Questions on the manipulation of List objects in Mathematica, and the functions used for these manipulations.
6
votes
2answers
163 views
Can Tuples help simplify this?
I have a list {1, 2, 3, 4} and wish to form
{{{1,2},{3,4}},
{{1,3},{2,4}},
{{1,4},{2,3}}}
I ended up doing it manually ...
6
votes
1answer
89 views
How does `LongestCommonSubsequence` work?
When I recently came across this posting about LongestCommonSubsequence I was curious about how the function worked.
...
6
votes
2answers
324 views
Elegant way to handle columns operations
Inspired by this question from @sjdh and by my recurrent use of columns operations in database sets, I was looking for one way to make columns operations more symetric, so I can handle with matrix and ...
5
votes
6answers
264 views
How to change specific values in a List
In a list lst I want to change all values > 255 to 255. Why is this code not working?
...
5
votes
4answers
211 views
5
votes
6answers
272 views
Creating sums of elements from a list
I have a list $(d_1, d_2, .. d_k)$ and I want to create all sums that I get for adding only two elements for my list $(d_1+d_2, d_1+d_3,...d_{k-1}+d_k)$. The ...
5
votes
7answers
327 views
How to pick an element of a row corresponding to the minimum in a particular column?
How can I find what value in column 1 of my list corresponds to the minimum value in column 2?
5
votes
3answers
226 views
Delete elements from a list really fast
I have this bit of code that works, but it's very slow when there are 600k elements in the list:
...
5
votes
2answers
143 views
How can I find if the number of subsets for a list is very large?
I'm trying to find the following;
Subsets[Range[2, 2300], {4}]
but my computer gave the following error:
...
5
votes
3answers
144 views
All values for a function with two arguments without Outer
Given two lists $l_a = \{a_1, a_2, a_3\}$, $l_b = \{b_1, b_2, b_3, b_4\}$ and some function $f$ accepting two arguments, how can I produce a list of all values $f(a, b)$ for $a\in l_a$ and $b \in l_b$ ...
5
votes
4answers
316 views
Leveling peaks in list
Consider the following:
data={2,2,2,5,3,3,3,6,1,1,1,0};
In[1]:=result=MyFunction@data
Out[1]:={2,2,3.5,3.5,3,3,4.5,4.5,1,1,1,0}
...
5
votes
3answers
213 views
Point belonging to a Disk in a Cartesian Coordinate System
The image below represents a human subject fixations while observing this abstract pattern for 3 seconds.
I would like to know how much time they spent looking at the actual disk. That is a red point ...
5
votes
2answers
231 views
5
votes
2answers
139 views
Threading behavior of SameQ vs Equals
I ran into a bug in my code today based on an errant assumption. Namely, I thought that:
...
5
votes
4answers
122 views
Tricky selection of elements based on their position
I need to make a list of some elements from a 4D array with dimensions 4x4x4x4. I need to select the elements based on their position in the following way: name an element e(x,y,z,w); I need all the ...
5
votes
4answers
229 views
Any rule of thumb for converting a simple mathematical expression into a pure function?
By simple mathematical expression I mean one with a few functions and few variables copied into MMA in a mechanical way from a math book.
For example this one, giving the distance between two points ...
5
votes
3answers
237 views
Replace “,” in a list with “.”
I have a list which consists of numbers which use comma (,) instead of dot (.) as their decimal point. I would like to replace the commas, but only those commas which are followed by more than five ...
5
votes
3answers
343 views
Ways to compute inner products of tensors
One way to evaluate the following sums is combining Table and Sum:
$u_{abcd} = \sum_{e=1}^3 v_{aeb}w_{ced}$
$q_{ab} = \sum_{d,e=1}^3 v_{d e a}w_{deb}$
It will look like
...
5
votes
4answers
228 views
Join lists in a list of lists
What's the best way to join two consecutive lists in a list of lists ?
For example if I have the list
x = {{1}, {2}, {3}, {4}}
and I want to get in x
...
5
votes
2answers
171 views
Concatenate strings from different levels of list
After a lot of trouble I've finally come pretty close to what I want to do. I now have a list that looks like this:
...
5
votes
2answers
181 views
5
votes
2answers
193 views
5
votes
3answers
151 views
Self-referential list or table of buttons
How could one create a table or list of buttons, such that clicking on a button removes the button itself from the list?
5
votes
3answers
143 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
2answers
165 views
How can I force points in a list to have distinct x values?
I have a list of points that may be used for linear interpolation using Interpolation and need to ensure that no two points have the same $x$ value, since, if they ...
5
votes
1answer
223 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 ...
5
votes
2answers
507 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.
...
5
votes
2answers
232 views
Splitting a list at a particular position
Is it possible to split a list into two lists at a specific position? The main list is for example:
data={{xa,ya},{xb,yb},{xc,yc},...,{xz,yz}}.
I want to split ...
5
votes
1answer
125 views
Bug in LengthWhile?
Is this a bug or my MMA went insane or I'm missing something?
LengthWhile[Reverse@IntegerDigits[1000], # == 0 &]
works fine, but with SameQ instead of Equal, ...
5
votes
3answers
212 views
Efficiently extracting an array subset given a separate array
I have two arrays. The first array consists of tens of millions of rows and three columns with a string, a number, and a string.
...
5
votes
2answers
134 views
Creating a list of PopupMenus that dynamically update an element in a list
So as the title says I am trying to make a list where each element is determined by a users choice of an element in a PopupMenu.
My first attempt:
...
5
votes
2answers
208 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 ...
5
votes
1answer
154 views
Problem with creating a large list of tuples
This is a follow-up question from Sum of Multinomial Coefficients
I have thought about the meaning of the formula I mentioned and, with help, I implemented the following code:
...
5
votes
1answer
121 views
question about Inner
question1:
(code 1 something wrong, Why? and how to overcome this to achieve the next code's effect)
...
5
votes
1answer
106 views
Is there an equivalent of “shiftdim” of MATLAB?
Recently I'm spending my time implementing some computer vision algorithms, which usually handle a large amount of data.
The problem I'm facing now is that I have to reform my video data to pass it ...
5
votes
1answer
167 views
DeleteDuplicates[] does not work as expected on floating point values
Here is my simple example, and in this case function DeleteDuplicates does not work as expected.
I want to FindRoot of my ...
5
votes
1answer
279 views
Get the coordinates from ContourPlot and RegionPlot
How do I get the coordinates from a contour plot I've done in Mathematica? For example, I have a two-variable function f[x, y], for which I can make a contour plot:
...
5
votes
1answer
142 views
How I can use Apply or Map with If?
I have list of objects:
list = {{1, 1, 0.05, 0.05}, {2, 2, 0.05, 0.05}, {13, 13, 0.05, 0.05}};
list = {{x,y,Vx,Vy},{x,y,Vx,Vy},...};
And
...
5
votes
1answer
92 views
Trouble with Flatten[]
I made this function:
f[g_] := Table[{ToExpression[ToString[x] <> ToString[n]], 1, 3}, {n, 1, g}]
When I evaluate the function with some g (say 8), I'll ...
5
votes
4answers
196 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 (...
5
votes
1answer
180 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?
...
5
votes
1answer
335 views
Computing the distance between two curves obtained from image data?
I've got some image data that I've binarized. The image contains two curves, from which I'd like to compute the distance between (eventually computing an average radius for each point in the ...
5
votes
3answers
98 views
Applying a function with the HoldAll attribute inside NestList
I'm trying to write an update function, which can be applied to a list and then to NestList it.
As the function has to manipulate the given variable I figured I ...
5
votes
1answer
105 views
nested use of Apply/Map/MapThread in pure functions
1. My main question
I have a function with two argument slots. I wan't to apply this function to 2 lists with different length's. I thought in this solution:
...
5
votes
1answer
105 views
RankedMax and Max do not behave identically
I have spent three hours debugging some code and at the end I think I found a bug. Otherwise I cannot explain what I am seeing:
When I want to use RankedMax on ...
5
votes
1answer
129 views
Numbering element in descending order
How can I numbering the elements in descending order of second elements of sublists?
For example, if I have the list which is
...
5
votes
2answers
215 views
Extracting parts of nested lists of rules using a syntax similar to JsonPath or Xpath
Given an example data set:
...
4
votes
4answers
267 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
288 views
Creating lists from a string, separating words from numbers
Say I have a string that contains numbers and words, such as this one:
string = "there are 1234 words and numbers 5678 in here $999";
How would I separate the ...
4
votes
4answers
211 views
Add/replace a rule in a list of rules based upon the rule's lhs without regard for the rule's rhs
I have the following lists of rules:
{x -> 1, y -> 2}
{x -> 1, y -> 2, z -> 4}
{x -> 1, y -> 2, z -> 2}
For each list of rules, I would ...
