Questions about how Mathematica evaluates expressions, tracing evaluation (Trace*), handling expressions in unevaluated form (Hold*, Unevaluated), nonstandard evaluation, etc.
24
votes
9answers
780 views
Injecting a sequence of expressions into a held expression
Consider the following toy example:
Hold[{1, 2, x}] /. x -> Sequence[3, 4]
It will give
Hold[{1, 2, Sequence[3, 4]}]
...
22
votes
3answers
725 views
Difference in Plot when using Evaluate vs when not using Evaluate
This example comes from
the Mathematica documentation for Plot
under Basic Examples.
Can someone please explain why these are each plotted as a different color in ...
24
votes
4answers
900 views
How do I evaluate only one step of an expression?
I am looking for a simple, robust way to evaluate an expression only one step, and return the result in a held form.
The definition of a single step is ambiguous, and this itself is probably worthy ...
32
votes
3answers
9k views
Get a “step by step” evaluation in Mathematica
Is it possible in Mathematica to get a step-by-step evaluation of some functions; that's to say, outputting not only the result but all the stages that have led to it?
Example : Let's say I want to ...
8
votes
3answers
991 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:
...
15
votes
5answers
677 views
How to pass a symbol name to a function with any of the Hold attributes?
Given a function with the attribute HoldFirst, HoldAll or similar, and a variable, list, how ...
12
votes
4answers
467 views
Making a symbol's new definitions be tried before all previously defined ones
Is there any way to "close" a package (or a symbol, or a context) in that if a user of the package adds definitions to the symbol they will be tried before the package defined ones, just like what ...
17
votes
2answers
357 views
Implementing a safe ValueQ that does not evaluate its argument
Mathematica has a built-in function ValueQ. Quoting the docs,
ValueQ[expr] gives True if ...
3
votes
3answers
241 views
Help on evaluation control?
I want to plot series of slopes:
x, 2x, 3x, 4x, 5x, 6x...
I'm trying to do this:
...
9
votes
4answers
239 views
How are parameters evaluated for a Plot in Manipulate
I am trying to get my head around how Manipulate evaluates functions in a Plot. I have read the introduction to Manipulate, and introduction to Dynamic, but I still ...
8
votes
2answers
185 views
19
votes
2answers
646 views
Mathematica for Computer Scientists
Background
I've coded in C/Scheme for 10 years. I have a BS CS.
I've recently become very interested in pattern-matching (lisp's match macro), and term rewriting systems, which brought me to ...
23
votes
2answers
684 views
Block attributes of Equal
I answered “Equating matrices (or higher order tensors) element-wise” with:
...
17
votes
3answers
403 views
How can I completely ban usage of some functions in output and mandate use of others?
For example, I hate that Mathematica uses Pochhammer symbol in outputs and prefer all the expressions in Gamma function. How can ...
4
votes
3answers
143 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:
...
0
votes
1answer
79 views
Slow Dynamic Button Does Not Print [duplicate]
Possible Duplicate:
Why do buttons with ChoiceDialog freeze the front end?
Why is SystemDialogInput failing to return a save file path?
Evaluation inside Button stops after some seconds
I ...
16
votes
2answers
280 views
Context unique to each group at a specified level
Recent versions of Mathematica provide the option of having a unique $Context for each cell group, via:
Evaluation > Notebook's Default Context > Unique to Each ...
8
votes
3answers
219 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 ...
10
votes
1answer
261 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 ...
12
votes
1answer
226 views
Using Evaluate and Evaluated -> True in Plot
A few use cases:
(1) This, as expected, returns (almost) immediately:
f[x_?NumericQ] := Evaluate[(Pause[1]; x)]
Plot[f[x], {x, 0, 1}]
(2) This takes ...
16
votes
3answers
447 views
Asynchronous evaluation: Is it possible?
Currently we have a limited number of ways to perform asynchronous evaluation.
The most common is through Dynamic and ...
10
votes
4answers
272 views
Recovering intermediate results from a Table after an interruption
I am interested in knowing if it is possible to recover intermediate results from a Table after an interruption.
Let us say I want to produce a really useful table
...
16
votes
1answer
354 views
How do you set attributes on SubValues?
SubValues, as discussed in a previous question, are declared as follows
...
16
votes
5answers
326 views
18
votes
2answers
188 views
Evaluation of OptionValue[]
I have a function that takes an OptionsPattern and I want to access several options, say a, and ...
15
votes
4answers
278 views
Function that caches when it returns unevaluated
How would you go about implementing a function that caches not the result but the fact that it didn't match a pattern? So that it doesn't waste time recomputing a complex time-consuming pattern ...
11
votes
3answers
241 views
Returning an unevaluated expression with values substituted in
Let's say I have a function
formula[x_List] := (x[[1]] - x[[2]]) + (x[[3]] - x[[4]]);
and I want to pass it a variable and get back the unevaluated formula with ...
8
votes
4answers
139 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 ...
5
votes
3answers
176 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 ...
4
votes
4answers
177 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 ...
6
votes
1answer
155 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 ...
5
votes
2answers
220 views
Normal[Series[ ]] does not give a normal expression
Let us say I want to show how the Taylor's series for Sin behaves:
...
3
votes
1answer
70 views
Evaluation order of Button[]'s action
it looks like it should be well known issue but I haven't found answer anywhere.
Please tell me why the result (after clicking) of this:
...
11
votes
3answers
230 views
Comparing Mathematica expressions like diff
I am looking for a way to compare (or "diff") two Mathematica expressions, similarly to how to diff utility can compare two text files and report the differences.
Has anyone already written such a ...
6
votes
1answer
142 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 ...
11
votes
5answers
455 views
How to determine how much of a table is generated?
If I'm generating a table from a slow function, like this:
foo = Table[SuperSlowExpression, {1000}]
is there a way to determine after submission (so it is too ...
7
votes
1answer
89 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 ...
9
votes
1answer
228 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 ...
15
votes
1answer
346 views
Wagner's trick to monitor FindRoot no longer works in Mathematica version 8
I recently learned that we can use Print as a monitoring tool. My favorite is Ted Ersek's example
...
13
votes
2answers
303 views
How to REALLY Hold arguments and capture FullForm?
I'm trying to write an expression deconstructor or FullForm-capturer; might even call it a parser, maybe, but that might be too glorious a word.
I got some great ...
13
votes
3answers
176 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 ...
10
votes
2answers
186 views
How can I access a variable in one evaluator from another evaluator?
I have two notebooks, each with a different kernel. Is there a way to grab the value of a variable in KernelA from KernelB?
10
votes
3answers
232 views
How can I hold UpValues but evaluate other expressions?
Consider these definitions:
own = "OwnValue";
down[_] = "DownValue";
sub[_][_] = "SubValue";
N[n] = 3.14;
_[___, up, ___] ^= "UpValue";
The attribute ...
8
votes
1answer
284 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 ...
7
votes
2answers
193 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 ...
11
votes
3answers
208 views
Numbers in alternate bases transcend the evaluator?
It looks to me like a number in a base other than base 10 gets evaluated before the evaluator ever gets a chance to be tweaked.
For example, FullForm[16^^abcdef] ...
10
votes
2answers
131 views
Subtle difference in conditional rules placement
I had gotten used to thinking of the placement of a condition in a rule definition as logically immaterial. In my specific example, namely
...
6
votes
2answers
248 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
4answers
234 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
1answer
177 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 ...
