# Questions tagged [list-manipulation]

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

358 questions with no upvoted or accepted answers
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387 views

### Strategies to find curves on messy data using FindCurvePath

I have a set of data points that correspond to roots of a transcendental equation with respect to a parameter (the abscissa) which, by the properties of the system, define continuous curves (might be ...
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115 views

### What changed in String handling between 11.0 and 11.3?

I was trying to optimize some String-handling code here using my standard trick of converting it to a list of bytes first and using basic list operations instead of ...
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75 views

### Different timing with large array assignment

Consider the following example n = 10^8; AbsoluteTiming[ A = ConstantArray[1., n]; // anyFunc; A[[2 ;; ;; 2]] = -1.; ] {0.388108, Null} ...
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### Predicting Pick performance

I'm trying to figure out how to use Pick most efficiently. I use it already in a number of places, but only ever in the context of ...
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152 views

### About efficiency of modifying large list in place

In Wagner's book "Power Programming with Mathematica" page 303. Wagner metioned do not "modify large list in place" He take two example ...
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74 views

### Why can't (partially) packed arrays be used in this example?

Bug introduced in 11.3 or earlier and persisting through 12.0 or later I didn't expect any difference between these: ...
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257 views

### How to create a prefix code

I am trying to create a demonstration of shannon code in mathematica. Given a list of lengths (l1, l2,..lk) I need to create a list of binary lists (or list of ...
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95 views

### Graph problem with IGraphM package

Consider the graph: ...
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75 views

### Join on QuantityArray

I need to merge two QuantityArray objects of mixed type. My first attempt looks something like: ...
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95 views

### SimilarityRules for SequenceAlignment of two lists

I am trying to use the SimilarityRules option of SequenceAlignment to customize its output. This works perfectly for strings, ...
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57 views

### Fastest way to change the values of an association

I have an association of the following form : assoc = Association[a -> {1, 2, 3}, b -> {5, 6, 7}] <|a -> {1, 2, 3}, b -> {5, 6, 7}|> (the size ...
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79 views

### What exactly is the difference between Part and Extract?

From the statement in the documentation: Extract[expr,{i,j,…}] is equivalent to Part[expr,i,j,…]. So basically Extract is just a listable version of ...
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113 views

### Graph Partition

Consider a graph 'g': ...
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83 views

### Why does Shortest not work inside SequencePosition?

Why does Shortest not work in a SequencePosition call? The following two commands return the same answer: ...
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60 views

### Speeding up MovingMap of Dot

I have the following code (showing an insignificant simplification) that I really need to run many times for different ps (its length can be 10^7 or so) and wSize. Is there a way to speed this up ...
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205 views

### Transpose nested Association

Say I have a nested Association like this one: ...
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276 views

### Extract a key from an Association?

I have some large nested expression containing associations and I'd like to extract the nth key inside one of them, using part syntax. Here's a tiny example: ...
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105 views

### How to deal with “Compile::cset” in Compile

When compiling below code Compile[{{data, _Integer, 2}}, Fold[Flatten /@ Flatten[Outer[List, ##, 1], 1] &, data]] I encountered error messages Compile::...
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91 views

### Optional with non-empty list requires PatternTest/Condition?

Arbitrary lists are easy to use with Optional: a~MatchQ~a[0, Optional[x_List, {}]] a~MatchQ~a[0, Optional[x:{___}, {}]] ...
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### Create an adequate 'training set' to train a ClassifierFunction which performs the role of the built-in SubsetQ

I am trying to "grow" my own SubsetQ function using Machine Learning methods. My cSubsetQ when given two lists (listA and listB)...
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195 views

### Bug in ListLinePlot[] when Filling up and down?

Bug introduced in 9.0 or earlier and persisting through 11.0.1 or later I'm trying to understand this apparently incorrect Filling behavior in ...
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177 views

### ParallelTable and expensive functions

I would like to build up a 2D-Table using ParallelTable as follows ...
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177 views

### Gather dependency on list order?

I get intrigued about Gather behavior on this post. I thought that Gather was invariant under list order? Here is the result I ...
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95 views

### Sowing Once into Each of Many Lists – Any Better than Append?

In general I know to try to use Reap and Sow over Append or AppendTo due to superior performance. However, Reap and Sow are notorious for being difficult to understand how to use and how they work ...
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66 views

### Why is ContainsOnly so slow for checking a list is full of zeros?

I'm on Windows 10 Mathematica 12.1.1. I would expect allocating a big ConstantArray and comparing would be slower but apparently it is not: ...
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53 views

### Transforming a checkerboard using Transpose and ReshapeLayers

In the paper for RealNVP, they introduce a "squeezing" operation that takes a matrix and transforms it to have more channels, according to the following transformation (before and after): As you can ...
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44 views

### Bags don't work in parallel code?

Appending to a list in parallel works: b = {}; SetSharedVariable[b] WaitAll[Table[ParallelSubmit[{p}, AppendTo[b, 1]], {p, 3}]]; (* b is now {1,1,1} *) But ...
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60 views

### Net layer that replicates last element

I am trying to design a neural network layer that takes in an array of dimensions n x 100, and produces an array of dimensions ...
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72 views

### Strange behaviour with Differences

I have some problems with the built-in function Differences. I do not understand how its behaviour comes about under the following circumstances: ...
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1k views

### Is it possible to reshape with one unknown dimension in Wolfram?

In Python I can reshape array remaining one dimension unknown, allowing Python to infer it automatically: ...
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207 views

### Finding possible lattice planes of a crystal structure

After generating a crystal structure from a crystallographic data and duplicating it to a larger crystal system I would like to find possible lattice planes of this crystal. It is well-known that the ...
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72 views

### Can a simple Part expression produce a list of elements from nested lists?

If list1d is a simple 1-d list, then list1d[[{i, j, k}]] evaluates to a list consisting of the ...
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56 views

### Applying a function along the desired dimensions of a n-dimensional array

I am trying to apply an arbitrary function f that takes several inputs and returns a scalar along some specific dimensions of an n-dimensionnal array. Here is an ...
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74 views

### Incrementally building a list without memory overhead

I have a task, where I have to combine pairs with a function, but the number of pairs I actually need is much smaller than the number of all pairs. The condition to keep the element is some function ...
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56 views

### Test which symbol of a list depend on some variable

Given a list of symbols, say {y[x],z[x],k} Is there a way to sort which depend on x and which don't ? Applied to the above list,...
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56 views

### Map entries of StructuredArray

I have the following defintion arr = SymmetrizedArray[{1, 2} -> 1, {4, 4}, Antisymmetric[{1, 2}]] which gives a ...
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111 views

### How to make reading from long lists faster?

Background I am using Mathematica to solve a traveling-salesman-like problem. I implemented two different algorithms that are able to solve this type of problem and both algorithms yield the same ...
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94 views

### How to ArrayFlatten some matrix which have repeated element

I have many such matrices(whose dimension are all 3*3.) like: ...
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98 views

### Mysterious Riffle::rspec warning

I have part of a code that looks like this: ...
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692 views

### Taking real and imaginary parts of indexed functions and speeding up ComplexExpand

I am setting up a large system of ODEs and in order to use the IDA method (which is sig. faster for my system and thus attractive), I must split my equations into real and imaginary parts. I am ...
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32 views

### Example of when explicitly using MapThread is better than giving a function the Listable Attribute

This answer says that Listable just makes the function automatically apply MapThread to its arguments. Does anyone have a simple ...
2answers
105 views

### Restrict kinds of heads when dealing with level concept

In Map[(# + 7) &, {a, 3 b, {c, d}, e}, {2}] Out {a, 10 (7 + b), {7 + c, 7 + d}, e} But I want feature like ListMap : ...
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48 views

### Handling lists indexed with integer partitions

I am trying to do some exploration around the multi-dimensional version of [truncated moment problem], where I work with various distributions (uni- or multi-variate), and use some truncated moment ...
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87 views

### Repeated calling of a ParametricNDSolve result in a faster way?

I recently learned of ParametricNDSolve and found that I need to compute it once and then call its result sol at different ...
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42 views

### Error using Standardize

I'm trying to standardize a set of data. ...
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76 views

### For any countable totally-ordered $A$, how can mathematica show my choice function gives a unique collection of finite sets whose union is $A$?

Suppose $A=\sqrt{\mathbb{Q}}_{>0}$. We want a unique $\bigcup\limits_{i\le j}A_i$ for $i,j\in\mathbb{N}$ such that $A_i$ is finite, $A_i\subseteq A_{i+1}$, and \$\lim\limits_{j\to\infty}\bigcup\...
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42 views

### How to use Cases to delete specific lists from a nested list?

To simplify things, let's say that I have a list: aList = {{1,2,3,4},{2,2,6,6},{6,9,8,7},{6,6,6,3}} and I want to use Cases to get rid of the lists that have more ...
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59 views

### How to work around TuringMachine's lack of a halting mechanism

It looks like the TuringMachine function doesn't have a halting mechanism. When I say "halting mechanism", I mean something that's built-in to the ...
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41 views

### ListContourPlot and ListPlot3D return empty (white) plots when scaled

using Mathematica 11.1.1.0. I could not find this topic before. Look at these simple data plots: ...
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88 views

### Using Fold[ or Map[ Better?

I'm working through MMA Cookbook by Mangano, page 54. He shows Map is a compact form of Fold function. Is using ...