378 views

### Find the identical rows in a matrix [duplicate]

Suppose I have the following matrix: ...
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 ...
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 ...
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 ...
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: ...
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 ...
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 ...
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: ...
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 ...
820 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 ...
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 ...
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: ...
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 ...
2k views

### Match opening and closing parentheses?

Suppose I have a List of the form: lst={"(", "(", "(", ")", "(", ")", ")", ")"} By running ...
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 ...
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 ...
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 ...
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. ...
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 (...
489 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 ...
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 ...
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. ...
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: ...
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: ...
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: ...
301 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 ...
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? ...
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: ...