Questions on correct (or improved) formulation of Mathematica code to achieve particular results. Use this tag (not "symbol") for questions about all those odd @@ /@ # & and ~ characters.
12
votes
4answers
940 views
Clear complains that a variable is not a symbol or a string?
Here's a small listing where I've used EscqEsc to typeset θ in the notebook:
...
39
votes
2answers
778 views
Flatten command: matrix as second argument
One thing I could never wrap my head around is how Flatten works when provided with a matrix as the second argument, and the Mathematica help isn't particularly ...
17
votes
1answer
253 views
Why doesn't PatternTest work with Composition?
While playing around with the solutions to this question, I've found some very strange behaviour:
...
28
votes
6answers
921 views
Functions vs. patterns
Every time I define a new function, I wonder which construct I should use: true functions obtained by using Function, or rule-based syntax. For example, these are ...
33
votes
2answers
696 views
Head and everything except Head?
I have been working on picking expressions apart using Head and Part and encountered a little mystery. Consider the canonical ...
5
votes
1answer
283 views
Coding mistake? [closed]
I have just started using Mathematica with v9.0. I am trying to follow a computation from a book on Fourier series with the function $f(x)=x$ on the interval $-\pi < x < \pi$.
Here is the code ...
19
votes
2answers
470 views
Meaning of backtick in floating-point literal
If I compute, say, 1/3//N, Mathematica displays
0.333333
as the result.
When I copy that output to use elsewhere,
the paste ...
22
votes
1answer
423 views
Convenient string manipulation
With Mathematica I always feel that strings are "second class citizens." Compared to a language such as PERL one must juggle a lot of code to accomplish the same task.
The available functionality is ...
28
votes
3answers
470 views
Usage of \[InvisibleApplication] and other related invisible characters
From the front end, \[InvisibleApplication] can be entered as Esc @ Esc, and is an invisible operator for @!. By an unfortunate ...
16
votes
1answer
277 views
Prefix operator with low precedence
The question is simple, but I will elaborate on the background as well for those interested in the idea:
How to define a new operator with specified precedence value?
Background
Mathematica was ...
4
votes
3answers
136 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:
...
4
votes
1answer
287 views
What is the difference between Set and SetDelayed?
I have a problem with defining a function and most of time I get confused by Set or = and ...
2
votes
4answers
242 views
Using the result of Solve in subsequent calculations
I'd like to know how to use the results of Solve in later calculations. Here's what I'm doing now:
This is my expression
...
8
votes
2answers
259 views
How can one define an infix operator with an arbitrary unicode character?
You can setup "UsefulFunction[a, b]" to use custom infix notation "a [LeftRightArrow] b" as follows:
...
8
votes
1answer
323 views
6
votes
2answers
472 views
What does # mean in Mathematica?
I asked Mathematica to compute the following
Solve[c (1-x)^2-x^(1/4) == 0, x]
and it returned this:
x = Root[#1^8 c^4 - 8 #1^7 c^4 + 28 #1^6 c^4 - 56 #1^5 ...
6
votes
3answers
462 views
Using D to find a symbolic derivative
I need to do the following:
Define a function
Take the derivative of this function and have a look at the symoblic representation
Substitute in some values
With the bonus that I want to use the ...
4
votes
2answers
270 views
Evaluating an If condition to yield True/False
I would like to decide whether an option passed to my custom function has the value Automatic or something else. This is my attempt:
...
8
votes
5answers
300 views
can the color in MeshStyle be specified by a ColorFunction, such as “SunsetColors” for example. If so, what is the correct syntax?
This is an example given in Help:
Plot3D[Sin[x y], {x, 0, 3}, {y, 0, 3}, MeshStyle -> Gray]
Could this be changed to something like
...
8
votes
2answers
176 views
Cases[data,Colon[key,_]] vs. Cases[data,key:_] toward XPath, XQuery
Exploring methods to index and search tree-structured key:value pairs via named-entity index (key paths) as opposed to Position-based indexing.
Using Rule as ...
8
votes
2answers
457 views
Collapse a section of a huge function
(I have recently taken to Mathematica... still a noobie)
Is there an elegant way of hiding/collapsing only a portion of a function (for example, in Matlab, within a function, one can collapse the for ...
9
votes
4answers
227 views
Calling Table with custom iterator
I often find myself in situations where I, for example, need to build a table for some expression, but want to set the number of points rather then the step size, so the code ends up looking like this
...
7
votes
3answers
192 views
how to read in a file in the same directory?
Now I am writing a small package. I have written several files and put them in the same directory. And some files need to read in or include another file in the same directory. But when I simply write ...
7
votes
3answers
266 views
Why can't D[] be used in place here? [duplicate]
Possible Duplicate:
General::ivar is not a valid variable when plotting - what actually causes this and how to avoid it?
Beginner question:
Why can't I use ...
7
votes
5answers
1k views
Notation of partial derivative
I want to write partial derivatives of functions with many arguments. Why is it that when I type
f[x,y] ctrl+6 (0,1)
it turns out to be bad syntax? The output of
...
4
votes
1answer
250 views
Path queries for tree-structured data
Can anyone suggest documentation or tutorials for developing path queries and indices for (XML-like) tree-structured data?
Suppose data is organized hierarchically in key->value pairs, eg:
...
20
votes
3answers
398 views
Parentheses in pure functions: # & vs. ( # &)
I've been using Mathematica for years, and over time I have developed the habit of using:
Select[data, (# == 1 &)]
instead of
...
17
votes
2answers
628 views
Code Readability and Object-Oriented Code
The basic problem I have been running into is making readable code, where in other languages I am more familiar with I might have been using class or struct like objects. For example, you can imagine ...
11
votes
3answers
204 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] ...
11
votes
7answers
396 views
Equating matrices (or higher order tensors) element-wise
Say I have two matrices (or, as in my case, higher order tensors) $A,B$, and I want to solve the equation $A=B$. To do so , I need a list of equations that equate entry-wise the elements of $A$ and ...
5
votes
2answers
215 views
Extracting parts of nested lists of rules using a syntax similar to JsonPath or Xpath
Given an example data set:
...
10
votes
2answers
329 views
How do you programatically load data into symbols?
I am trying to convert a list of string names into symbols, which will then be used to store data. I have 24 files (where the name of each file is a member of the list mentioned above) that I need to ...
4
votes
1answer
183 views
Upon opening my notebook, I must evaluate cell for code to work properly
I am new to Mathematica. Once I evaluate the cell containing the code, my Manipulate works as desired; however, if I open the file and do not evaluate the cell, the Manipulate goes haywire. Can ...
10
votes
4answers
293 views
Extracting equations from Piecewise expressions
Say I have a PDF:
PDF[LogNormalDistribution[1.75, 0.65], x]
Calculating it, Mathematica gives me an expression that looks like this:
I want to extract the ...
7
votes
1answer
229 views
Setting the DifferenceOrder Option
I've been playing around with Method in NDSolve[...] and can't quite seem to figure out how to force ...
6
votes
2answers
196 views
What's the purpose of the Unique function?
As the title says: What's the purpose of Unique?
I understand that it generates some unique symbol, but when and for what is it to be used? Are there applications ...
5
votes
2answers
133 views
Splicing a list of arguments into a function with Sequence
Not sure if this has been asked, but I have a fairly simple operation that I don't know the syntax for. Say I have an array with some values, and a function f that accepts an arbitrary number of ...
5
votes
3answers
147 views
Syntax for prefix mode with multiple arguments using @ shorthand
Is it possible to use the @ symbol with multiple arguments? The Prefix command suggested not. If so, why?
5
votes
2answers
316 views
Constructing a Summation with a Variable Number of Inner Sums
I have a question regarding multiple sums. And my second index depends on the first index. Here it is:
$$
\sum_{d=1}^{P}e^{-d}\sum_{\substack{1\leq k_{1}\leq Q \\ 1\leq k_{2}\leq Q \\... ...
5
votes
1answer
168 views
Why does the Front End group backslashes into pairs?
What is \\ for? Help finds nothing on it. An odd number of backslashes greater than one are grouped by two and the last one is displayed with a different color. ...
2
votes
2answers
190 views
local variable naming & symbolic argument
I'm stuck here with my variable assignment in this scenario
...
1
vote
1answer
277 views
Why is Euler's number an uppercase “E” and not a lowercase “e”?
Why is Euler's number an uppercase "E" and not a lowercase "e"?
Isn't Euler's Number normally expressed with a lowercase "e"?
