Questions on the manipulation of List objects in Mathematica, and the functions used for these manipulations.

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1
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1answer
36 views

Let's do this list-rearrangement prettier [duplicate]

Suppose that i have a list in this format= $list=\{ \{a,obj1 \} ,\{c,obj2 \}, \{a,obj3 \}, \{b,obj4 \} \dots\}$ I want to construct a new list in this way: $ newlist= \{ \{ \{a,obj1\} , \{a,obj3\} ...
3
votes
5answers
188 views

Determine if all the zeros in a list occur at the end

I have a collection of (short) lists of nonnegative integers. For each, I would like a predicate that decides if all of the zeros in the list appear at the end. For example, ...
0
votes
0answers
32 views
6
votes
3answers
188 views

How to build a matrix from a set of vectors and other matrices using ArrayFlatten

I want to build a 6 by 3 matrix, from the following: one vector is 3 by 1, then a matrix, which is 3 by 2. These will go to the top 3 rows. Now the bottom 3 rows, I'll have 3 vectors each is 3 by 1. ...
2
votes
3answers
110 views

How to extract both first and last rows of each group from unbalanced panel data

This is a follow up question form my last question click here. I have unbalanced panel data such as : ...
5
votes
2answers
213 views

Generate cyclic list from a list

Given a list (for example, {3, 5}) I would like to make a function genCyclic[n_, list_] which generates a cyclic patterns of ...
0
votes
0answers
41 views

How to make Position work right? [on hold]

I have a variable called t[1]. This happens: a = { t[1], 1}; Position[a, 1] (* {{1, 1}, {2}} *) How do I make Position find the 1 and not variables that ...
1
vote
0answers
56 views

Find algorithm or generator formula with automated procedure and pattern recognition

I am trying to find a generator formula for the discriminant of a polynomial and believe it would be an interesting problem for the Mathematica community to know how to automate evaluation of certain ...
5
votes
4answers
206 views

How can I get a list of the positions of the running minima in a list?

I want the positions of the (sequential) running minima in a list, that is, the positions of entries that have the minimum value of all the elements in that list, up to and including that entry. For ...
3
votes
3answers
113 views

List manipulation: Dropping first or last row or column of a matrix

How do I drop the first/last row or first/last column from a matrix? The general case is answered here but I am new to Mathematica and it the arbitrary case is unnecessarily complex. Suppose I am ...
0
votes
1answer
34 views

Elements inside arrays seem to be strings rather than signed numbers

When I have code like b = Array[Subscript[be, #] &, {3, 1}] b = b /. {Subscript[be, k_] :> Subscript[-be, k] /; k > 1} I am noticing that Mathematica ...
4
votes
3answers
145 views

Replace elements in matrix to make it symmetric using automated procedure

I am generating a matrix using (mat2 = Array[Subscript[a, ##] &, {4, 4}]) // MatrixForm but would like to force Mathematica to make it symmetric. Thereby I ...
0
votes
2answers
40 views

Constructing a table from a constraint on another table

I have a 1000X6 table, where in each row the first three columns are dependent on the second three. I then make smaller tables of 1000X2 from these first three columns and basically ...
1
vote
3answers
149 views

Defining functions when there are a lot of parameters

Suppose I have two lists of parameters: $\delta=\{\delta1, \delta2, …\}$ and $\gamma = \{\gamma1, \gamma2, …\}$. The question is: how can one use $\delta$ and $\gamma$ (the parameter vectors) within ...
1
vote
2answers
59 views

Inconsistency in arithmetic on list elements [closed]

As expected, {a,b,c} - {a,a,a} (* {0, -a+b, -a+c} *) Or the same result can be obtained with less typing, thanks to implicit duplication of the subtrahend: ...
2
votes
2answers
90 views

Inverse of Times->Sequence

Suppose there is a list list={abc} then the replacement Times->Sequence gives ...
6
votes
3answers
315 views

Functional equivalent of a For loop referring to consecutive elements in a list

I have implemented this function which smooths the values in a list by, if it encounters a zero, replacing this entry by the average of its two neighbours (apart from the first and last values of the ...
1
vote
1answer
32 views

Replacing Logic operator List and generate combination of variables

so I'm trying to do the following starting from a list of inequalities: ...
12
votes
1answer
207 views

The efficiency compare between Flatten[#, 1] & and Join @@ # &

Err..Often I met the situation to join lists at the first level and I used to just Flatten[#, 1] &@ them. However, I found(when glance over the mathematica.stackexchange.com) someone else prefer ...
0
votes
0answers
20 views

Manipulating nested matrices columnwise [duplicate]

I have a project that requires analyzing large volumes of excel sheets at the same time. I have been having trouble figuring out how to index the imported files and manipulating them. The way I am ...
6
votes
4answers
155 views

Duplicate mapped elements in Riffle

What is the best way to do this? in: {a, b, c} {d, {e, f}, g} out: {{a, d}, {b, e}, {b, f}, {c, g}} ...
5
votes
2answers
142 views

What does IndexBy do and how exactly does it work?

I just saw the function IndexBy in the autocomplete suggestions and I was curious to know exactly how it works. Unfortunately, there's no trace of it in the ...
8
votes
5answers
389 views

How to randomly select subsets

I have a list of around 300 elements. I want to sample subsets of length 25 such that my samples are all distinct. My first inclination was to use something like ...
3
votes
1answer
62 views

Output function control and manipulation for efficient computation

I have taken note of a similar post related to modifying function parameters. I tried to adapt Kuba's solution to this post but I can't get it to work. I will illustrate my problem using a similar ...
2
votes
3answers
99 views

Plot a table of lists

I want to make a point plot of a table with lists in it. Assume I have data like this: data={{1,2,3,4},{5,6},{1,5},{2,4}}; and now I want to make a point plot ...
0
votes
2answers
43 views

Table results within loops

I have the following code. ...
0
votes
1answer
68 views

Iterating over a 2D list [closed]

I want to collect the result of a function in a data structure. In java the loop looks like the following code: ...
0
votes
1answer
51 views

Compute the evolution of several quanties from a data file

This question is somehow a part II of this: Part I. Here comes a small data sample as a minimal working example: ...
1
vote
2answers
84 views

How to select elements from a list by condition?

Considering I have a list: F = {f1, f2, f3, f4} I want to perform exponent on the list F = Power[#, 2]& /@ F Now I ...
1
vote
4answers
113 views

Pure function, one slot refers to List-position

I used all my google-fu trying to find how to make this work; read the docs as well. What have I missed? I feel sure this can't be a new question, but I can't find anything Here is is: ...
0
votes
0answers
29 views

Remove list head to insert as arguement [duplicate]

I'm trying to use a set of terms, e.g., {x,y,z}, as an argument for another function with doesn't take in a list but a series of variables (e.g. need ...
5
votes
2answers
895 views

Dramatic speed difference of code on Matlab and Mathematica

Background: I was trying to convert a Matlab code (fluid simulation, SPH method) into a Mathematica one, but the speed difference is huge. Matlab code: ...
1
vote
0answers
53 views

List manipulations with ; and ;; [closed]

I have the following list: L = {{a1,a2,a3,a4},{b1,b2,b3,b4},{c1,c2,c3,c4}} By using just the ; or ...
0
votes
2answers
80 views

Making table and plotting results of NDSolve problem

I am trying to solve two equation to find /[Phi][x] and plot h vs. /[Phi][x] for different values of L.I tried by writing as: ...
0
votes
1answer
61 views

Code optimization, Pad missing keys and Total part of values [closed]

Code optimization, Pad missing keys and Total part of values ...
1
vote
2answers
60 views

Encoding variables

I'm trying to fit a Logistic Model using LogitModelFit in Mathematica. The data I'm using contain qualitative variables that I have to encode. I've searched in ...
2
votes
1answer
107 views

Functional programming approach to avoid traditional loops

I'm currently learning from the wonderful post: Where can I find examples of good Mathematica programming practice? And i'm correcting my habits using Nest/Map/Thread instead of loops. The problem ...
0
votes
0answers
47 views

Appending elements to a list in parallel table

Is there any way to store values in a list that is being computed in parallel? For example, if ParallelTable is computing some ...
0
votes
2answers
118 views

Contract two lists

Suppose I have two lists of the equal length in the form powers= {{rho th},{mu},{beta},{nu},{r},{}} ...
0
votes
0answers
22 views

Iterating a model with 2nd order difference equations using Manipulate [duplicate]

I have a model including first and second order difference equations. It works for the case of first difference equation, which is below: ...
0
votes
0answers
33 views

Error with AppendTo [duplicate]

I am trying to write a function that builds a graph, or - better said - adds a layer of leaves to a pre-existing graph. Basically, I hand to the function three lists. ...
0
votes
0answers
25 views

Extracting subscript and superscript [duplicate]

I have an expression in the form Power[Subscript[g,a],mu] and I need to extract "a" and "mu" from this expression. It should be very simple but I could not find ...
4
votes
2answers
94 views

Finding column-wise ranks of values in a multidimensional list

I often have to find ranks of values in a list. However, Mathematica does not have built-in functions for that, or may be I'm just not aware of them. I can easily find ranks of a row/column vector. ...
1
vote
1answer
147 views

LinearModelFit[] of simulated data

I'm trying to simulate Hooke's law, f = k * x, so I generate data with the code: ...
1
vote
1answer
68 views

Importing, sorting and exporting lists

I have several TSV lists (.txt) of something like 2000*2000 numerics (e.g. 1.8645) to import ...
0
votes
0answers
64 views

Autocorrelation of 3D Points

I'm trying to compute the autocorrelation of a list of points (x,y,z) in 3D. Can the CorrelationFunction or maybe ListCorrelate ...
-1
votes
1answer
41 views

Convex Combinations of List Elements

Let's say I have two lists $L_1=\{a_1,...,a_5\}$ and $L_2=\{b_1,...,b_10\}$. Is there an easy way to make new lists from convex combinations of these two lists' entries? Let me be more specific: 1 ...
1
vote
6answers
149 views

Converting two lists into a list of tuples [duplicate]

I wonder how I can turn the list $\{a_1,...,a_n\}$ into the new list $\{(a_1,1),...,(a_n,n)\}$, i.e $a_k \mapsto (a_k,k)$, how do I do that? I figured the function Tuples might work, but it just ...
0
votes
1answer
36 views

Delete String sub list

I have a list with several sub lists containing either strings or integers. I'd like to delete all list with strings. The following list ...
3
votes
1answer
63 views

Finding pairs where the intersection of them is empty set from a nested list

It would be better to give an example first. Suppose I have a nested list of the form list = {{ 1,2,3,6}, { 3,6,10,20}, { 4,7,11}, { 10,20}}; where each element ...