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

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12
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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 ...
12
votes
5answers
632 views

Filtering elements from a list that appear only once

I want to be able to select elements from a list that only appear once. I originally had this: Select[Table[Count[list, i], {i, list}], # == 1] But the issue is ...
12
votes
7answers
475 views

Product of elements of a list

Sum is for functions; Total is for a list. Product is for a function; what is the command ...
12
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5answers
748 views

What's the cleanest functional-programming style way to perform this list operation?

I am looking for the correct method for accomplishing the unknown piece in the following code snippet: rolls = Tuples[Range[1, 4], {2}] ...
12
votes
5answers
2k views

How can I simulate this animation of checking for prime numbers?

I can only implement a very simple one, I want to make my code look like that. Any better ideas? ...
12
votes
9answers
1k views

How to partition a list of numbers in two lists of numbers such that each lists elements sums up to the same number?

I have the following list of squares. {1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256} Their sum is $1496$, I want to make two partitions ...
12
votes
6answers
826 views

Deleting Lonely Numbers From a List

Suppose I have a list of integers like so: list = {1,2,5,6,8,10,11,12,14,16,17,18,19}; I'd like a function that will take this list and return a new list with ...
12
votes
3answers
258 views

Using 0 to create new columns in a list with Part

I just discovered a nice way to create intermediary columns in a list, and I would like to know if I can have some problem with it. It Works like this: Imagine I have this list, 5x3: ...
12
votes
5answers
468 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 ...
12
votes
4answers
432 views

How to enumerate all possible binary associations?

Suppose I have a list of symbols like: {a,b,c,d} I would like to enumerate all possible binary associations (combining symbols and/or sublists pairwise): ...
12
votes
4answers
645 views

What's the function that takes X[Y[z___]] to Y[X[z___]]? (“swap heads”)

I've seen the operation described in the title before (basically, swap the heads of the first two levels of an expression), but I can't find it... Does anyone know the function (or idiom/incantation) ...
12
votes
3answers
840 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? ...
12
votes
1answer
440 views

How can I collect data for visualization of quick sort?

I know how to implement quick sort in Mathematica. But I don't know how to get the temp data for visualization, because I used recursion. I want the effect similar to this: ...
12
votes
4answers
2k views

Reshape vectors exactly like in MATLAB

This gives a 64×1 column vector in Mathematica: ...
12
votes
3answers
356 views

Taking Top K Occurrences

Having millions of numbers in a list, I want to take top K occurrences. MMA Commonest is very slow so I write my own versions: ...
12
votes
3answers
508 views

Map a function of several arguments

I have three lists, e.g. list1 = {{a,b,...}}; list2 = {{1,2},{3,4},...}; list3 = {{x,y},{z,w},...}; and a function ...
12
votes
4answers
376 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: ...
12
votes
4answers
471 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 ...
12
votes
5answers
417 views

Faster way to convert triangle list to edge list with no duplicates

I have a list of triangles returned by a Delaunay triangulator, in the following format: ...
12
votes
4answers
552 views

Calculate proportion of values for dates

At work, we were discussing when is it the best time to change to winter tires for bikes and/or cars. Using WeatherData[] and ...
12
votes
3answers
768 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: ...
12
votes
1answer
602 views

When to use @@@ vs. /@ [duplicate]

So much thanks to Szabolcs in improving sequence.. for complete explanation. I am so glad to read all explanation which fundamentally remove the problem. But in number 4 of its explanation pointed to ...
12
votes
2answers
197 views

Faster trimming of leading/trailing zeroes in rectangular array

Given a (large) rectangular array, e.g. (small for example case): ...
12
votes
2answers
203 views

why MeanFilter is nearly 100 times slower than MovingAverage for 1D array?

I used to average over my data with MeanFilter before I learn about MovingAverage and thought that these two functions are similar except that MeanFilter is more powerful for its ability to deal with ...
12
votes
4answers
406 views

Equivalent Nested Loop Structure

Consider the following examples: ...
12
votes
1answer
281 views

Mapping on leaves

Say I have some nested structure, such as {a,{{b,c},d,{e,{f,g}}}}, and I want to apply a function $q$ to each of the leaves; that is, I want the output to be ...
12
votes
3answers
925 views

How can I remove the singleton dimensions of an array? [duplicate]

If I have an array A, for which Dimensions[A] is {9, 9, 1, 1, 3, 4}, how can I convert ...
12
votes
2answers
257 views

Efficient conversion between two partition representations

I have a list of distinct expressions. We can represent a partitioning of list in two ways: As a list of sublists ...
12
votes
1answer
520 views

How can I define a Listable function only apply to a vector?

Consider the function f which should behave as f[{{1, 2}, {3, 4}, {5, 6, 7}}] ...
12
votes
3answers
816 views

Simpler way to fill date gaps with zero values

One basic problem when working with data like sales and dates is the case where you dont't have sales in all days, so you have to fill it to take information as average or to make a plot. See this toy ...
12
votes
2answers
1k views

Filtering and Replacing outliers

I have a set of data that measurements of temperature vs time (date). It is easy to realize when the sensor went wrong as one might see a blip on the data. How to teach to Mathematica to: 1) Detect ...
12
votes
2answers
779 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 ...
12
votes
1answer
771 views

How to extract and replace submatrix of a doubly-periodic matrix?

How can I effectively extract and replace a rectangular subpart of a toroidal matrix, i.e. one where boundaries are connected at opposite ends? Since the submatrix can overhang the edges, a simple ...
12
votes
1answer
188 views

ListConvolve Memory Hemorrhage

Bug introduced in 10.0.0 and fixed in 10.2.0. I wrote a routine that convolves an impulse response array with a test array and checks for the RMS difference between that convolution and a data ...
12
votes
2answers
399 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 ...
12
votes
0answers
223 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 ...
11
votes
10answers
676 views

Elegant way to map a function to only the last element of each sublist [duplicate]

Suppose that I have the following list, called test: test = {{{a1}, {a2, a3}}, {{b1}, {b2, b3}}, {{c1}, {c2, c3}}}; Now ...
11
votes
6answers
1k views

Multiplying two dimensional data by a constant

I have a list of lists of the form: {{1, 2}, {2, 4}, {2, 8}} But I want to multiply only the second dimension of that data by a constant. I know I could do this ...
11
votes
11answers
480 views

Replace elements in list not matching elements of another list at the same position

I would like to compare two arrays: m = {a,b,c,d,d,f}; n = {b,b,a,d,d,e}; and get: {0,b,0,d,d,0} I have tried all kinds ...
11
votes
4answers
5k 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 ...
11
votes
4answers
3k views

What is the cleanest way to prevent divide-by-zero warnings?

If I evaluate {1, 2, 3, 4}/{5, 6, 0, 8}, obviously I get one warning: ...
11
votes
7answers
3k views

Combination and Permutation

How could I obtain the list of all the groups of 5 numbers taken from Range[12] such that the 2 lists have an empty intersection : ...
11
votes
7answers
1k views

How can I Flatten this List with Inner List of Lists?

Given the following List ...
11
votes
4answers
630 views

In a list of points, how to efficiently delete points which are close to other points?

Consider a list of points: pts = Partition[RandomReal[1, 10000], 2]; ListPlot[pts] I'd like to delete points so that the minimum distance between two points is ...
11
votes
10answers
577 views

How to pick increasing numbers from the list

How to pick increasing numbers from the list. lst = {5, 3, 6, 2, 7, 4, 8}; out: {5,6,7,8} So many interesting ...
11
votes
5answers
3k views

How to find rows that have maximum value?

Suppose if I have following list { {10,b,30}, {100,a,40}, {1000,b,10}, {1000,b,70}, {100,b,20}, {10,b,70} } How to find rows that have max value in ...
11
votes
10answers
805 views

Finding neighbors from list

Say you are given the following list: list={{3, 2, 1}, {3, 4, 2}, {2, 4, 1}, {1, 4, 3}}; Two numbers $x,y$, not necessarily distinct, are neighbors if one of the ...
11
votes
5answers
4k views

How to create a list of pairs from 1d list(s)?

Say I've got two 1-dimensional lists A = {1,2,3} B = {a,b,c} How do I create a list of all pairs from list A such that the ...
11
votes
7answers
246 views

DateList coupled with String Operations

Through some webscraping, I have the following list: ...
11
votes
4answers
405 views

What's fastest way of defining 10^5 down values?

I want to define isGood[___] = False; isGood[#] = True & /@ list where list is a list of several million integers. ...