Linked Questions

6
votes
4answers
378 views

Find the identical rows in a matrix [duplicate]

Suppose I have the following matrix: ...
1
vote
4answers
266 views

Using Select in Subsets [duplicate]

I have a list: list = {{7, 6, 14, 14, 4}, {4, 9, 8, 11, 14}, {20, 12, 12, 11, 3}, {7, 4, 11, 7, 20}, {2, 13, 17, 11, 19}} I did a test to identify where ...
535
votes
19answers
135k views

Where can I find examples of good Mathematica programming practice?

I consider myself a pretty good Mathematica programmer, but I'm always looking out for ways to either improve my way of doing things in Mathematica, or to see if there's something nifty that I haven't ...
70
votes
7answers
6k 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 ...
21
votes
13answers
4k 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: ...
10
votes
4answers
6k 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 ...
21
votes
6answers
3k views

Looking for a way to insert multiple elements into multiple positions simultaneously in a list

I would like to take the following data: insertValues = {r, x}; insertPositions = {{1}, {5}}; originalList = {a, b, c, d, e, f, g}; and generate output that ...
47
votes
3answers
1k views

Why is the new PositionIndex horribly slow?

This issue has largely been mitigated in 10.0.1. New timings for the final test below are: ...
21
votes
7answers
2k 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 ...
25
votes
5answers
814 views

Partitioning a list when the cumulative sum exceeds 1

I have a long list of say 1 million Uniform(0,1) random numbers, such as: dat = {0.71, 0.685, 0.16, 0.82, 0.73, 0.44, 0.89, 0.02, 0.47, 0.65} I want to ...
16
votes
6answers
532 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 ...
11
votes
9answers
461 views

Positions of elements in the initial flattened list in a split list

Say, we have a list: m1 = {2, 2, 7, 0, 7, 7, 2, 2, 2} It can be split easily: ...
18
votes
5answers
876 views

Easiest/most efficient way to delete duplicates from one list at the positions of another?

This question is somewhat related to How to efficiently find positions of duplicates? , although I'm interested in something a little bit simpler since I'm not really interested in a list of all the ...
13
votes
5answers
2k views

Match opening and closing parentheses?

Suppose I have a List of the form: lst={"(", "(", "(", ")", "(", ")", ")", ")"} By running ...
10
votes
7answers
757 views

intersection between two 2D arrays with labeled data is slow

Here is how to Generate data the function is below with two images to generate data img1 img2 ...
9
votes
4answers
2k 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 ...
12
votes
6answers
650 views

How do I prune a list of pairs of numbers so that each number occurs on only one pair?

How do I select only the first pair of numbers in which a number occurs from a list of pairs of numbers? For example, I have ...
8
votes
6answers
529 views

Find positions of duplicates that are neighbours

Assuming the following list {1,2,3,3,5,6,3} I would like to get the position of values that are next to each other and are duplicates. That is, the output should be {3,4}. Thank you for any help. ...
10
votes
4answers
422 views

Efficient way of identifying the indices of first occurrences

It seems to me like identifying the first occurrence of unique elements should take about the same amount of time as it takes to delete duplicates. My data sets are much larger than this (...
16
votes
3answers
488 views

Retaining and reusing a one-to-one mapping from a sort

Imagine I sort the elements of an array $Q$ by some means. Perhaps I sort strictly by the elements in the first column, perhaps I use SortBy[...] to sequentially sort by the elements in successive ...
8
votes
5answers
263 views

While partitioning the elements in a list using GatherBy, can I correspondingly partition the elements of an unrelated list?

If we have a list like the following: list1 = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12}; And we use GatherBy to subpartition is ...
11
votes
5answers
1k views

Relational joining of tables

A common thing to want to do with data is to combine it (at least I thought this was a common thing). In SQL there is the idea of table joins "select id,v1,v2 from A,B where A.id=B.id" kind of thing. ...
17
votes
1answer
363 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: ...
7
votes
4answers
187 views

Summing elements in a list by groups indicated by a list of indices

I have two lists, one containing values, the other indices. Now I want to accumulate the values that have the same corresponding index. So for example: ...
4
votes
6answers
412 views

Randomly distribute numbers to lists

I have 4 empty lists. I would like to randomly distribute numbers from 1 to 10 to these 4 empty lists. How can I add integers to lists randomly? For example: ...
8
votes
2answers
300 views

Interpolating four-dimensional data

In this case it is not easy to work with a MWE, so I shall use the actual data which contain initial conditions $(x,y,p_x,p_y)$. Let's see how the data look on the $(x,y)$ plane ...
2
votes
2answers
1k views

Find equal elements on a list

Consider the following code to produce the sequence $x_1,\ldots,x_{n+1}$ where $x_i=11\cdots1$ ($i$ digits 1). Is there an easier way to do this? ...
3
votes
3answers
146 views

How to get a description of the positions of similar elements in a list

I want to find a function F to represent the positions of identical elements in a list, which means, for example: ...
4
votes
3answers
338 views

Modify BinLists to give elements indices in list instead of giving the list elements belonging to each bin

How could I use BinLists to give elements indices in list instead of giving the list elements belonging to each bin ? Thanks.
5
votes
2answers
143 views

Determining lists with equal elements in the same position

I would like to define a function with the following properties: Given a list of equal length sub-lists ls={{a,a,a},{a,b,a},{a,b,c}}; Define a function ...
4
votes
2answers
201 views

position of intersection of two lists

I have the two lists: ...