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

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11
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3answers
565 views

How to add lists with unequal length?

Given lists of unequal lengths, I want to add them together element-wise, treating missing elements as zero. For example, {1, 2, 3} + {a, b} + {x} should give ...
11
votes
3answers
300 views

What is the best way to flatten a linked list with lists as values?

By following the advice on this site I have ended up with a linked list like the following: acc = {{1, 2}, {{2, 3}, {{3, 4}, {{4, 5}, {{5, 6}, {}}}}}}; This was ...
11
votes
5answers
450 views

How would you efficiently implement BinListsBy?

Let's say I have a list like this: data = {{21, "Alice"}, {27, "Bob"}, {22, "Charles"}, {34, "Dora"}, {75, "Eliza"}} containing ages and names of persons. What I ...
11
votes
3answers
726 views

Specifying string patterns in DeleteCases

It seems that DeleteCases is not compatible with string patterns, at least directly (see, for example, this question). Is this true? If so, why is this the case? ...
11
votes
4answers
319 views

Thread a function over a list and with a non-atomic 2nd parameter?

From the documentation, Thread's behavior on functions where the first parameter is a List and the second is an atomic expression, is this: ...
11
votes
6answers
396 views

For a given expression: if it appears, remove it, but if it is absent, add it

While reformatting Szabolcs's code from (42660) I noticed this interesting operation: ...
11
votes
4answers
346 views

MapThread with non-rectangular lists

MapThread works fine and dandy with rectangular list structures: MapThread[f, {{{a, b}, {c, d}}, {{1, 2}, {3, 4}}}, 2] ...
11
votes
3answers
270 views

Threading over a list of lists of lists and a list of atomic expressions

Although I solved the following problem using MapThread, I'm curious about why Thread does not work as I expect it to here. ...
11
votes
3answers
193 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. ...
11
votes
3answers
642 views

Delete duplicates in a list, depending on the sequence of numbers

Below, list is a representative sample of my list, which contains lists of integers. I would like to be able to input: ...
11
votes
2answers
2k views

How to use “Drop” function to drop matrix' rows and columns in an arbitrary way?

The built-in function "Drop" can delete a Matrix's row and column. Typical syntax for "Drop" is as follows: Drop[list,seq1,seq2...] But what if I want to drop a ...
11
votes
2answers
189 views

Simple way to concatenate Dataset columns

Consider these two datasets: d1 = Transpose@Dataset[<|"a" -> Range[5]|>]; d2 = Transpose@Dataset[<|"b" -> Range[5, 1, -1]|>]; {d1, d2} What's ...
11
votes
2answers
187 views

Faster trimming of leading/trailing zeroes in rectangular array

Given a (large) rectangular array, e.g. (small for example case): ...
11
votes
1answer
228 views

Map efficiently over duplicates in list

I am interested in mapping a function, f over a (long) list which contains duplicates. Since this function takes a while to evaluate I would like to make use of ...
11
votes
1answer
253 views

How to find longest monotonic sequence?

I have a list, for example list = {2, 1, 3, 5, 4, 6} How to find longest ascending sequence of this list? There are two meanings of this question: Find the ...
11
votes
2answers
677 views

Vlookup function as Excel in Mathematica

How can we create a Vlookup function, similar to what we have in Excel? So if we have a table a and another b, we could add ...
11
votes
2answers
369 views

Solving the magic backyard puzzle with MMA

I was trying to solve this puzzle using mma It looks like the generalization of the magic square. Anyway here my attempt:- There are four columns $(x,y,z,w)$ and nine rowa which are denoted by the ...
11
votes
0answers
184 views

Strategies to find curves on messy data using FindCurvePath

I have a set of data points that correspond to roots of a transcendental equation with respect to a parameter (the abscissa) which, by the properties of the system, define continuous curves (might be ...
10
votes
9answers
591 views

How to find range in which a number falls, from given list of numbers?

How can one find the range in which a number falls, from given list of numbers? ...
10
votes
8answers
2k views

Map and Apply a function on a nested list

I have a list like this: {{1,2}, {4,2}, {6,4} ... } I want to replace every second number with a function of that number. For e.g. ...
10
votes
4answers
4k views

How can I multiply matrix and vector element wise like Numpy?

I have an $M \times K$ matrix $C$ and a length $K$ vector $v$. I want to form the $M \times K$ matrix $S$ where $s_{ij} = C_{ij}/v_j$. To do this in numpy I simply divide the two objects like this ...
10
votes
3answers
2k views

Most Efficient Way to Calculate the Product of All Items in a List?

Say that I have a list of returns on a financial asset over time and want to know the total compounded return. To get the answer, I need to add 1 to each return and then calculate the product of all ...
10
votes
4answers
290 views

Searching linked lists that contain lists?

Following the advice I've read here and other sites, I've been trying to use the Mathematica equivalent of a linked lists... ...
10
votes
5answers
567 views

How do you efficiently return all of a List but one element?

I have a problem that I am working on that requires that I compare several lists of overlapping data points. As I traverse one of the lists, I'd like to only compare those data to the other lists, and ...
10
votes
4answers
667 views

How to rebuild original data from a histogram?

I have data in the following form. I have a list of values and a list of frequencies that indicate how often each value has occurred. Example list of values: ...
10
votes
5answers
528 views

How to find first list element that differs from average of N previous elements by more than a given amount?

Given a list of real numbers, I need to write a function that returns the value of the first element in the list that differs from the mean of the previous $N$ elements by greater than $P$ standard ...
10
votes
5answers
634 views

How to subtract one list from another, treating each elements as distinct, assuming the smaller list is a subset of the larger list? [duplicate]

Suppose that I have two lists of different sizes, the smaller of which is a subset of the larger, e.g. a = {"A", "A", "A", "B", "B", "C"} b = {"A", "B"} How can ...
10
votes
3answers
749 views

Count consecutive positions with the same value

I want to program a function that counts the successive positions with the same value: In[]= ConsecutiveValues[{1, 1, 1, 1, 2, 2, 1, 0}] Out[]= {4, 2, 1, 1} In ...
10
votes
5answers
417 views

Finding time-series direction reversal of certain magnitude

I'd like to find the points of a time-series that are a certain distance away (in value, not in time) from the previous maximum, which I consider a reversal. For example, for ...
10
votes
4answers
759 views

Using Position (which requires a pattern) in the same manner as Select (which requires a criterion)

I am using Mathematica7. I have a list of lists (creatively named list in the following sample code) containing data. I would like to find the positions of the ...
10
votes
8answers
385 views

Fast way to get positions of “boxed” array elements?

Given a 2D array (arbitrary numeric / evaluates to numeric elements, simplified for examples, and can be arbitrary dimensions so long as the operation makes sense, I'm working with 2K X 2K in my ...
10
votes
6answers
342 views

Using MapIndexed only at certain elements of a list

MapIndexed is a very handy built-in function. Suppose that I have the following list, called list: ...
10
votes
3answers
353 views

Modifying strings in a list efficiently

I have a list of strings $\{s_1,s_2,\ldots,s_n\}$. I would like to to produce the following list of strings: $$ \{s_1<>1,\ 1<>s_2<>1,\ldots,\ 1<>s_n\} $$ In other words, I want ...
10
votes
5answers
412 views

How to randomly select subsets

I have a list of around 300 elements. I want to sample subsets of length 25 such that my samples are all distinct. My first inclination was to use something like ...
10
votes
4answers
336 views

Complement of multisets

Given lists $a$ and $b$, which represent multisets, how can I compute the complement $a\setminus b$? I'd like to construct a function xunion that returns the ...
10
votes
3answers
355 views

Looking for an elegant way to construct this tensor-product-ish list

I would like to make the following matrix: ...
10
votes
4answers
372 views

Graphically approximating the area under a curve as a sum of rectangular regions

I am completely new to Mathematica. Basically I was trying to write a code to plot a function and draw the approximate area by rectangles. To be more precise, plot a function ...
10
votes
2answers
313 views

Using Gather to rearrange my data

I have a list of elements: {{1, 2}, {1, 5}, {2, 6}, {3, 7}, {4, 8}, {5, 6}, {5, 9}, {9, 13}, {10,11}, {11, 12}, {11, 15}, {15, 16}} I have to divide it into ...
10
votes
5answers
449 views

ListPlot under conditions

I have a data file containing several columns and thousands lines of data. Below I present a small portion of it, in order to express my issue: ...
10
votes
3answers
186 views

Pick with nested list

I have a list of pairs of values. Some of those pairs are invalid based on another list. I would like to select only the valid pairs. ...
10
votes
8answers
519 views

Forming an upper triangular matrix from the elements of a list

I asked this question before, but i was closed because it the question was not comprehensible enough. So i reworded it, hope that's ok. I have a list with 6 elements. ...
10
votes
4answers
418 views

How to pass a list of arguments into HoldAll

I have a list of arguments (which in reality is lengthy): arguments = {a, b, c} arguments2 = {a_, b_, c_} f[Sequence@@arguments2] := a + b + c Note: It seems ...
10
votes
3answers
346 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: ...
10
votes
4answers
793 views

Efficiently Visualising Very Large Data Sets (without running out of memory)

I have put a few really hard problems in combinatorics up against Mathematica 8. I'd have to say that it works really well, until you want to view the data. If you look at my question Advanced ...
10
votes
2answers
567 views

Faster “stuttering” accumulate?

Given some list of arbitrary precision numbers, e.g. (actual lists are 1M+ elements long): test={0, 2, 2, 47839283, 2, 0, 0, 2, 0, 1, 2, 0} I need to accumulate ...
10
votes
2answers
405 views

Select cases from a list

I have a list in the form : {1,1,1,0,-1,-1,1,1,1,0,0,0,0,-1,-1} No I'm trying to select only those sequences which start with 1 have a zero (or many zeros) ...
10
votes
4answers
444 views

Building a tree

Given a list of word characters, such as this one, I'd like to build a tree, similar to this makeTree function, but with the tree in a different format. So, for an input such as ...
10
votes
5answers
555 views

Best way to modify values in a list of rules?

Recently I had to solve a problem similar to this: Let's say I have a list of list of rules ...
10
votes
2answers
3k views

How to get Matlab data imported with the same dimensions?

I have some Matlab image data with the following dimensions (output is from Matlab): >> size(im) ans = 86 86 3 45 The data imports just fine ...
10
votes
2answers
465 views

Rearranging a List

I am trying to rearrange/sort a list i.e. {{1,2},{10,140},{43,10},{1,140},{43,2}} into ...