All Questions
589 questions
260
votes
9
answers
19k
views
What are the use cases for different scoping constructs?
Mathematica includes three functions that I know of which can be used to effectively (if not actually) define a variable and give it a value within a local scope: ...
- 4,961
90
votes
6
answers
4k
views
How to avoid nested With[]?
With[
{v1 = #},
With[
{v2 = f[v1]},
g[v1, v2]
]
]
How to avoid nested With[] like the above? I'd like to use <...
- 8,674
57
votes
2
answers
2k
views
How safe is the use of Block and Internal`InheritedBlock
For many questions on this site answers are given which suggest to use Block and, especially for more advanced topics, ...
- 23.6k
52
votes
1
answer
2k
views
Local variables in Module leak into the Global context
From the Mathematica documentation on Module:
Module allows you to set up local variables with names that are local to the module.
But after using ...
- 623
51
votes
5
answers
3k
views
Enforcing correct variable bindings and avoiding renamings for conflicting variables in nested scoping constructs
Using global variables the following turns an "expression" into a Function:
expr = 2 x;
Function[x, Evaluate[expr]]
Of course ...
- 4,064
50
votes
1
answer
1k
views
What does Internal`InheritedBlock do?
What does the function Internal`InheritedBlock do? How is it different from the regular Block?
- 7,164
42
votes
4
answers
3k
views
Why modules with no variables?
I was reading some code, in particular, recipe 4.13 on unification pattern-matching in Sal Mangano's Mathematica Cookbook, and there were many instances of Modules with no variables in them, such as
...
- 8,763
33
votes
2
answers
3k
views
What are some advanced uses for Block?
I read the answers to this question (What are the use cases for different scoping constructs?) and this one (Condition, Block, Module - which way is the most memory and computationally efficient?).
...
- 4,742
32
votes
2
answers
943
views
What causes Module variables to leak?
Sometimes, after writing code like
f[] := Module[{a}, ...]
and running f[] multiple times, there will be many ...
- 236k
32
votes
1
answer
964
views
Is the renaming mechanism of `With` flawed?
I'm confused about the renaming mechanism of With:
...
- 2,814
31
votes
1
answer
11k
views
Module with "local functions"
I am trying to use a Module having "local functions", i.e., those which I need to define only inside this module.
So I tried this:
...
- 1,011
29
votes
4
answers
1k
views
SetOptions locally?
Is there a way to SetOptions locally? For example, I have a Module and inside it I am doing a lot of ...
- 11.5k
29
votes
2
answers
835
views
What is the purpose of Internal`LocalizedBlock?
I have come across the (internal) use of the function:
Internal`LocalizedBlock
I am trying to determine its purpose. It seem to behave like ...
- 273k
29
votes
1
answer
445
views
Strange behavior with Cases and Position with SetDelayed
I feel like I must be missing something simple and obvious here, but this has me scratching my head.
This works as expected:
...
- 1,783
26
votes
1
answer
514
views
Bug with Block & Lookup: Scope Variable Leak
After debugging a large chunk of code, I could identify this MWE.
...
- 26.4k
26
votes
0
answers
537
views
Compile not correctly initializing a variable defined inside Module
Bug introduced in 8.0, fixed in 10.4, intentionally reintroduced in 11.0 and persisting through 13.0.0 or later
In the following example, inside the minimumX's <...
- 6,715
23
votes
3
answers
1k
views
Nested definition: How can I define a function with a passed-in expression?
Here's a simplified version of what I'm trying to do:
...
- 19.2k
23
votes
1
answer
481
views
Bug with iterated With: Referring to function arguments invalidates previous definitions
Thank you for your report. I have forwarded your report to the appropriate team for review. - WRI
...
- 5,779
22
votes
2
answers
910
views
Should I always be using Module in functions unless I explicitly want global variables?
I define functions a ton. In the past, I've mostly just defined my functions like:
myFunc[var1_]:=(
var2=var1^2;
Return@var2;
)
And I haven't run into too ...
- 3,102
22
votes
2
answers
1k
views
I define a variable as local to a module BUT then the module uses its global value! Why?
I have a question about modules and local variables.
Here's my example:
h = 5;
Module[{a, h}, a[h_]= h^2; a[4]]
(*Out[2] = 25*)
I expected the module to ...
- 323
22
votes
2
answers
1k
views
Module trash collection behaviour
I have asked this question before on the Wolfram community forum (http://community.wolfram.com/groups/-/m/t/83845?p_p_auth=Gxoxe65B) but not with great success.
Module creates temporary variables of ...
- 3,302
21
votes
3
answers
2k
views
Variable scope similar to other languages
Mathematica, unlike many other languages, defines the variables in the global scope unless it is explicitly asked to define a variable in a local scope, e.g., using ...
- 4,069
21
votes
1
answer
288
views
"With" as part of the "Dynamic Interactivity Language"
I noticed that in Mathematica's Function Navigator With is listed as being part of the "Dynamic Interactivity Language".
It is not the first (or even the fourth, ...
- 66.1k
21
votes
1
answer
286
views
Why are delayed definitions in scoping functions not documented?
As outlined here, := may be used in scoping constructs like With to give delayed definitions, but this is undocumented.
This ...
- 273k
21
votes
1
answer
689
views
ClearAll::clloc - "Cannot clear local variable x" in Block[{x}, ...] - why?
Block[{x}, ClearAll@x]
ClearAll::clloc: Cannot clear local variable x.
The documentation just says
An error occurs because ClearAll cannot be used with a ...
- 5,779
21
votes
1
answer
332
views
Integrate - Why is the integration variable not local
I have noticed something odd:
Let
f[t_]:=Integrate[x, {x, 0, t}]
h[t_]:=Sum[x, {x, 0, t}]
Now, enter
...
- 34.4k
20
votes
3
answers
1k
views
How does $CellContext` work?
The only official info about $CellContext` I was able to find is:
placeholder for the context of a symbol inside Dynamic
...
- 138k
19
votes
4
answers
643
views
Internal DynamicModule steals scope of external DynamicModule
I have two dynamic objects, one nested in the other. Both have their own functions defined, as follows. I have simplified them to the point where they don't make much sense, but can still illustrate ...
- 47.2k
18
votes
2
answers
1k
views
Syntax highlighter shows error for Manipulate when it's inside a Block or a Module
I'm putting my new copy of Mathematica 10 through its paces and I noticed a weird change in the colouring scheme that I find very annoying, and I'd like to get some insight into why it's happening.
...
- 10.3k
18
votes
2
answers
562
views
Garbage collection for lexical closures
Mathematica is known not to garbage collect temporary variables in certain cases. I am particularly interested in the very useful programming pattern known as a lexical closure. A somewhat trivial ...
- 12k
18
votes
1
answer
670
views
A With expression local variable specification without head List
I just noticed a new form of With in its documentation:
With[$x=x_0$, $expr$]
specifies that all occurrences of the symbol $x$ in $expr$ should be replaced ...
- 15.1k
17
votes
1
answer
8k
views
Returning a value from a Module
In the following simple example
MyMod := Module[{i},
Do[
Return[1],
{i, 3}
];
Print["test"]
]
I expected the module to exit on ...
- 1,094
17
votes
3
answers
2k
views
What is the right way to construct DynamicModule
When I'm creating DynamicModules for CDF purposes I'm usualy basing on my experience with that, particular goal and content rather than on deep insight.
I want to ...
- 138k
17
votes
1
answer
541
views
Is the sharing of variables in Module/Block within Compile documented behavior?
Today I noticed something, I think for the first time.
When used inside Compile variable values within Module (and ...
- 273k
17
votes
1
answer
564
views
Modules that initialize themselves on first call
I use a lot of functions that extract a specific data item from a file with many data items. I want these functions to load data (slow) and return the item (fast) on first call, but just return the ...
- 5,482
17
votes
2
answers
433
views
Messages leak out from Block
Bug introduced in 8.0 or earlier and fixed in 10.1
When assigning message to a symbol localized by Block, I would expect that this message is not accessible ...
- 15.1k
16
votes
4
answers
3k
views
Subscripts - Why do I see the error "only assignments to symbols are allowed" when using a Module and not otherwise?
When I type
Remove["Global`*"]
foo[] := Module[{Subscript[a, 1] = "x"}, 0];
foo[]
I get the expected error "only assignments to symbols are allowed". I ...
- 150k
16
votes
1
answer
483
views
StringReplace, ReplaceAll and Rule interact in a bizarre way
I have the following pair of things:
ClearAll[foo, labeledFoo];
labeledFoo = {"FooBarBazQuux", foo};
This works like you'd expect:
...
- 18.6k
15
votes
1
answer
880
views
Do Table iteration variables need to be localized using Module?
Lets say I define a Module, that internally use iterating in a Table or Do, say something ...
- 2,499
15
votes
2
answers
562
views
Why can't I inject expressions in Compile using (only) With
Generally speaking, With can be used to inject into arbitrarily held expressions, for example:
...
- 7,753
15
votes
1
answer
357
views
Working with DynamicModule: SetDelayed + OwnValues
Shortly, definitions in form of e.g. x:= RandomReal[] have to be avoided inside DynamicModule[{x}, ...]. Why?, this ...
- 138k
15
votes
2
answers
358
views
Is anonymous pure function a scoping construct?
In recent thread was raised the question: why anonymous pure functions Function[body] (or body &) do not rename symbols in ...
- 62.3k
14
votes
4
answers
535
views
Using With to scope over pure functions
Consider this example: given an option for a function func as "x" :> (a&), how can one assign different values to ...
- 47.2k
14
votes
2
answers
3k
views
Scoping in assigning a derivative
While answering another question, I stumbled upon a problem I cannot easily resolve.
To assign the derivative of a function to another function, typically one can do this with a ...
- 1,085
14
votes
2
answers
552
views
Is Mathematica's lexical scoping broken?
Why do these two lines output different things (b and 0, respectively)? Is it a bug?
...
- 1,327
14
votes
1
answer
122
views
13
votes
4
answers
1k
views
What is the accessibility of a package's `Private` context variables?
I've been reading up on how Mathematica handles contexts, $Context, $ContextPath, and a few of the tutorials they have on ...
- 489
13
votes
2
answers
458
views
RecurrenceTable iteration variable not localized? Bug or user error?
I recently lost a lot of time on this and need to understand whether I have misunderstood MMA operations or there's a problem. I am using MMA 10.1 on Windows 7 64-bit.
The essence of the issue is ...
- 2,570
13
votes
3
answers
513
views
Why does Module slow down StringMatchQ by 180 times?
I knew that Module is slower than With or Block, often by several times. But it surprises me ...
- 2,789
13
votes
1
answer
978
views
Context of localised (dynamic) symbols
When you localise a symbol with Module, the created unique symbol is usually in the current context. There seems to be an exception to this which has further ...
- 113k