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

learn more… | top users | synonyms (3)

6
votes
5answers
322 views

How to sum over duplicates?

I have a list of the form: {{1, 0, 1}, {2, 1, 2}, {1, 0, 3}, {2, 4, 2}, {2, 1, -1}, {1, 0 ,0}} that I would like to sort such that I calculate the sum over the ...
19
votes
7answers
612 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 ...
9
votes
2answers
326 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 ...
4
votes
6answers
224 views

Filter list with different list in it

I have a list like: {{{4, 14}, 1}, {{4, 15}, 1}, {{4, 16}, 1}, {{4, 17}, 1}, {{4, 18}, 1}, {{4, 14}, 3}, {4,15}, {{4, 16}, 2},{4,18}} Now I want to ...
3
votes
2answers
168 views

Subscript and Part: What's going on in this example?

Example I came across the following simple example from A Beginner's Guide to Mathematica, by McMahon and Topa, which I believe was written circa version 5.2. (FWIW, I'm using v9.) ...
2
votes
1answer
435 views

RowReduce: Solving for the resource vector (a, b, c) in Augmented Matrix

Here are two examples: RowReduce[{{3, 1, a}, {2, 1, b}}] evaluates to {{1, 0, a - b}, {0, 1, -2 a + 3 b}} but ...
1
vote
3answers
1k views

How to create functions of arbitrary number of variables? [duplicate]

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 $z_2$? ...
7
votes
5answers
681 views

remove -1 values from a list

I would like to turn a list which looks like this one: {{-1, -1, -1}, {-1, 2, 3}, {-1, -1, 2}, {2, 3, 4}} into one which looks like that one: ...
2
votes
2answers
132 views

Total elements of a list that have a common first element [duplicate]

Possible Duplicate: List-operations only when restrictions are fulfilled (Part 1) I have a set of data like: ...
4
votes
2answers
347 views

Rearranging a matrix

I would like to convert a matrix which has a structure like this: {{"A", 10, "D", 1},{"B", 3, "A", 2},{"C", 7, "B", 3},{"D", 6, "C", 4}}//MatrixForm//Print; ...
3
votes
1answer
252 views

Implementing Position

As an assignment I need to re-implement Position. I've managed to do this except for one small thing, the resulting list is not in the current format. The following ...
2
votes
2answers
272 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 ...
5
votes
3answers
556 views

Combining two lists of different dimensions into a list of all combinations of points?

I have two ranges of values, one from 0 to 20 and the other from 0 to ...
7
votes
3answers
841 views

Filtering elements from a list

Suppose that I have a list {{{2, 1}, {4, 3}, {2, 4}}, {{2, 1}, {4, 3}, {3, 1}}, {{2, 1}, {2, 4}, {3, 1}}, {{4, 3}, {2, 4}, {1, 2}}} I want to make a new list ...
2
votes
2answers
269 views

Simple manipulation of imported data

I am importing 2 columns of numeric data from a data file in the way defined in this answer: ...
39
votes
8answers
2k views

Levels: how do they work?

I cannot understand how Mathematica manages levels, and so it's always a painful try-and-fail to use Flatten. Can someone please give me a very clear definition? ...
6
votes
2answers
452 views

Map a function over the columns of an M x N array

I have a $N \times M$ list of lists, or rather, a list of records and I want to map a function over its columns (e.g. find the mean temperature, age, etc.). Map ...
7
votes
4answers
232 views

Button's command is not evaluated in iteration

I wish I didn't feel like a newbie after several months, but... If I enter the following: Table[ Button[ "Number: " <> ToString @ i, Print @ i], {i, 1, 5}] ...
3
votes
1answer
464 views

Exporting/Importing a Table of complex numbers

I'm generating a long table of list of the form: PN={{1,2,1+i},{3.5,2.6,2}...},{...},... Using: Export["PN.dat", PN, "Table"] ...
8
votes
3answers
164 views

How can I use the overhang argument to Partition to avoid using PadLeft and PadRight here?

Say I have a list of values v = {v1, v2, v3, ... }. I need to partition these values to perform operations with a sliding window (think ...
2
votes
1answer
222 views

Computing closest set of points to each point in large set — running out of memory (arrays unpacking)

I have a large set of points in 3D and I'm trying to identify all the points that lie within a certain distance of each point. Then using this data store the vector between the pairs of points. I ...
5
votes
2answers
574 views

Finding the intersection of two date lists

I've got two lists that look like this ...
3
votes
1answer
459 views

Removing an ordered pair from a list of ordered pairs if the second element in the list is Less than a Value

I have a list of ordered pairs from a piezoelectric sensor sampled at 2kH. The first element is time and the second, amplitude. I'm trying to remove all ordered pairs in the list of ordered pairs when ...
5
votes
1answer
302 views

An easier and more efficient way to combine lists?

I currently have a two lists. The first list contains independent variables $x$, and the second list contains dependent variables in the form of $\{\{f[1][x]\},\{f[2][x]\},...,\{f[n][x]\}\}$. I want ...
4
votes
1answer
245 views

speed up iteration with conditionals plus optimize memory usage

Given list1 and list2 whose elements are vectors of a certain (fixed) dimension, I am interested in the behaviour of a scalar ...
4
votes
3answers
292 views

Extracting Pareto elements from a list

How can I extract the Pareto elements from a list? For example, for that list: ...
7
votes
4answers
196 views

Generating all matrices with 1 (possibly) replaced by -1

I have a matrix $M$, whose dimension I am unsure of, which has only $\lbrace0,1\rbrace$ entries. I would like to generate all the possible matrices that result from changing (some subset) of the $1$'s ...
3
votes
3answers
246 views

Are table headings functional?

Are table headings for aesthetics only? If I have: ...
8
votes
3answers
877 views

Analytically finding critical values in a hysteresis curve made with ListPlot

Let me start with a super basic explanation of what I'm trying to do, and I will follow with a more detailed discussion afterwards. From data in the form shown below (a ...
9
votes
6answers
248 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 ...
4
votes
4answers
192 views

clean functional way to get first n rows that yield maximum rank

I have a matrix A and want the matrix consisting of the first n rows of A, having the same rank as A, where n is minimal. More generally, I want the shortest "start piece" of a list, such that some ...
8
votes
3answers
512 views

Sum over binary digits of integer

This might seem like a simple enough question but Mathematica seems to simplify inadequately here: How would you sum over the digits of an arbitrary binary number? I already tried this: ...
0
votes
2answers
79 views

Extract a List of Co-ordinates from a List of Complex Solutions

Let's say I have this list of solutions from a previous computation: ...
2
votes
3answers
506 views

Generating pairs of additive and multiplicative factors for integers

Given an integer $n$, I want to get two lists: a) the set of pairs of the divsors $a,b$ into exactly two factors $n=a\cdot b$, b) the set of pairs $a,b$ of two summands $n=a+b$. The code I ...
19
votes
6answers
1k 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: ...
1
vote
1answer
157 views

Optimization of correlation calculation

How can I make the following line of code run faster? Is there a way to do this calculation as a matrix vs vector than vector vs vector? In the code below, f is a ...
4
votes
2answers
530 views

Optimize inner loops

I'm Mathematica newbie so please be gentle :) I have this, heavily non-optimized part of code, which I would like to speed up. I have put all matrices to be RandomReal, but in my code they take ...
4
votes
1answer
260 views

Performance of a Do-Loop of variable length

If I'm changing a basis depending on a (non quadratic) matrix $\mathbf{M}\in\mathbb Z^{d\times k}$ and some $\mu\in\mathbb N^k$, i.e. i have an array a of the same ...
1
vote
2answers
442 views

Specifying parts from lists for plotting using Show

I have the following data: ...
2
votes
0answers
356 views

Generating a function which outputs possible chemical reactions

I want to make a list of chemical reactions and I write them down in a $\require{mhchem}\LaTeX$ format. They are of the following form $$NA_n^i+MB_m^j \rightarrow \hat NA_{\hat n}^{\hat i}+\hat ...
0
votes
1answer
206 views

Assign the result of Solve in a list [duplicate]

Possible Duplicate: Assign the results from a Solve to variable(s) I want to assign the results of a Solve command in a list. For example Solve[-((287 s^3)/50) == 0, s] returns {{s -> 0}, ...
4
votes
6answers
256 views

Sorting Matrix elements

I have matrix in as shown, consisting of real numbers and 0. How can I sort it to become out as shown? ...
7
votes
3answers
356 views

Operations on elements of ragged lists

Suppose I have a list of lists with different lengths, sorted from longest to shortest ...
5
votes
5answers
706 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 (...
6
votes
1answer
330 views

Why is MapIndexed better than mapping over a range?

Background I'm working on an application in which I need to create and control two sets of locators. I know from reading the Mathematica documentation and certain posts on Mathematica.SE that this ...
28
votes
4answers
1k views

Filtering beat-to-beat heart rate data

From an experiment, I have a dataset of beat-to-beat heart rate data: a list of the time between each heart beat in [ms]. The data is measured using an infrared optic sensor at the finger tip. The ...
11
votes
3answers
188 views

Why can't I use Sequence to perform a Select like task? [duplicate]

Suppose I wanted to write down a list of $p^2$, for $p$ a prime between $1$ and $20$. I would expect Table[If[PrimeQ[k], k^2, Sequence[]], {k, 1, 20}] to work. ...
3
votes
1answer
293 views

Operations on a List of Vectors (Norm)

I have a List containing vectors. The coordinates are random numbers: r = {RandomReal[{-1, 1}, n], RandomReal[{-1, 1}, n], RandomReal[{-1, 1}, n]}; Now I want to ...
0
votes
2answers
122 views

Matrix conditional operation

Say I have a matrix of: tmp1 = 5; tmp2 = 5; tmp3 = RandomChoice[{0, 1, 2, 3, 4, 5}, {tmp1, tmp2}]; MatrixForm[tmp3] How to do a conditional operation of elements ...
7
votes
2answers
3k views

Building matrices by attaching vectors (columns) and matrices

Having the need to attach a column to a matrix or to join matrices to make longer rows is an operation that I use very frequently and I find the Join function ideal ...