# Tagged Questions

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

409 views

### For a given expression: if it appears, remove it, but if it is absent, add it

While reformatting Szabolcs's code from (42660) I noticed this interesting operation: ...
184 views

### Faster position-based duplicate removal in a ragged array?

Take a ragged 2D array: rArray={{4, 2, 1, 0}, {2, 2}, {0, 0, 3, 3, 3}, {4, 0}, {3, 4}} I need to "deduplicate" this, where a duplicate is any element of a ...
305 views

### Threading over a list of lists of lists and a list of atomic expressions

Although I solved the following problem using MapThread, I'm curious about why Thread does not work as I expect it to here. ...
199 views

### Why can't I use Sequence to perform a Select like task? [duplicate]

Suppose I wanted to write down a list of $p^2$, for $p$ a prime between $1$ and $20$. I would expect Table[If[PrimeQ[k], k^2, Sequence[]], {k, 1, 20}] to work. ...
2k views

### Estimating the full width half maximum of a peak

I have a dataset of which I'm attempting to determine the full width at half maximum. A picture of the data is given below, and the dataset itself can be found here (pastebin). Now, this set has a ...
579 views

### Find all the possible ways of partitioning a list into a set of pairs of element

I have a list {x1,...,xN} where N is even, and I need to find all the possible ways to split it into pairs of elements, e.g. the ...
233 views

### Map efficiently over duplicates in list

I am interested in mapping a function, f over a (long) list which contains duplicates. Since this function takes a while to evaluate I would like to make use of ...
307 views

### How to find longest monotonic sequence?

I have a list, for example list = {2, 1, 3, 5, 4, 6} How to find longest ascending sequence of this list? There are two meanings of this question: Find the ...
194 views

### Behavior of Inner depends on List dimensions?

Consider the following case: ...
102 views

### Speed up Dynamic for graphical interface

I am trying to create a user interface where I can click on certain points in a set of data and save them into a list that can be used later in my code. The problem is it is currently sluggish. Is ...
249 views

### Why does Outer sometimes return a packed array, and sometimes not?

For example In[342]:= test3 = Flatten[Outer[Plus, Range[1000], Range[1000]], 1]; // AbsoluteTiming Out[342]= {0.0198735, Null} returns PackedArray (...
274 views

### Can I return lists with different dimensions from a compiled function?

Sometimes I'll need several lists with different dimensions to be returned by a compiled function, but compiled function will fail if an irregular list contained: ...
138 views

### Bad interaction between Join and ListPlot (or Plot)

When you write this code: tab = {{1, 1}}; tab = Join[tab, {{2,2}}]; you expect to get the following table: {{1, 1}, {2, 2}} ...
1k views

### Flatten at a certain level

In an earlier question the brainteasing fourth syntax variant of Flatten was discussed. I see that with ...
154 views

### Is there is a design problem or documentation problem with FirstCase?

In version 10 the new function FirstCase can take up to 4 arguments (plus option). The third argument is a default value to be returned if there is no match: ...
445 views

### How to merge Datasets representing matrices with named rows/columns?

I am working with square (non-symmetric) matrices with named rows and columns. The data I receive is sometimes messy and rows/columns may come in various permutations. I usually need to semi-...
849 views

### Best way to emulate MATLAB's bsxfun using Map and similar functions

Given a matrix, we want to subtract the mean of each column, from all entries in that column. So given this matrix: ...
1k views

### Position of First nonzero number

I want to know is there any one line code to find position of first nonzero number without calculating position of all non zero numbers. For example, ...
666 views

### How to find range in which a number falls, from given list of numbers?

How can one find the range in which a number falls, from given list of numbers? ...
682 views

### Flatten nested lists

I have a list as follows lis= {a, {{{b}, {c,d,e}}, {{f}, {g,h,i}}}} which I would like to flatten to ...
511 views

### Unique Element in List

I have a list a, and want to find out whether there is a unique element in the list. For instance a={x,y,z,y,x} has an unique ...
632 views

### How to use Map inside MapThread?

Given this input lst1 = {{a, b, c}, {d, e, f}}; lst2 = {1, 2}; and the goal is to generate this output ...
685 views

### Matrix indexed by one number

I'm trying to find a generic way (for arbitrary dimensions) to create a matrix like so a[1] a[2] a[3] a[4] i.e where it is indexed by one number, instead of say ...
308 views

### Searching linked lists that contain lists?

Following the advice I've read here and other sites, I've been trying to use the Mathematica equivalent of a linked lists... ...
645 views

### Creating a simple function to compute the average of the difference between pairs of elements in an array

I have a list of the form: {{{1, 2}, {3, 4}}, {{5, 6}, {7, 8}}, {{9, 10}, {11, 12}}} How can I create a function which will take each set at the highest level of ...
2k views

### How to sum up subelements of a list of unknown length

I'm simply looking for the {}+{} function for sublist elements of a list: (Parallel sums of elements) ...
668 views

### How do you efficiently return all of a List but one element?

I have a problem that I am working on that requires that I compare several lists of overlapping data points. As I traverse one of the lists, I'd like to only compare those data to the other lists, and ...
778 views

### How to rebuild original data from a histogram?

I have data in the following form. I have a list of values and a list of frequencies that indicate how often each value has occurred. Example list of values: ...
587 views

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### Partitioning a list by recursion (Programming Paradigms via Mathematica (A First Course))

I am working through the course Programming Paradigms via Mathematica. One of the homework exercises for the section Recursion I: Passing the Buck is as follows: Use recursion to partition a list:...
631 views

### Span a function across several consecutive elements in a list

Suppose that I have a list called list. I would like to span an arbitrary function f across specified consecutive elements in <...
596 views

### How to find first list element that differs from average of N previous elements by more than a given amount?

Given a list of real numbers, I need to write a function that returns the value of the first element in the list that differs from the mean of the previous $N$ elements by greater than $P$ standard ...
272 views

### Extract left-upper triangular data from a square matrix-form list

Hopefully, this question is not too basic or obscure. I'd like an idea on how to extract this {{1}, {5, 2}, {9, 6, 3}, {13, 10, 7, 4}} from ...
1k views

### How to subtract one list from another, treating each elements as distinct, assuming the smaller list is a subset of the larger list? [duplicate]

Suppose that I have two lists of different sizes, the smaller of which is a subset of the larger, e.g. a = {"A", "A", "A", "B", "B", "C"} b = {"A", "B"} How can ...
944 views

### Count consecutive positions with the same value

I want to program a function that counts the successive positions with the same value: In[]= ConsecutiveValues[{1, 1, 1, 1, 2, 2, 1, 0}] Out[]= {4, 2, 1, 1} In ...
439 views

### Finding time-series direction reversal of certain magnitude

I'd like to find the points of a time-series that are a certain distance away (in value, not in time) from the previous maximum, which I consider a reversal. For example, for ...
337 views

### Efficient way to make subsets of list with placeholders

I have an arbitrary list of unique elements: lst = {a, b, c, d} Documentation allows finding subsets with same number of elements, say ...
202 views

### Timing problem with ReplaceRepeated

I have the following data: ...
426 views

### Repeating Digits

I have written ...
431 views

### Fast way to get positions of “boxed” array elements?

Given a 2D array (arbitrary numeric / evaluates to numeric elements, simplified for examples, and can be arbitrary dimensions so long as the operation makes sense, I'm working with 2K X 2K in my ...
424 views

### Modifying strings in a list efficiently

I have a list of strings $\{s_1,s_2,\ldots,s_n\}$. I would like to to produce the following list of strings: $$\{s_1<>1,\ 1<>s_2<>1,\ldots,\ 1<>s_n\}$$ In other words, I want ...
339 views

### Split according to List and apply rule

I have a list which looks some thing like this {a1, a2, a3, a4, a5, a6, a7, a8, a9, a10} Now I want to split it according to lengths ...
444 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 ...
390 views

### Complement of multisets

Given lists $a$ and $b$, which represent multisets, how can I compute the complement $a\setminus b$? I'd like to construct a function xunion that returns the ...
670 views

### Count consecutive occurrences in a list above a certain value

I am trying to tally and sort the number of consecutive occurrences in a list above a certain value. For instance: ...
950 views

### how to efficiently apply function to all pairs of a set (and collect the results)

To build a graph, I need to apply a function f[a_, b_] to all pairs of a list (3500 elements). The function itself returns a link ...
346 views

### Using Gather to rearrange my data

I have a list of elements: {{1, 2}, {1, 5}, {2, 6}, {3, 7}, {4, 8}, {5, 6}, {5, 9}, {9, 13}, {10,11}, {11, 12}, {11, 15}, {15, 16}} I have to divide it into ...
295 views

### Convert elements of list to string

how easily can I convert elements of this list to string. If I have output like this: {{M, 1, 0, 0, 1, 1, 1, 0}, {A, 0, 0, 0}} And I want something like this: <...