# Tagged Questions

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

6answers
4k views

### Elegant operations on matrix rows and columns

Question The Mathematica tutorial has a section 'Basic Matrix Operations', describing operations like transpose, inverse and determinant. These operations all work on entire matrices. I am missing a ...
3answers
343 views

### Using pure functions in Table

I need a table with the elements made of pure functions and list elements. This is a simplified example: I need a list as: ...
3answers
1k views

### Delete duplicate elements from a list

If a list contains duplicate elements, for example list = {a, 1, 5, 3, 5, x^2, x^2}, how can the duplicate elements be removed? The result would be ...
4answers
650 views

### Partitioning with varying partition size

How can I partition a list into partitions whose sizes vary? The length of the $k$'th partition is a function $f(k)$. For example: if $l = \{1, 2, 3, 4, 5, 6\}$ and $f(k) = k$. Then the partitioning ...
6answers
996 views

### How to select minimal subsets?

I am a newbie, so please point me in the right direction if you feel this question has been answered somewhere else before. Here goes: Suppose I have a list like this: ...
5answers
721 views

### Partition a set into subsets of size $k$

Given a set $\{a_1,a_2,\dots,a_{lk}\}$ and a positive integer $l$, how can I find all the partitions which includes subsets of size $l$ in Mathematica? For instance, given ...
2answers
464 views

### Transpose uneven lists

Is there a quick method to transpose uneven lists without conditionals? With: Drop[Table[q, {10}], #] & /@ Range[10] Thus the first list would have the ...
12answers
812 views

### Map a function across a list conditionally

It seems that this is a really basic question, and I feel that the answer should be obvious to me. However, I am not seeing. Can you please help me? Thanks. Suppose I have a list of data ...
6answers
811 views

### Mathematica Destructuring

Context I'm writing a function that look something like: ...
7answers
481 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
890 views

### Local max/min of Mathematica data sets

Is there a way in Mathematica to find the local maxima of a set of points? Suppose you have ...
3answers
406 views

### Efficient way to combine SparseArray objects?

I have several SparseArray objects, say sa11, sa12, sa21, sa22, which I would like to combine into the equivalent of {{sa11, sa12}, {sa21, sa22}}. As an example, I ...
3answers
367 views

### How do I use Map for a function with two arguments?

I'm a newbie who tries to be a good boy, and use Map instead of writing out a list of functions. I have a table I want to Map ...
2answers
414 views

### Finding all partitions of a set

I'm looking for straightforward way to find all the partitions of a set. IntegerPartitions seems to provide a useful start. But then things get a bit complicated. ...
6answers
2k views

### Does Mathematica have advanced indexing?

I have two $M \times K$ arrays $L, T$ where I would like to set all the elements in $L$ to zero whenever the corresponding element of $T$ is greater than 15. The ...
2answers
590 views

### Tiling a square

I wondered if there was a way to automate the process of finding a way to tile a tile into a square. The idea is to represent the tile with a matrix of 0s for blank space and 1s for filled spaces ...
6answers
1k views

### How to visualize/edit a big matrix as a table?

Is it possible to visualize/edit a big matrix as a table ? I often end up exporting/copying big tables to Excel for seeing them, but I would prefer to stay in Mathematica and have a similar view as in ...
5answers
954 views

### What is the most efficient way to add rows and columns to a matrix?

Say I have a matrix m and a vector v. ...
6answers
891 views

### Find zero crossing in a list

I'm looking for a function that finds the index of the zero-crossing points of a list. Before I go making my own subroutine to do this, I was wondering if anyone knows of any built-in Mathematica ...
6answers
847 views

### Insert $+$, $-$, $\times$, $/$, $($, $)$ into $123456789$ to make it equal to $100$

Looks like a question for pupils, right? In fact if the available math symbol is limited to $+$, $-$, $\times$, $/$ then it's easy to solve: ...
7answers
975 views

### How to apply or map a list of functions to a list of data?

Say I have a group of functions: f1[a_] := a * -1; f2[a_] := a * 100; f3[a_] := a / 10.0; and some data in a list: ...
7answers
684 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: ...
2answers
770 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 ...
2answers
688 views

### Extracting values from nested rules in JSON data

I have been using Mathematica to analyse some data from the StackExchange API. It is conveniently available in JSON form, which Mathematica interprets as replacement rules. However, some of the rules ...
8answers
536 views

### How do you check if there are any equal arguments(even sublist) in a list?

I would like to set up a function which has to return True if at least two arguments of a given List are equal. So if I give {1,4,6,2} to the function it has to ...
4answers
325 views

### How can I remove B -> A from a list if A -> B is in the list?

I have a list of transformations like this: list = {"A" -> "B", "B" -> "A", "C" -> "D"} As this is used to plot an undirected graph with ...
7answers
287 views

### How do I obtain an intersection of two or more list of lists conditioned on the first element of each sub-list?

Given two lists like list1 = {{1, 1}, {2, 4}, {3, 9}, {4, 16}}; list2 = {{2, 6}, {3, 9}, {4, 12}, {5, 15}}; I would like to produce an output like ...
6answers
294 views

### List-operations only when restrictions are fulfilled (Part 1)

Consider the following: ...
5answers
248 views

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

I have a list of pairs, for example: ...
3answers
545 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 ...
11answers
639 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 ...
5answers
957 views

### Finding all elements within a certain range in a sorted list

Suppose we have a sorted list of values. Let's use list = Sort@RandomReal[1, 1000000]; for this example. I need a fast function ...
7answers
809 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 ...
4answers
383 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: ...
3answers
203 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: ...
2answers
505 views

### Finding a subsequence in a list

I have a list and I want to find (in this particular case the first) appearance of a any of some subsequences, of possible different lengths. None of the subsequences is a subsequence of each other. ...
6answers
397 views

### How to Map a subset of list elements to a function?

How would you, given a list {1, 2, 3, 4}, apply a function f to 1 and ...
8answers
422 views

### Applying And to lists of Booleans

I'd like to take {True,True,False} and {True,False,False} and apply And to get ...
7answers
604 views

### Selecting a sublist based on Length

If you have a simple list of lists as follows: test = {{1, 2}, {4, 5, 6, 7}, {5, 4, 3}} How do you ask Mathematica to return the sublist of greatest length? ...
4answers
361 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 ...
5answers
747 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 ...
6answers
679 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 ...
1answer
177 views

### DeleteCases messing with my mind

I'm losing my mind. Please tell me my laptop is doing weird things and not me. Or do I need to get committed someplace? ...
2answers
207 views

### From a list of dates get a list of the last date available in a each month

I wondered if anyone has another or even a more direct way of finding the last dates of each month available from a list of successive dates? I currently do the following (note: nothing special about ...
5answers
264 views

### Assigning a particular value to array elements

I have an array of 10000 elements. I want to randomly assign energy to these 10000 elements using Gaussian or Exponential distribution, such that each time a particular element is selected its energy ...
3answers
211 views

### How to create functions of arbitrary number of variables?

In the following code what would be the simplest way to generalize it to say some $N_f$ number of $z$ instead of just $z_1$ and $z_2$? ...
3answers
336 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 ...
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 ...
3answers
343 views

### How to efficiently find positions of duplicates?

Is there an efficient way to find the positions of the duplicates in a list? I would like the positions grouped according to duplicated elements. For instance, given ...
4answers
365 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 ...
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