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4
votes
1answer
87 views

An alternative to subscripts?

Are there any good alternatives to using subscripts to keep track of and uniquely identify many variables? I know that subscripted variables behave badly. Thanks!
2
votes
2answers
89 views

Define a 4d matrix without for loop

I was wondering if there is a way to avoid for loop here: I want to define a 4d matrix H(i,j,k,l), and each entry in the matrix is calculated through a complex function, say Do_H(i,j,k,l), depending ...
5
votes
4answers
96 views

Multi-index variable depending on signature of permutation of indices

I do have some multi-indexed variable, e.g. like this \begin{align*}f_{123} &= 1\\f_{345} &= 1/2\end{align*} where $f$ is antisymmetric under permutation of any pair of indices, i.e. e.g. $...
0
votes
0answers
57 views

Workaround of non-integer indexed objects? [duplicate]

When using indexed object, I found the behavior of value 0.7 as the index quite strange, then did the following test. The first output cell was executed directly as the second input cell. Have not ...
-1
votes
3answers
221 views

What would be a good way to implement index notation?

So I need to calculate a 3x3 rotation operator (basically a matrix). I have an index-notation expression, and I was wondering if with pre-calculated theta, K1, K2, and K3, could Mathematica compute ...
4
votes
1answer
66 views

Assign zeros to large list of indexed variables

I have a big list of the form list={P[1,1],..,P[i,j]} and would like to assign 0 to each of them, but ...
7
votes
1answer
175 views

Why does this simple sum function fail to compile?

Consider the following compiled function, which takes a $12 \times 5$ array $x_{ij}$ of real numbers and computes the triple sum $$ \sum_{k=1}^5 \sum_{i=1}^{12} \sum_{j=i+1}^{12} x_{ik} x_{jk}. $$ <...
19
votes
3answers
600 views

What are the requirements for a well behaved indexed variable? Subscript, ToExpression, Downvalue?

On writing this answer I needed to call a function, (NonlinearModelFit) with an unknown number of parameters. We have learned that we should not use ...
-1
votes
1answer
55 views

Position not working? [closed]

I have the list: myList={0,0,0,a,b,c,0,0,d} I need to find the positions of all elements that are non-zero. I do like this: ...
2
votes
1answer
93 views

How should I enter indexed terms? For example, constants $n_k$ for $k\in\{1,\,\ldots,\,N\}$

How should I enter $\quad \quad \sum^{N}_{k=0}{f(n_k)}$ into Mathematica? More generally, how should I work with the indices? Take the following as an example. I know how the ...
26
votes
2answers
2k views

Why do Mathematica list indices start at 1? [closed]

In most programming languages, container indices start at 0. This is not random or hardware-related; for example Dijkstra's article explains why zero-based indices make sense. What are the reasons ...
1
vote
1answer
85 views

Tiny question about Indexed[x,n]

Maybe I should apologize for this silly question, but the documentation for Indexed is very short and not explicit. I have an equation with variables ...
2
votes
1answer
146 views

How to swap tensor indices without permutation?

Say I have a tensor of rank R, How to swap $i^{th}$ and $j^{th}$ tensor index? I do not want to use TensorTranspose because it requires to write down the entire permutation, i.e. TensorTranspose[T,{1,...
0
votes
1answer
1k views

Minimizing using indices, error: Part::pkspec1: The expression cannot be used as a part specification

I want to use Minimize where the variables to minimize are indices pointing into an array. Here a MWE that hopefully shows what my problem is. ...
1
vote
1answer
129 views

Summation notation

I have the current setup for calculating Allan Deviation: which was coded as: Sqrt[Total[Differences[y]^2]/(2 (M - 1))] However, after doing some research, ...
1
vote
1answer
144 views

How to evaluate sum with different coefficient in each term?

I would like to know if there is a syntax that allows me to enter a sum that has coefficients that vary for every term? I have no interest in evaluating them numerically, but rather to keep them as ...
-1
votes
1answer
91 views

Beginner Question: Assigning an indexed Variable to a given list

I am pretty new to the game and have a problem, which i am sitting on for hours. 1) I created a random Matrix 5x5 Q=Table[Q[i,j]=RandomReal[{0.6,1}],{i,5},{j, 5}] ...
4
votes
0answers
112 views

Using indexed array elements as integration dummy variables with EvaluationMonitor. Bug?

As part of a routine that must cope with integration of varying numbers of dimensions, I would like to use indexed variable names (e.g., x[0], x[1],...) as dummy integration variables. However, it ...
2
votes
3answers
81 views

Referring to the components of a vector within a sum

I feel like I'm missing something obvious here, but.... I'm playing around, trying to do the standard deviation for my stats homework (don't worry, I already worked out the answer by hand), and I ...
-2
votes
1answer
126 views

Recreating in Mathematica a matrix created in Python [closed]

I need to recreate in Mathematica this matrix that I created in Python: ...
1
vote
0answers
51 views

Index change in a recursive definition

Assume I have the following recursive definition: ...
25
votes
7answers
760 views

Faster position list construction from a “take instructions” list

Take some list, e.g., {1,2,3,4,5}, and a "take instructions" list, say {1,1,2,1,1}. The latter is read as "take the first ...
0
votes
1answer
74 views

Recurrence evaluation at specific index

I have the following set of equations. I want to solve a[n] and f[n] for the indices n=1:5. Note that the relation a[2n+2] involves the derivative of f[2n] and the relation for f[2n+2] involves the ...
0
votes
1answer
68 views

Loops over multiple variables (negative i_min)

Can someone explain why i ranges from -3 to 3 in the output of Do[Print[{i, j}], {i, -3, 3}], but not in Do[Print[{i, j}], {i, -3, 3},{j,-i,i}]? I was thinking for the second input I would get {-3,-...
0
votes
1answer
169 views

Array element problem in optimization

I have the following piece of code in Mathematica: ...
2
votes
4answers
449 views

How do I find the position of the maximum value in each column of a table?

Given a table of values, I know how to find find maximum values, but how do I find the position of the maximum value in each column. For example, given ...
1
vote
1answer
259 views

Define a Recurrence Table involving other tables

I'm relatively new to using Mathematica. Previously I used Matlab, but lately I noticed that the numbers i got in both of them do not match (mostly it seems that Matlab lacks precision...). The ...
6
votes
1answer
142 views

Unexpected behavior from Indexed (threading and messages)

I was going to include Indexed in this answer, thinking that it operated much as a List-only ...
3
votes
1answer
224 views

Collect terms in a sum

Suppose I have this expression ...
9
votes
3answers
389 views

Numbering nested list according with Depth (or Level)

(This question is slightly similar with Sequentially numbering a nested list, but not exactly the same.) What I want Say I have a nested list ...
9
votes
6answers
505 views

Efficiently finding the positions of a large list of targets in another, even larger list

I have a big list. It contains nearly 60,000 sub-lists. It's structured like bigList = {{x1,y1,z1},{x2,y2,z2},{x3,y3,z3},.....,{x60000,y60000,z60000}}; I have ...
31
votes
9answers
844 views

Sequentially numbering a nested list

I've got a nested list {a, b, {c, d}, e, {f, {g, h}}} which I want to magically transmogrify to ...