Questions tagged [list-manipulation]

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

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19
votes
4answers
588 views

Is there a faster way to create a matrix of indices from ragged data?

I have data that is given as a list of ordered pairs mixed with scalars. The pairs can contain infinite bounds. My goal is to convert the data into an index used in future computations. ...
19
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1answer
276 views

Why changing list's head takes time?

Consider AbsoluteTiming[Range[10^7];][[1]] 0.035000 and AbsoluteTiming[HoldComplete @@ Range[10^7];][[1]] 0.725000 ...
19
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1answer
518 views

Are there advantages to using additional arguments of common functions rather than alternative ways of calculating with lists?

(Apologies for the long question title.) One of the interesting, if sometimes confusing, things about Mathematica is that there is always more than one way to do things. Even intermediate users can ...
19
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1answer
545 views

The efficiency compare between Flatten[#, 1] & and Join @@ # &

Err.. Often I met the situation to join lists at the first level and I used to just Flatten[#, 1] & @ them. However, I found (when glance over the mathematica....
18
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11answers
2k views

How to subtract the column means from each row of a matrix?

Given a matrix, we want to subtract the mean of each column, from all entries in that column. So given this matrix: ...
18
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7answers
14k views

Mathematica: finding min/max in list

I have a list of coordinates like this one: {{x1,y1},{x2,y2},{x3,y3},...,{xn,yn}} I need to get the minimum and the maximum of all x-values and the minimum and ...
18
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6answers
6k 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 ...
18
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7answers
2k views

How to map and sum a list fast?

The following takes a lot of memory and is slow: AbsoluteTiming[Total[Map[#^2 &, Range[1, 100000000]]]] I suspect this is because it first calculates and ...
18
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9answers
2k views

Position of the first nonzero number

I want to know is there any one line code to find position of first nonzero number without calculating position of all non zero numbers. For example, ...
18
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5answers
2k views

Add a vector to a list of vectors

What is the syntax to add v1 to each vector in v2? I know it has to be simple, but I really have searched and not found it. <...
18
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7answers
623 views

How to replace all elements of a list by a rule “element” -> “element_”

I have a list: {a, b, c, d, e, f, g, h, i, j} The goal is to get: {a_, b_, c_, d_, e_, f_, g_, h_, i_, j_} I can replace ...
18
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8answers
5k views

extract values from replacement list

Solve returns a list of replacement rules In: Solve[x + y == 3 && x - y == 6, {x, y}] Out: {{x -> 9/2, y -> -(3/2)}} I am only interested in the ...
18
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8answers
1k views

How do I delete all items that occur more than once?

DeleteDuplicates works fine but leaves a single copy of the duplicated item. I need to remove all items that occur more than once i.e. {{1,2},{1,2},{3,4}} -> ...
18
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5answers
2k views

Picking random items out of a list only once

I'm trying to create a function that randomly returns a value from a list but remembers the values that have been given before. At the end when the list is empty it should return an empty list. ...
18
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6answers
4k views

Max of a table/list with indeterminate values

Suppose I have a Table: tab = {1., 2., 3., Indeterminate} When I type Max[tab] I get ...
18
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6answers
1k views

How do I replace a missing value in a column with the value immediately above throughout a table?

I have a large table (7000 rows × 17 columns) of terse textual data. In many of the columns, empty entries have been replaced with "." as a marker. Working from the top of the columns downward, I want ...
18
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3answers
2k views

Fastest way to check for List membership

I have a set of say 100 numbers {1,3,7,11,19,...3971}. All elements are previously determined. I want to check whether ...
18
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4answers
1k views

“Indexing” a vector

I need to "index" a vector according to the first occurence of its distinct elements. I have written ...
18
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4answers
2k 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 ...
18
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4answers
681 views

Splitting a list

Please consider the following list: data={1, 0, 0, 0, 2, 5, 2, 3, 0, 0, 3}; Now I would like to split the list into 0-sequences and Not-0-sequences as following: ...
18
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4answers
1k views

Exclude data from a list

Let's create some random sample data data = Table[{RandomReal[{-10, 10}], RandomReal[{-10, 10}]}, {i, 1, 300}]; Then we plot them using ...
18
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5answers
859 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 ...
18
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5answers
3k views

List comprehension in Mathematica

Python supports a concept called "list comprehensions". Following is sample example (written in Python syntax) ...
18
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2answers
767 views

Part extraction vs functions like First, Last?

There are many ways to extract different parts of lists in Mathematica. For example, the first part of a list v can be accessed either as ...
18
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2answers
5k 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 ...
18
votes
1answer
2k views

How to split a Dataset into training and testing for machine learning?

Suppose I have iris data In the Dataset Format. How to split the iris data Into training and testing for Machine learning? For example,Transform the data for Classify.
17
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7answers
4k views

How do I check if any element in a list is positive?

As a simple example of what I would like to do, suppose I have a list a of all real numbers. I would like to perform a simple check to see if some element of ...
17
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7answers
5k 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 ...
17
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7answers
947 views

How to find all the positions of max value of a list efficiently?

For Example, list=RandomInteger[{1,100},2000]. Yes,I know Position[list,Max[list]] can do. But it's based on pattern matching!...
17
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10answers
3k views

Match patterns on list of Strings

Consider the following: data={"AB","CD","AF"}; Now I would like to delete all String from data which starts with "A". Result:...
17
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5answers
4k views

How can I get exactly 5 logarithmic divisions of an interval?

I'd like to get exactly 5 divisions from x to y on a log scale. Can FindDivisions do this?
17
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5answers
1k views

Delete duplicates from list of lists as if on a necklace

Given a list of lists of integers, all of the same length, I want to regard two elements as equivalent if one is the rotated and/or reflected version of another (that is, if the two lists are the same ...
17
votes
3answers
1k views

Performance of Select

I have a dataset of 3D coordinates with a length of about $ 4\times 10^6 $. From this volume I am sequentially selecting coordinates along one axis and manipulating this subset. My question: Can the ...
17
votes
5answers
552 views

Partitioning a list of numbers the Mathematica way

I have two sorted lists, one list will be called the "fences" and the other the "values" Fences could be: $\{1, 5, 9, 14\}$ Values could be $\{-1, 1, 3, 4, 6, 9, 10, 13, 14, 15\}$ I want to ...
17
votes
7answers
596 views

Splitting a list by specifying section headers

I have a list of strings called mylist: mylist = {"[a]", "a", "a", "[b]", "b", "b", "[ c ]", "c", "c"}; I would like to split ...
17
votes
5answers
3k 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 ...
17
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6answers
750 views

A quicker than outer

I need to get a matrix $\{a(x_i-x_j)\}$, where $x_i$ form a partition of an interval, $a(x)$ is a given function. I use ...
17
votes
4answers
5k views

Function to subdivide interval into n evenly-spaced points

[This post needs better tags than I could come up with. Edits to the tags would be particularly welcome.] I realize that it is trivial to define a function that takes an interval (i.e. two endpoints,...
17
votes
9answers
1k views

Splitting a list into increasing sublists

I have a list of numbers (say, for example, {1,2,3,4,3,2,1,2,3,4,3,2}), and I want to split it into sublists so that each sublist is the (smallest) sublist whose ...
17
votes
4answers
348 views

Setting parts of a list

Suppose I have list a = Range[10] {1, 2, 3, 4, 5, 6, 7, 8, 9, 10} in which I want to set some elements to be a list ...
17
votes
7answers
835 views

Elegantly pairing up mismatched lists

HistogramList returns a list of bin boundaries and a list of counts. There is one more boundary than counts, and I'd like to pair them up so I can feed it into <...
17
votes
3answers
1k views

How to group and join points

It seems there are many similar questions on Mathematica.SE, but none addresses (to the best of google) exactly my problem. I have the following data: ...
17
votes
1answer
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: ...
17
votes
8answers
1k views

Enumerating tuples from a larger space such that all pairs of values are present at least once

I am trying to write a function that will accept a list of N lists and returns a list of N-tuples that represent all pairs of values from the original N lists at least once. To illustrate with an ...
17
votes
2answers
627 views

Count number of sublists with a total not greater than a given max

Suppose I have a list of positive integers: data={1, 1, 2, 3, 3, 3, 5, 5, 5, 7, 7, 8, 8, 9, 10, 10, 12, 16, 23} I want to count the number of subsets up to ...
16
votes
7answers
2k views

Deleting sublists from lists

I have a list consisting of symbols and integers. I would like to delete the integers from the list only if a symbol precedes and follows each integer: ...
16
votes
5answers
738 views

Interlacing a single number into a long list

This seems like it should be a simple question, but I am running into some difficulty in doing this with Mathematica. Right now, I have a list like this: ...
16
votes
10answers
854 views

Count the sequences in an array

Is there a short and easier way to count number of sequences in a list? Let's say I have three lists: list1 = {0,0,0,0,0,0,0,1,1,1,1,1,0,0,0,0,0,0,0} ...
16
votes
10answers
764 views

Distributing elements across a list of lists

I have two lists: u = {{1, 3}, {2, 6}, {3, 9}} v = {0, 4} and I want to obtain this list from them: ...
16
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6answers
5k views

Counting negative values in list

I would like to count the negative values of a list. My approach was Count[data, -_] which doesn't work. How can I tell Mathematica to count all numbers with ...