Questions tagged [indexed-variable]
Questions about indexed variables and related alternative methods to allow assigning indexed names to objects.
58
questions
2
votes
2
answers
152
views
Lorentz indices contraction
I have a complex expression involving second-rank tensors and Kronecker Deltas. How can I instruct Mathematica to utilize the Kronecker Deltas and perform proper index replacements? For example, ...
- 173
4
votes
1
answer
98
views
Using Indexed variables in Compile?
This follows a: useful post about Compile from about 10 years ago
In that post there is a comment:
You can not really use indexed variables in Compile, although it may appear that you can.
For ...
- 3,968
2
votes
2
answers
91
views
Symbolic Indexing on Symbolic Arrays
Is there a way to use symbolic indices on symbolic vectors? The most barebones example I can think of is the following:
aV = Array[a,3];
aV[[m]]
which evaluates to ...
- 121
1
vote
2
answers
54
views
How to extract manipulated indexed variables from Manipulate
I am struggling with getting values of variables that are manipulated with Manipulate:
The code:
...
- 3,733
1
vote
0
answers
41
views
Update some elements of a symbolic matrix
I am trying to build, update and manipulate a binary matrix in an optimization problem using NMinimize.
The variable to be optimized on is a vector ...
- 185
5
votes
1
answer
59
views
Assumptions about indexed variables
I am interested in the most efficient way to impose assumptions on indexed variables. In the following stylized problem, the following code
...
- 187
0
votes
1
answer
79
views
How to specify and use indexed variables of undefined quantity?
These are toy examples intended to elicit general knowledge on indexed variables of undefined (symbolic) quantity; please feel free to offer better examples.
Suppose I wish to "solve" this: $...
- 2,560
0
votes
1
answer
71
views
Replace elements of indexed list by list of rules
I have an array bs that contains many indexed elements b[ ]. I want to define the values of bs by a list of replacement rules. ...
- 563
1
vote
1
answer
74
views
Indexed array cannot be named by the name of its elements
Indexed Arrays or lists can be created by
Array[k,3]
(*{k[1], k[2], k[3]}*)
However I would like to name this list also by k, i....
- 563
2
votes
2
answers
342
views
Would it be poor advice to assign roman numerals as top variables?
For example like so.
x = 10;
xi = 11;
xv = 15;
Is there an automated way to spit these numbers out maybe 1-1000.
- 7,575
1
vote
1
answer
221
views
xAct: How to identify and split module dummy indices?
Hi I'm using the package xAct but I'm running into an issue. It seems that before using //ChristoffelToGradMetric the christoffel connection has indices (which can ...
- 21
1
vote
1
answer
167
views
Counting multiple $-sign string indices in output and replacing them by normal indices
I'm using the package xTensor in xAct. My issue is that inside a module indices are replaced by strings of the form $##### which make the final output unreadable. In a related post Print expressions ...
- 21
5
votes
5
answers
897
views
List of letters longer than the alphabet
I would like to create a list of the first $n$ letters of the english alphabet. Importantly, I would like that in case $n$ is bigger than the size of the alphabet, the letters will start repeating in ...
- 191
2
votes
1
answer
110
views
Handling dummy variables 2
This is a detailed exploration of my previous post,
There was some comment by @Nasser and @yarchik, I post my module in detail
...
- 803
0
votes
1
answer
106
views
Creating and defining a family of functions with indexed variables?
I am trying to do the following: Create a list as
Table[Subscript[\[Beta], i] _, {i, 1, dim}]
And then use this list to define a function:
...
- 5,121
5
votes
3
answers
161
views
Listing all Index Possibilities Given a Vector and Indexed Set
Let's say I am given a vector $v=(5,2,-2,-4,3)^T$ and $\alpha \in \{-1,1\} $. I want to find all indices $(\alpha,i)$ such that $\alpha v_i >0$, where $v_i$ is the $i$th entry of the vector $v$ ...
0
votes
1
answer
75
views
Avoiding For loops to perform an assignment to a variable with indices of user-choice [duplicate]
I am looking for an efficient way to perform the following operation without For loops
...
3
votes
2
answers
200
views
Replacing multiple variables according to the index
I'm currently working on the Polya's Enumeration Theorem implementation in Mathematica. As an example, of what I want to do, here's a formula I'm working with:
$$P_{C_{12}}(x_1,\ldots,x_{12})=\frac{1}{...
- 119
3
votes
2
answers
77
views
Solve with a non-numerical-index constant
I have 4 different constants in a given expression. Due to their physical interpretation, it makes sense to label them with non numerical indices, such as $r_+$.
The problem is that they prevent even ...
- 51
0
votes
2
answers
88
views
Extract subscript of variable in product
My input is a product of the variable x with index:
x[1]^2 x[5] x[7]
How do I extract the indices of the above product to get ...
- 13
1
vote
1
answer
397
views
How to assign values of variables with index
I'm trying to plot the following function
$V(\{r_j\})=\sum^n_{i<j,j=1}
\frac{1}{|r_i-r_j|^2}$
in Mathematica.
For the expression of this function, I wrote
...
- 11
3
votes
0
answers
153
views
Summation variables aren't recognised as dummy variables
I'm trying to write this expression
in Mathematica, and calculate the following quantity
However, when I tried the following
...
- 229
0
votes
2
answers
168
views
Indexed variable - Function that contains summation
I have the following problem:
I have a function that looks like $f(c_1 , c_2 ) = c_1 + c_2 + c_3$.
The variables are $c_1$ and $c_2$, $c_3$ is a known constant. I would like to define a function
in ...
4
votes
1
answer
200
views
Differentiation by indexed variable in equation of Christoffel Symbols
I am very new to Mathematica, and am trying to use it to compute Christoffel symbols for a certain manifold. All this requires is taking some indexed sums of derivatives, but it builds up on several ...
- 43
1
vote
2
answers
124
views
how to create a list of machine-made index names
How can I create such lists of variables:
list1={sus1[t],sus2[t],sus3[t],...,sus99[t]};
list2={inf1[t],inf2[t],inf3[t],...,inf99[t]};
Consecutive ...
- 5,508
6
votes
1
answer
266
views
Undefined Indexed Variable
I searched already a lot about indexed Variables, and it tends to be the most applicable way to use tensor notation. But I am having a hard time to solve for undefined indexed variables:
Here my ...
- 187
1
vote
1
answer
123
views
Can I overload Part for specific heads (a la overloaded array subscripting)? [closed]
Mathematica/Wolfram Language newbie here.
The behavior I am after is to be able to "overload" the subscripting of me-defined objects/Heads and thereby use them as if they were ...
user75893
2
votes
0
answers
89
views
Referring to matrices with index [closed]
For exampe I have two matrices, with index 1 and 2
Mat[1] = ConstantArray[0, {2, 2}]
Mat[2] = ConstantArray[0, {2, 2}]
I wanna assign new value with
...
- 741
3
votes
2
answers
281
views
How to use a list of indexes as a variables inside a Module?
This is a trivial function, just to show that Module does not recognize the list generated by Table.
...
- 31
1
vote
1
answer
89
views
How does Mathematica evaluate sums where the upper limit of the index is not an integer?
I am interested in the way Mathematica evaluates sums where the upper limit of the index is not an integer - i.e., sums that are some sense mathematically meaningless. For example:
...
- 2,141
3
votes
2
answers
337
views
Define implicit dummy index?
I've been looking for a solution to this question for a while. I was wondering if anyone is aware of a way of defining symbolic dummy indices to implement Einstein's convention.
Let me explain with an ...
- 157
0
votes
1
answer
269
views
Subscripts use to make expression readable [duplicate]
I am trying to write a function in mathematica which depends on different parameters. For example
$$f(x_1, x_2, x_3, x_4, x_5)= 2x_1+\sin(x_2)+x_3*x_4-x_5^{-1}$$
(My actual function has 12 such ...
6
votes
2
answers
334
views
How to create a nested list of variable depth with indexed elements
i want to obtain this definition of a nested list
t = Table[C[i, j], {i, 3}, {j, 3}]
with arbitrary depth,
...
- 804
3
votes
0
answers
180
views
Parallelize import of many files
I'm trying to speed up the import of a large number of files, using ParallelTable to store them in a indexed variable, eqDump. The files are in the folder "Frames" and are named "Conf_Run.1000", "...
- 171
1
vote
1
answer
281
views
Getting a stored variable from its value
Given a numerical value, is it possible to check whether that value is stored in (or defines) some variable in the notebook?
Maybe if I could list all of the defined variables, then I could simply ...
- 4,179
6
votes
2
answers
1k
views
Defining a function of arguments with sub- and superscripts
I want to define a function that takes, several, let's say two arguments, both of which have several sub- and suberscripts. For example a function might have the signature
$$ f(x_i,q_{j,k}) = \mathrm{...
- 333
1
vote
1
answer
57
views
Manipulating variables indexed by a list
I have a variable g, indexed by a list:
g[{1,1}]=11;
g[{1,2}]=12;
g[{2,1}]=21;
g[{2,2}]=22;
I would like to move things around as follows:
...
- 877
6
votes
1
answer
2k
views
Best way to write an equation with subscripts and superscripts
This post is not about a specific Mathematica problem but about to develop the best way to enter mathematical equations with subscripts and superscripts. I have a ...
- 5,508
3
votes
1
answer
1k
views
xAct, xTensor: How to avoid clash of indices?
Please refer to the picture below. In the first line, I define the angular momentum vector $\vec{L} = \vec{R} \times \vec{P}$ using the Levi-Civita tensor $\epsilon^{i}_{jk}$. The definition relies on ...
- 651
1
vote
2
answers
245
views
Collecting terms from expression with indexed functions
Say I have an expansion of terms containing functions y[j,t] and its derivatives, indexed by j with the index beginning at 0 ...
- 115
0
votes
1
answer
47
views
Getting named functions from an sequence of parameter values applied to a parametric function [closed]
The general question is how can I loop on names and indices?
An example:
I have
pfun = ParametricNDSolveValue[{y'[t] == a y[t], y[0] == b}, y, {t, 0, 10}, {a,b}]
...
- 1,265
0
votes
2
answers
599
views
Summation over an index and a set
I have an index CC and a set of values CM, and c is an element of CM and ...
- 5,508
2
votes
1
answer
125
views
Loop with indexed function not defined for some index values [duplicate]
If anyone cares, this is supposed to be a very simple implementation of a Metropolis algorithm for a uniform distribution. I want to plot the variance of the sampled values as a function of an ...
9
votes
1
answer
990
views
Einstein summation convention for symbolic vector calculus
I am trying to do some vector calculus in Mathematica in index notation form because it gives a clear result that can be compared to pen and paper calculations. Since there is no built in Einstein ...
- 765
0
votes
2
answers
269
views
Sum over integer partition with variable function argument
Define
$$\hat{X}(Y) = [X,Y]
$$
I have known matrices $S_i$ and $V$. I am trying to use Mathematica to define a function which calculates
$$
\sum_{\substack{n_1, \ldots, n_k>1\\ n_1+\ldots n_k = ...
- 253
1
vote
1
answer
237
views
Create list of values with arbitrary index and the use it in a function
I have the following generating functions:
$l_{2i-1}=l_{1}-(i-2)(w+s)$ with $i\geq 2$ and
$l_{2i}=l_{2}-(i-1)(w+s)$ with $i\geq 2$, so the first one is for odd index and the second for even index. ...
4
votes
1
answer
70
views
Taking a Part of a Part of a Variable
Say I have Part of a variable x.
Part[x, 1]
Then, I'll get an error message, as the ...
- 1,585
2
votes
2
answers
70
views
Naming indexed output streams
I need to write into multiple files in a Do loop.
Hence, I tried to open multiple streams:
...
0
votes
1
answer
57
views
How to truncate equation with multivariables
I am trying to set certain terms zero. How to set terms with greater than 'a3' and 'an3' zero in the following equation:
...
2
votes
0
answers
115
views
ParallelDo with doubly-indexed iterators
What is the reason that ParallelDo works with singly-indexed iterators, but not doubly- or more-indexed iterators? For example, the code
...