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

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10
votes
3answers
314 views

How to “ignore” an element of Map or MapIndexed

Say I have some function that I'm applying every element in a list to... if that element matches some criteria: ...
10
votes
3answers
559 views

What is the most efficient way to map a linked list?

In order to work with lists effectively using common recursive patterns from functional programming, it's often most convenient to worked with the "linked list" representation, that looks like this: ...
10
votes
4answers
723 views

Efficiently Visualising Very Large Data Sets (without running out of memory)

I have put a few really hard problems in combinatorics up against Mathematica 8. I'd have to say that it works really well, until you want to view the data. If you look at my question Advanced ...
10
votes
2answers
550 views

Faster “stuttering” accumulate?

Given some list of arbitrary precision numbers, e.g. (actual lists are 1M+ elements long): test={0, 2, 2, 47839283, 2, 0, 0, 2, 0, 1, 2, 0} I need to accumulate ...
10
votes
2answers
338 views

Select cases from a list

I have a list in the form : {1,1,1,0,-1,-1,1,1,1,0,0,0,0,-1,-1} No I'm trying to select only those sequences which start with 1 have a zero (or many zeros) ...
10
votes
4answers
409 views

Building a tree

Given a list of word characters, such as this one, I'd like to build a tree, similar to this makeTree function, but with the tree in a different format. So, for an input such as ...
10
votes
4answers
312 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] ...
10
votes
3answers
391 views

Zero-dimensional matrices

In a recent Mathematica project, $(n \times 0)$- and $(0 \times n)$-dimensional matrices have suddenly become a frustratingly common edge case for me. For instance, consider the following two ...
10
votes
2answers
3k views

How to get Matlab data imported with the same dimensions?

I have some Matlab image data with the following dimensions (output is from Matlab): >> size(im) ans = 86 86 3 45 The data imports just fine ...
10
votes
1answer
202 views

Unexpected result from Flatten

Consider the following three evaluations involving Flatten: Flatten[f[f[f[x, y], 1, 2], f[x, y]], Infinity, f] f[x, y, 1, ...
10
votes
3answers
388 views

Generating Linear Extensions of a Partial Order

Given a set $S$ and a partial order $\prec$ over $S$, I'm looking for a way to "efficiently" generate a list of linear extensions of $\prec$. Suppose the partial order is given by a ...
10
votes
1answer
585 views

Modifying a List in a function in place [duplicate]

An example will be most specific: func[list_, column_] := list[[All, column]] = Map[#*2 &, list[[All, column]]]; This throws errors. I want to avoid doing ...
10
votes
1answer
222 views

Drop selection of columns from a ragged array

I have an application where I need to drop some arbitrary list of columns from a ragged array (where of course the shortest array rows have at least all the specified columns). E.g., given a ragged ...
10
votes
3answers
241 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: ...
9
votes
8answers
525 views

Adding elements to a collection of lists

I would like to add element a at the start of a series of lists. For example I have a collection of lists called ...
9
votes
8answers
643 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, ...
9
votes
7answers
385 views

How to efficiently Append a result of an operation on each element of a list to itself

I'm looking for the best function to apply the product of the last two elements of sublist elements to each element: Example: ...
9
votes
4answers
4k views

How can I multiply matrix and vector element wise like Numpy?

I have an $M \times K$ matrix $C$ and a length $K$ vector $v$. I want to form the $M \times K$ matrix $S$ where $s_{ij} = C_{ij}/v_j$. To do this in numpy I simply divide the two objects like this ...
9
votes
4answers
277 views

Make a ragged list rectangular by trimming instead of padding

I have a ragged list ragged = {{a,b,c,d,e},{x,y,z}} that I would like to trim (on the right) to be rectangular. The desired result is ...
9
votes
4answers
578 views

Find positions of items in a list based on neighbours

How can I select items from a list where the selection criterion depends on the neighbouring items? For example select all indices where a change has occurred with respect to the previous element in ...
9
votes
5answers
545 views

Counting the population of integers

Suppose that myData is a list of sublists. Each sublist has a length of one or greater and contains any number of replicates of the integers 1, 2, 3, and 4. I ...
9
votes
6answers
2k views

extract values from replacement list

Solve returns a list of replacement rules In: Solve[x + y == 3 && x - y == 6, {x, y}] Out: {{x -> 9/2, y -> -(3/2)}} I am only interested in the ...
9
votes
4answers
1k 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) ...
9
votes
5answers
327 views

How can I remove matching sublists from my list?

I want to use DeleteCases (or any appropriate function) to remove elements of a list, which in turn are lists of fixed size. The rule I wish to apply is that any ...
9
votes
5answers
2k views

How to find rows that have maximum value?

Suppose if I have following list { {10,b,30}, {100,a,40}, {1000,b,10}, {1000,b,70}, {100,b,20}, {10,b,70} } How to find rows that have max value in ...
9
votes
5answers
425 views

Take both of the diagonals through an element in a matrix

I have a matrix like this: Reverse@Array[List, {8, 8}] // MatrixForm I need a function that given an element of this matrix will return all the elements ...
9
votes
3answers
267 views

Splitting a list in which figures vary from negative to positive

Consider the following list: data={4078, 256, -1266, -388, 1290, 1368, 1446, 1524, 2}; I would like to split any kind of such list always, when figures turn form ...
9
votes
5answers
365 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 ...
9
votes
4answers
619 views

Using Position (which requires a pattern) in the same manner as Select (which requires a criterion)

I am using Mathematica7. I have a list of lists (creatively named list in the following sample code) containing data. I would like to find the positions of the ...
9
votes
5answers
369 views

Repeating Digits

I have written ...
9
votes
8answers
355 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 ...
9
votes
6answers
307 views

Using MapIndexed only at certain elements of a list

MapIndexed is a very handy built-in function. Suppose that I have the following list, called list: ...
9
votes
3answers
240 views

What is the best way to flatten a linked list with lists as values?

By following the advice on this site I have ended up with a linked list like the following: acc = {{1, 2}, {{2, 3}, {{3, 4}, {{4, 5}, {{5, 6}, {}}}}}}; This was ...
9
votes
5answers
371 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: ...
9
votes
6answers
406 views

Performing Computations on Sets

I would like to find a permutation of $\quad S=\{\frac{1}{10}, \frac{1}{2}, \frac{4}{7}, \frac{3}{5}, \frac{2}{3} \}\quad$ that maximizes the sum of theses elements raised to unique powers: ...
9
votes
5answers
586 views

Fast way of finding position of Interval containing an Integer

I have a large list (about 2.2 million) of pairs of integers. Each pair defines an Interval[{}]. Given some integer x, I need to ...
9
votes
5answers
704 views

Coalition Games

Suppose an election produces the following result: result = {{"a", 120}, {"b", 80}, {"c", 50}, {"d", 30}}; Letters are parties and numbers seats. To form a ...
9
votes
6answers
242 views

Is there a better way to add a “key” for the list of GatherBy?

In ruby , 1.upto(10).to_a.group_by{|x| x%3} gives {1=>[1, 4, 7, 10], 2=>[2, 5, 8], 0=>[3, 6, 9]} I would like use ...
9
votes
5answers
260 views

Fill out blanks with a upcoming number in a list?

I have a bunch of arrays looking like this: {"", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", "", ...
9
votes
5answers
344 views

Finding first instance of a relationship between elements in two lists

Suppose I have two lists, say: A = {1,5,10,200,50,7} B = {4,3,19,78} For each element in A, I would like to find the index of the first element in B which ...
9
votes
2answers
215 views

How to find synchronization offset?

I have 2 sets of data containing delays (audio / video). ...
9
votes
4answers
848 views

Passing large list by reference

I have the following problem: I would like to control evaluation of a variable that points to a list. For example, frequently in the code I have functions of the form that are supposed to work on ...
9
votes
2answers
463 views

How to represent a list as a cycle

I want to select distinct cycles (represented by List). The cycle {a, b, c, d} must be the same as {b, c, d, a} or ...
9
votes
2answers
450 views

Rearranging a List

I am trying to rearrange/sort a list i.e. {{1,2},{10,140},{43,10},{1,140},{43,2}} into ...
9
votes
2answers
581 views

Fluorescence Correlation Spectroscopy Simulation

I am simulating Fluorescence Correlation Spectroscopy which basically involves tracking the random motion of particles in a box with periodic boundary conditions and then calculating their intensity ...
9
votes
2answers
252 views

Combining nested lists that meet certain criteria for a permutation

I have the following list representing a permutation on 26 characters: ...
9
votes
1answer
193 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 ...
9
votes
1answer
338 views

Two ways of map a function on the list: Which one is faster?

There are 2 ways to make a function listable on the list: Use Map or use SetAttributes. My question is, which one is faster, ...
9
votes
2answers
327 views

Speed up 4D matrix/array generation

I have to fill a 4D array, whose entries are $\mathrm{sinc}\left[j(a-b)^2+j(c-d)^2-\phi\right]$ for a fixed value of $\phi$ (normally -15) and a fixed value of $j$ (normally about 0.00005). The way ...
9
votes
2answers
296 views

How to partition a list and leave in the last sublist which is of different length? [duplicate]

How to partition a list and leave in the last sublist which is of different length? In[75]:= Partition[{1,2,3,4,5},2] Out[75]= {{1,2},{3,4}} I want it to be ...