# Tagged Questions

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

3answers
351 views

### Find duplicates in list of InfiniteLine

MMA 10 introduced a new function, which can be very convenient: InfiniteLine. Of course, two infinite lines can be described by different arguments: for example <...
6answers
1k views

### Generating an Ulam spiral

An Ulam Spiral is quite an interesting construction, revealing unexpected features in the distribution of primes. Here is a related topic with one answer by Pinguin Dirk, who has provided one ...
8answers
546 views

### Splitting a list into increasing sublists

I have a list of numbers (say, for example, {1,2,3,4,3,2,1,2,3,4,3,2}), and I want to split it into sublists so that each sublist is the (smallest) sublist whose ...
4answers
413 views

### MapThread with non-rectangular lists

MapThread works fine and dandy with rectangular list structures: MapThread[f, {{{a, b}, {c, d}}, {{1, 2}, {3, 4}}}, 2] <...
3answers
497 views

### Generating Tuples with restrictions

I am interested in a certain subset of all tuples. Generating all with Tuples and then filtering is wasteful, and will "blow up" very quickly. For a concrete ...
6answers
581 views

### Elegantly pairing up mismatched lists

HistogramList returns a list of bin boundaries and a list of counts. There is one more boundary than counts, and I'd like to pair them up so I can feed it into <...
1answer
1k views

### Solving and Animating Three Body Problem

I am attempting to solve a three-body problem using the Lagrange formalism, with a 1/r potential. I started off by defining T and U (kinetic and potential energy) as follows: ...
4answers
1k views

### List comprehension in Mathematica

Python supports a concept called "list comprehensions". Following is sample example ...
2answers
576 views

### Are there advantages to using generalized Part extraction instead of specialized functions like First, Last?

There are many ways to extract different parts of lists in Mathematica. For example, the first part of a list v can be accessed either as ...
2answers
453 views

### Conditional Gathering of lists

Just need a little help with the GatherBy / SplitBy function(s). I have a list of random numbers here: ...
3answers
766 views

### How to zero (or replace) the diagonal of a square matrix?

Zeroing the diagonal of a square matrix is an operation I need frequently, but somehow I still haven't managed to find an elegant solution that satisfies all these requirements: works with both ...
2answers
307 views

### Which elements of a large array are part of a set?

I have a large integer packed array arr. My array is multidimensional, but for simplicity let's consider 1D arrays for now. ...
2answers
219 views

### Why does Part return empty list for non-existing elements in ;; specification?

When using the ;; specification Part will return empty list for first index above a list dimensions. For example: ...
3answers
741 views

### How to partition a list to make each subset's size equal and mean as close as possible

Suppose we have 3 lists each with 18 numbers like this: ...
2answers
251 views

### Bug with SparseArray selector in Pick

Bug introduced in version 8 or earlier and fixed in 10.0 I have created a notebook with two cells. This is the content of the first: ...
3answers
536 views

### Solving the magic backyard puzzle with MMA

I was trying to solve this puzzle using mma It looks like the generalization of the magic square. Anyway here my attempt:- There are four columns $(x,y,z,w)$ and nine rowa which are denoted by the ...
5answers
623 views

### Interlacing a single number into a long list

This seems like it should be a simple question, but I am running into some difficulty in doing this with Mathematica. Right now, I have a list like this: ...
9answers
532 views

### Distributing elements across a list of lists

I have two lists: u = {{1, 3}, {2, 6}, {3, 9}} v = {0, 4} and I want to obtain this list from them: ...
6answers
752 views

### How to find the distance of two lists?

I have two lists as the followings: ab = {1, -1, -1, -1, 1, 1}; ac = {1, -1, -1, 1, 1, 1}; How I can find the difference (more precisely, the edit distance) ...
9answers
718 views

### Thread over list in different levels

I have these two lists, both with the same length in the first dimension: ...
6answers
664 views

### How to find all the positions of max value of a list efficiently?

For Example, list=RandomInteger[{1,100},2000]. Yes,I know Position[list,Max[list]] can do. But it's based on pattern matching!...
6answers
3k views

### Counting negative values in list

I would like to count the negative values of a list. My approach was Count[data, -_] which doesn't work. How can I tell Mathematica to count all numbers with ...
12answers
681 views

### Join lists with nested list

Is there a way of smarter way of joining list of the form l1 = {a,{b,c}}; l2 = {d,{e,f}}; l3 = {g,{h,i}}; To obtain ...
8answers
544 views

### How to neatly get the sum of symmetric elements in a list?

The task is to compute symmSum[{a, b, c, d, e, f}] (*==> {a+f, b+e, c+d} *) symmSum[{a, b, c, d, e}] (*==> {a+e, b+d, c} *) My clumsy solution is ...
5answers
400 views

### Pick elements from list “separately”

I have the following list I want to pick elements from: mylist = {1, 5, 7, 3, 4, 7, 2, 9, 9, 1, 10, 12, 2, 64, 34, 64} I have another list with 0's and 1's which ...
6answers
2k views

### Shuffle product of two lists

I want to do the following: I have two lists {a_1,...a_n}, {b_1,...,b_n} and I would like to build now all shuffles out of this. This means all unions of these ...
6answers
826 views

### What's the cleanest functional-programming style way to perform this list operation?

I am looking for the correct method for accomplishing the unknown piece in the following code snippet: rolls = Tuples[Range[1, 4], {2}] ...
8answers
1k views

### DeleteCases on list of Strings

Consider the following: data={"AB","CD","AF"}; Now I would like to delete all String from data which starts with "A". Result:...
4answers
405 views

### A replacement problem

list = {1, x, 3, x, 4, x, x, 3, 4, 3, x, 2, 3, x, 8, 7}; If there is exactly one integer between two x I want to replace it with y: ...
7answers
401 views

### Get the first element after the first sequence of length N of consecutively increasing values

I'm looking for an efficient way of extracting the first element after the first sequence of N consecutive elements in which the values are increasing. If these are the data of a toy example: ...
4answers
929 views

### Implementing a function which generalizes the merging step in merge sort

One of the key steps in merge sort is the merging step. Given two sorted lists sorted1={2,6,10,13,16,17,19}; sorted2={1,3,4,5,7,8,9,11,12,14,15,18,20}; of ...
7answers
858 views

### A function that accepts a pair or a list of pairs

Probably a duplicate, but it's not easy to search on "lists". I have a function that accepts a list of lists, say {{a1, b1}, {a2, b2}, {a3, b3}} and performs some ...
5answers
300 views

Test case: ...
5answers
313 views

### Apply multiple functions to parts of a nested list [duplicate]

data = {{1, a, x, "one"}, {2, b, y, "two"}, {3, c, z, "three"}} I want to apply a list of four functions {f, g, h, m} one ...
7answers
1k views

### How to Set parts of indexed lists?

I would like to assign a list to an indexed variable and then change it using Part and Set like this: ...
3answers
609 views

### Determining all possible traversals of a tree

I have a list: B={423, {{53, {39, 65, 423}}, {66, {67, 81, 423}}, {424, {25, 40, 423}}}}; This list can be visualized as a tree using ...
2answers
622 views

### Reproducing nested loops using Map?

I keep producing bits of code like the following: ...
3answers
470 views

### Quick multiple selections from a list

What is the fastest way to make multiple selections from a list? Compiled methods included. For example, here are two methods for selecting a subset, compared:- ...
2answers
3k views

### How to use “Drop” function to drop matrix' rows and columns in an arbitrary way?

The built-in function "Drop" can delete a Matrix's row and column. Typical syntax for "Drop" is as follows: Drop[list,seq1,seq2...] But what if I want to drop a ...
2answers
259 views

### Part like Delete: How to delete list of columns or arbitrarily deeper levels [duplicate]

On trying to write this answer I reached the frustrating realization that I didn't have an efficient way to delete a list of columns or deeper level components in a simple way as ...
1answer
264 views

### Ordering function with recognition of duplicates

Fairly often I have a need to get the Ordering of an expression but with recognition of duplicates. For example: ...
4answers
292 views

### Fast Selections Testing on Inequality

Is there a functional idiom for fast selections testing on inequality? For example, finding values from list a & b where the key is less than or equal to a value, in this case 4 or 9. An ...
2answers
252 views

### Fastest way to sum the upper triangle

I feel like this is an recurring question: if there's a symmetric matrix whose diagonal is not all 0, how could I get the sum of the part of it that's above the diagonal as fast as possible? Small ...
1answer
617 views

### How can I define a Listable function only apply to a vector?

Consider the function f which should behave as f[{{1, 2}, {3, 4}, {5, 6, 7}}] ...
2answers
164 views

10answers
647 views

### How can I generate a nested array like this one?

Well, this is how the array looks like: I came up with this code: n = 10; Nest[ArrayPad[#, 1, 1 - #[[1, 1]]] &, {{1}}, n] /. {1 -> "*"} Any better ...