
votes
2answers
419 views
Speeding up mathematica by subsitituting numerical values
According to this Wolfram Blog post, one can speed up Mathematica code by substituting numerical values as soon as possible.
How does one substitute before the main expression is evaluated? The ...
6
votes
1answer
135 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 ...
6
votes
0answers
69 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 ...
5
votes
3answers
227 views
First evaluation fails
I am fairly new to Mathematica (version 7), so please bear with me...
I have duplicated, in Mathematica, a program that I originally wrote in C (it generates OEIS sequence A191837). The problem is ...
5
votes
2answers
216 views
Normal[Series[ ]] does not give a normal expression
Let us say I want to show how the Taylor's series for Sin behaves:
...
5
votes
1answer
90 views
“And” to continue evaluation after “False”
Below, is it possible to print No and have x be 2. That is, I'm looking for an And-like function that looks at all arguments ...
5
votes
2answers
58 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 ...
5
votes
1answer
179 views
Parts of Module body evaluated in external scope?
I have an expression that suggests that some expressions in a module body are dragging in definitions from outside the scope in a surprising way. First, consider a symbolic constant, ...
5
votes
3answers
167 views
HoldFirst and inserting additional options into a Grid of Graphics
This is related to my earlier question, but is specific to an issue I have encountered with the use of the HoldFirst
First, let's create some fake data for ...
5
votes
1answer
92 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 ...
5
votes
2answers
167 views
Why doesn't Evaluate appear to work in this RegionPlot example with MatchQ?
I am trying to plot the region over which a function of two (real) parameters a and b is real-valued, using ...
5
votes
4answers
196 views
Prevent Part[] from trying to extract parts of symbolic expressions
If you have a list, e.g.
{1, 2, 3}
then you can extract the $k$th part using Part (...
5
votes
3answers
229 views
Delayed evaluation of assignment
I have a set of PDE's that depend on parameters. They depend on many parameters, but for simplicity we can here assume that they depend only on one, say a.
I want ...
5
votes
2answers
192 views
Manipulate Evaluation Order Problem
I seem to be getting some unintended results from a nested Manipulate that I have not been able to resolve. I boiled down the problem I'm having to a simplified ...
5
votes
1answer
112 views
Impossible to bypass evaluation on returned values?
Following all the great advice on this other question, I'm now fluent with HoldAllComplete and Unevaluated on the input side of ...
5
votes
1answer
132 views
How to simplify expression and use HoldForm at the same time?
Hopefully a simple problem for the experts here.
I have a need many times to build a symbolic expression for an equation (say a PDE) to use for plot labels in a low level function inside Manipulate, ...
