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

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20
votes
3answers
733 views

Emulating R data frame getters with UpValues

What's the best way to emulate R's data frames functionality? This includes the ability to select rows and columns in a 2-dimensional table by the string identifiers positioned typically in the first ...
14
votes
3answers
2k views

K-means clustering

In MATLAB, there is a command kmeans() that divides an array into $k$ clusters and calculates the centroid of each cluster. Is there any command in Mathematica to ...
17
votes
5answers
553 views

Map-Thread-Through-Apply a list of functions onto a list of (lists of) values

I have a list of functions: fns = {f, g, h} and a list of triples: ...
11
votes
2answers
1k views

Simple algorithm to find cycles in edge list

I have the edge list of an undirected graph which consists of disjoint "cycles" only. Example: {{1, 2}, {2, 3}, {3, 4}, {4, 1}, {5, 6}, {6, 7}, {7, 5}} Each ...
17
votes
7answers
926 views

Selectively Mapping over elements in a List

I am using the following code to easily generate a row of images of all eight planets of our Solar System: ...
14
votes
4answers
525 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 ...
14
votes
4answers
358 views

“Unflattening” a list

Imagine we have a list with some hierarchical structure: list = {{{2,3},{2,4}},{{{3,4}}},{{5,6},{7,8}}}; Which we then flatten to some level in order to process ...
11
votes
2answers
611 views

How do I find the elements in a list that return the highest value for a function?

Suppose we have a list of values and a function f. I want to find which of the elements maximizes the return value of the function in Mathematica. Let call the ...
5
votes
5answers
490 views

Replace values which obey certain criteria

I'd like to replace all values in a list which obey one or more criteria with another value. Example: Replace all values>30 by 30. data={{3,35},{2,7}} ...
13
votes
9answers
502 views

Thread over list in different levels

I have these two lists, both with the same length in the first dimension: ...
11
votes
3answers
478 views

Simpler way to fill date gaps with zero values

One basic problem when working with data like sales and dates is the case where you dont't have sales in all days, so you have to fill it to take information as average or to make a plot. See this toy ...
7
votes
3answers
391 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 ...
5
votes
2answers
453 views

Finding the intersection of two date lists

I've got two lists that look like this ...
9
votes
2answers
398 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 ...
7
votes
4answers
370 views

Find list position and mapping list

Sorry, my English is not very good. I have two lists: ...
11
votes
5answers
943 views

Best way to extract values from a list of rules?

Mathematica has a lot of list manipulation functions, and, also because I don't work with lists often, at times I'm a bit lost. I'll find a way, but I'm sure it's not the most efficient. Case in ...
6
votes
5answers
363 views

Computing the equivalence classes of the symmetric transitive closure of a relation

I have a list of pairs, for example: ...
14
votes
6answers
1k views

Max of a table/list with indeterminate values

Suppose I have a Table: tab = {1., 2., 3., Indeterminate} When I type Max[tab] I get ...
9
votes
5answers
467 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 ...
5
votes
5answers
332 views

Elementwise join

I have two tensors of arbitrary but equal rank n (and equal dimensions): A and B, and I want to get a third tensor of rank n + ...
5
votes
4answers
399 views

Prevent Part[] from trying to extract parts of symbolic expressions

If you have a list, e.g. {1, 2, 3} then you can extract the $k$th part using Part (...
2
votes
2answers
182 views

Position function not always retuning an answer even with no apparent problems

I'm having some problems with Position. Sometimes it will give an empty list instead of the actual position of the element I am looking for when that element is ...
16
votes
4answers
467 views

Accessing list elements by name

First, a bit of a long introduction to my problem: I only have a few weeks of Mathematica experience. I am creating a mathematica application that calculates some material properties of steel based ...
12
votes
11answers
686 views

How to get list of duplicates when using DeleteDuplicates?

This might be easy, but can't find a way to use DeleteDuplicates to get also list of the actual duplicates. Example: ...
20
votes
7answers
1k views

How to Derive Tuples Without Replacement

Given a couple of lists like a={1,2,3,4,6} and b={2,3,4,6,9} I can use the built-in Mathematica symbol ...
13
votes
1answer
168 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: ...
13
votes
6answers
817 views

Splitting up delimited data in lists

One task that I frequently find myself doing in Mathematica is splitting lists into lists of sublists, using specific elements to define the break-points. This is particularly useful with imported ...
8
votes
1answer
136 views

Quickly selecting for points in a set that are within a critical distance of points in another set

Imagine two lists of two-dimensional coordinates: listA = RandomReal[{0,100},{202,2}]; listB = RandomReal[{0,100},{97,2}]; I'm attempting to quickly generate a ...
19
votes
5answers
635 views

MapThread over sublists of different length

I have a list which looks like this: l={{1,0,3,4},{0,2},{0,0,1,3},{1,2,0}}. Now I would like to count how many 0s the sublists contain in the first, second,... ...
10
votes
1answer
433 views

How to monitor the progress of Map?

I have a function doSomethingComplicated[...] that takes about 10s on average to evaluate. My list, listOfArgs has about 10000 ...
9
votes
4answers
244 views

How to pass a list of arguments into HoldAll

I have a list of arguments (which in reality is lengthy): arguments = {a, b, c} arguments2 = {a_, b_, c_} f[Sequence@@arguments2] := a + b + c Note: It seems ...
8
votes
3answers
478 views

Checking for duplicates in sublists

I have a list with contains entries that are length 8 sublists,i.e. L1={{a1,b1,c1,d1,A1,B1,C1,D1},{a2,b2,c2,d2,A2,B2,C2,D2},...} and so on. Now, in my problem two ...
18
votes
11answers
885 views

Generating an ordered list of pairs of elements from ordered lists

I have a pair of ordered lists. I want to generate a new ordered list (using the same ordering) of length n by applying a binary operator to pairs of elements, one from each list, along with the index ...
14
votes
6answers
1k 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 ...
14
votes
2answers
727 views

Is there an equivalent to MATLAB's linspace? [duplicate]

Does Mathematica have an equivalent to Matlab's linspace? I want to make a list with "start", "stop" and "number of points". What is the neatest way of doing this?
14
votes
7answers
5k views

Mathematica: finding min/max in list

I have a list of coordinates like this one: {{x1,y1},{x2,y2},{x3,y3},...,{xn,yn}} I need to get the minimum and the maximum of all x-values and the minimum and ...
13
votes
5answers
856 views

From a list to a list of rules

Starting from these two lists, var = {a, b, c} values = {{1, 2, 3}, {4, 5, 6}, {7, 8 , 9}} how can I generate a list of rules? ...
10
votes
4answers
323 views

How to add lists with unequal length?

Given lists of unequal lengths, I want to add them together element-wise, treating missing elements as zero. For example, {1, 2, 3} + {a, b} + {x} should give ...
10
votes
3answers
264 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: ...
9
votes
4answers
582 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 ...
15
votes
7answers
1k views

How do I check if any element in a list is positive?

As a simple example of what I would like to do, suppose I have a list a of all real numbers. I would like to perform a simple check to see if some element of ...
12
votes
4answers
280 views

The gap between MapAt and ReplacePart for 2D data tables with headers

Consider a relational table derived from survey data, where each column ("001-01" ...) represents a responder and each row (...
9
votes
6answers
258 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: ...
8
votes
4answers
278 views

Gather list elements by index

The following list has some elements that are labeled. For example {1, 2} -> 1, {-1, 3} -> 3, etc: ...
8
votes
3answers
496 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: ...
7
votes
4answers
428 views

Prepend 0 to sublists

My question is similar to this one, but my goal is to prepend a single 0 the each sublist, not incrementally many 0's. The file I'm working is a CSV containing around 50K sublists of length 35. I've ...
4
votes
4answers
360 views

Replace For-loop with functional code

I am very new to Mathematica. I have code written using for loops. I want to rewrite it using Map/Thread but I am not sure how ...
4
votes
2answers
1k views

Is it possible to compute trapezoidal rule numerical integration?

Is it possible to compute trapezoidal rule numerical integration? I know that Mathematica has Interpolation, and that a list of points can be interpolated and then ...
11
votes
3answers
451 views

Delete duplicates in a list, depending on the sequence of numbers

Below, list is a representative sample of my list, which contains lists of integers. I would like to be able to input: ...
9
votes
7answers
358 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: ...