# Questions tagged [list-manipulation]

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

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### What is a Mathematica packed array?

A simple sounding question with a few sub questions: What is the difference between unpacked vs packed array? Are packed arrays more space efficent, how much so? Are packed arrays more time ...
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### Why does MatrixForm affect calculations?

This is a really newbie question, but it has me confused. Why does this code work without // MatrixForm and doesn't work with ...
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### Elegant operations on matrix rows and columns

Question The Mathematica tutorial has a section 'Basic Matrix Operations', describing operations like transpose, inverse and determinant. These operations all work on entire matrices. I am missing a ...
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### Function in Table

I need a table with the elements made of pure functions and list elements. This is a simplified example: I need a list as: ...
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### Flatten command: matrix as second argument

One thing I could never wrap my head around is how Flatten works when provided with a matrix as the second argument, and the Mathematica help isn't particularly ...
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### Partitioning with varying partition size

How can I partition a list into partitions whose sizes vary? The length of the $k$'th partition is a function $f(k)$. For example: if $l = \{1, 2, 3, 4, 5, 6\}$ and $f(k) = k$. Then the partitioning ...
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### Combining values in one column (or part) when values in another column (or part) match

Consider the following: ...
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### How to efficiently find positions of duplicates?

Is there an efficient way to find the positions of the duplicates in a list? I would like the positions grouped according to duplicated elements. For instance, given ...
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### Find subsequences of consecutive integers inside a list

I have a list which is something like this: {3,4,5,6,7,10,11,12,15,16,17,19,20,21,22,23,24,42,43,44,45,46} What I'd like to to is get the intervals which are in ...
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### Does Mathematica have advanced indexing?

I have two $M \times K$ arrays $L, T$ where I would like to set all the elements in $L$ to zero whenever the corresponding element of $T$ is greater than 15. The ...
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### Mathematica Destructuring

Context I'm writing a function that look something like: ...
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### What is the most efficient way to add rows and columns to a matrix?

Say I have a matrix m and a vector v. ...
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### Removing elements from a list which appear in another list

There are two lists {a, b, c, a, d, a, e} and {a, c, a}. I need to remove those elements from the first list which appears in a second list, to get {b, d, a, e}
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### 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? ...
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### Partition a set into subsets of size $k$

Given a set $\{a_1,a_2,\dots,a_{lk}\}$ and a positive integer $l$, how can I find all the partitions which includes subsets of size $l$ in Mathematica? For instance, given ...
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### How can I make threading more flexible?

Threading automatically with Listable functions requires the argument expressions to have the same length (or for one of them to be atomic). For nested lists the ...
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### How to remove redundant {} from a nested list of lists?

There are numerous examples here, whose end result is the removal of empty brackets {} and empty lists. I still can't find an example of simply removing redundant ...
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### Finding all elements within a certain range in a sorted list

Suppose we have a sorted list of values. Let's use list = Sort@RandomReal[1, 1000000]; for this example. I need a fast function ...
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### How do I convert an argument list to a sequence of arguments?

I think this is a basic question, but I am having difficulty finding the answer in the documentation. Thread is not what I am looking for, I think. Suppose that I ...
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### Are there guidelines for avoiding the unpacking of a packed array?

Packed arrays are very useful because they save memory and generally allow speedier and more efficient calculation times. If a list of data currently stored as a packed array is unpacked, it can slow ...
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### How to delete elements from list1 appearing in list2?

I'm new to functional programming of Mathematica and trying to remove one list of assorted elements from another. However I only find functions working with the Index rather than the values itself: ...
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### Sort lists according to the order of another

I have three parallel lists (i.e. the elements in position i of each list are related). I want to sort the first list using the function Sort and make the same ...
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### 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 ...
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### How to Set parts of indexed lists?

I would like to assign a list to an indexed variable and then change it using Part and Set like this: ...
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### How to select minimal subsets?

I am a newbie, so please point me in the right direction if you feel this question has been answered somewhere else before. Here goes: Suppose I have a list like this: ...
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### Map-Thread-Through-Apply a list of functions onto a list of (lists of) values

I have a list of functions: fns = {f, g, h} and a list of triples: ...
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### How to represent a list as a cycle

I want to select distinct cycles (represented by List). The cycle {a, b, c, d} must be the same as {b, c, d, a} or ...
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### Find zero crossing in a list

I'm looking for a function that finds the index of the zero-crossing points of a list. Before I go making my own subroutine to do this, I was wondering if anyone knows of any built-in Mathematica ...
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### How to make MapAt work with Span?

Span (;;) is very useful, but doesn't work with a lot of functions. Given the following input ...
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### Data interpolation and ListContourPlot

I am fairly new to Mathematica and I have two quick questions on using it for a Hydrology and Hydrogeology class. One is about data interpolation and interpolating without any data defined in an area. ...
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### How to Map a subset of list elements to a function?

How would you, given a list {1, 2, 3, 4}, apply a function f to 1 and ...
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### Permutations[Range[12]] produces an error instead of a list

This input: Permutations[Range[12]] Results in this (error) output: ...
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### Optimising 2D binning code

I have a set of (x,y,z) data, 45,000 to be precise and I want to bin the z values in 256 equidistant bins based on their (x,y) values. The final array should be a set of 256x256 array with each slot ...
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### 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 ...
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### Map a function across a list conditionally

It seems that this is a really basic question, and I feel that the answer should be obvious to me. However, I am not seeing. Can you please help me? Thanks. Suppose I have a list of data ...
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### How can I remove B -> A from a list if A -> B is in the list?

I have a list of transformations like this: list = {"A" -> "B", "B" -> "A", "C" -> "D"} As this is used to plot an undirected graph with ...
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### Delete duplicate elements from a list

If a list contains duplicate elements, for example list = {a, 1, 5, 3, 5, x^2, x^2}, how can the duplicate elements be removed? The result would be ...
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### How to 'merge' a list like FromDigits, but with a mixture of numbers and symbols?

How do I go from {C,4,G,5,S,7} to C4G5S7
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### Applying And to lists of Booleans

I'd like to take {True,True,False} and {True,False,False} and apply And to get ...
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### 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 ...
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### 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 ...
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### Finding all partitions of a set

I'm looking for straightforward way to find all the partitions of a set. IntegerPartitions seems to provide a useful start. But then things get a bit complicated. ...
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### A function that accepts a pair or a list of pairs

Probably a duplicate, but it's not easy to search on "lists". I have a function that accepts a list of lists, say {{a1, b1}, {a2, b2}, {a3, b3}} and performs some ...
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### 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 (...
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### Using sets of arguments from a list

Please Consider : list={{5,2,6},{2,8,3}} I need to get the product of all the element within list : As of now I do this : ...
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### Contracting with Levi-Civita (totally antisymmetric) tensor

I have an array $v_{ijk}$ which is effectively a rank-$3$ tensor with dimensions $3\times3\times3$, and I need to contract it with $e^{ijk}$, i.e. evaluate $v_{ijk} e^{ijk}$ (see Einstein convention)....
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### Transpose uneven lists

Is there a quick method to transpose uneven lists without conditionals? With: Drop[Table[q, {10}], #] & /@ Range[10] Thus the first list would have the ...
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### 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. ...
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