# Tagged Questions

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

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### Data Table Manipulation in Mathematica

I am a statistician searching for an efficient way to select rows or columns from a table of data in Mathematica. Let me pose the question in 2 parts with a SQL-style table of data: ...
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### How to apply or map a list of functions to a list of data?

Say I have a group of functions: f1[a_] := a * -1; f2[a_] := a * 100; f3[a_] := a / 10.0; and some data in a list: ...
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### How to get list of duplicates when using DeleteDuplicates?

This might be easy, but can't find a way to use DeleteDuplicates to get also list of the actual duplicates. Example: ...
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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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### 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: ...
724 views

### Sort+Union on a list

Probably this is too trivial question for this forum but for some reason I am not getting it on my Mathematica 7 so far. I have a list ...
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### 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: ...
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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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### Combining the second elements of a set of ordered pairs based on their first element [duplicate]

I have a large set of ordered pairs and I am trying to add the second elements together for any group of pairs that has the same first element. For example my data looks like: ...
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### Looking for a way to insert multiple elements into multiple positions simultaneously in a list

I would like to take the following data: insertValues = {r, x}; insertPositions = {{1}, {5}}; originalList = {a, b, c, d, e, f, g}; and generate output that ...
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### How do I split up a curve into chords of equal length?

I have a curve that is defined as f[x] and what I'm attempting to do is to divide the curve into equal straight lengths for a number of segments of my choosing that I've defined as nSeg. I've created ...
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### Undocumented form for Extract

Prompted by a comments conversation here, here's an interesting (and often performance enhancing) use of Extract: ...
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### Emulating R data frame getters with UpValues

What's the best way to emulate R's data frames functionality? This includes the ability to select rows and columns in a 2-dimensional table by the string identifiers positioned typically in the first ...
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### Replace values which obey certain criteria

I'd like to replace all values in a list which obey one or more criteria with another value. Example: Replace all values>30 by 30. data={{3,35},{2,7}} ...
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### Best way to extract values from a list of rules?

Mathematica has a lot of list manipulation functions, and, also because I don't work with lists often, at times I'm a bit lost. I'll find a way, but I'm sure it's not the most efficient. Case in point,...
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### How do I use Map for a function with two arguments?

I'm a newbie who tries to be a good boy, and use Map instead of writing out a list of functions. I have a table I want to Map ...
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### “Unflattening” a list

Imagine we have a list with some hierarchical structure: list = {{{2,3},{2,4}},{{{3,4}}},{{5,6},{7,8}}}; Which we then flatten to some level in order to process ...
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### how to efficiently apply function to all pairs of a set (and collect the results)

To build a graph, I need to apply a function f[a_, b_] to all pairs of a list (3500 elements). The function itself returns a link ...
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### Extracting values from nested rules in JSON data

I have been using Mathematica to analyse some data from the StackExchange API. It is conveniently available in JSON form, which Mathematica interprets as replacement rules. However, some of the rules ...
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### Efficient way to combine SparseArray objects?

I have several SparseArray objects, say sa11, sa12, sa21, sa22, which I would like to combine into the equivalent of {{sa11, sa12}, {sa21, sa22}}. As an example, I ...
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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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### 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 ...
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### Behavior of Inner depends on List dimensions?

Consider the following case: ...
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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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### Permutations[Range[12]] produces an error instead of a list

This input: Permutations[Range[12]] Results in this (error) output: ...
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### 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 + 1,...
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### How to use Union on list of lists without sorting them first?

If I do ClearAll[a, d] lsts = {{a, d}, {a, d}}; Union[lsts] I get the expected answer {{a, d}} but if I do ...
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### Finding all length-n words on an alphabet that have a specified number of each letter

For example, I might want to generate all length n=6 words on the alphabet {A, B, C} that have one ...
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### another way to do this mapping of a two argument function

Sorry, I'm just starting to learn mathematica. I have the following two-argument function: h[{x_, y_}] := x ^ y When I do the following: ...
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### Counting the population of integers

Suppose that myData is a list of sublists. Each sublist has a length of one or greater and contains any number of replicates of the integers 1, 2, 3, and 4. I ...
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### How to find the position of elements in a list satisfying criteria

Say I have a list x={2,4,6,8,10} and I want to find out the positions of the elements that are greater than 7. ...
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### Finding Local Minima / Maxima in Noisy Data

I'm trying to find local minima / maxima in noisy data, consisting of data values taken at certain time intervals. Ideally, the function should take a pair of lists (one containing time values and one ...
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### Instruct a Table to only evaluate until a condition is fulfilled

I am looking for the way of building a Table of pairs of numbers in a fast way. My true table evaluates huge functions, and I see no way and reason to show those cumbersome expressions here. Let us ...
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### Two ways of map a function on the list: Which one is faster?

There are 2 ways to make a function listable on the list: Use Map or use SetAttributes. My question is, which one is faster, ...
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### Sequentially numbering a nested list

I've got a nested list {a, b, {c, d}, e, {f, {g, h}}} which I want to magically transmogrify to ...
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### How do I find the elements in a list that return the highest value for a function?

Suppose we have a list of values and a function f. I want to find which of the elements maximizes the return value of the function in Mathematica. Let call the ...
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### 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? ...
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### 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 ...
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### Replace For-loop with functional code

I am very new to Mathematica. I have code written using for loops. I want to rewrite it using Map/Thread but I am not sure how ...
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### The gap between MapAt and ReplacePart for 2D data tables with headers

Consider a relational table derived from survey data, where each column ("001-01" ...) represents a responder and each row (...
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### Manipulating a sublist based on position

I have a huge list with huge sublists of the form list={{a,b,c,d},{e,f,g,h},{i,j,k,l}} I am looking for a way to manipulate these sublists based on the positions ...
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### Threading elements over corresponding elements in the second list

I often need to merge two lists like the following: $A=\{\{0.1\},\{0.2\},\ldots,\{1\}\}$ $B=\{\{x_1\},\{x_{21},x_{22},x_{23}\},\{x_{31},x_{32},x_{33}\},\ldots,x_N\}$ in a way to ideally obtain a ...
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### How to switch from List of rules to Association? [duplicate]

Considering we have a list of rules: ...
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### Replace very small numbers by zero in the output of a list

I have a long nested list, but many elements are numbers of order 10^-18 or less, e.g. ...
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### How to sum over duplicates?

I have a list of the form: {{1, 0, 1}, {2, 1, 2}, {1, 0, 3}, {2, 4, 2}, {2, 1, -1}, {1, 0 ,0}} that I would like to sort such that I calculate the sum over the ...
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### How to sum over a List

list = {11.5575, 11.397, 5.52734, 4.0878, 2.54815, 1.86652, 2.55028, 2.14952, 1.6242, 1.34117} I have a list of numbers. How do I make a function that creates a ...
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### Convert head Times to List

I can convert an expression with the head of Times,for example, 12*12.5*13*13.5*14*14.5 to {12, 12.5, 13, 13.5, 14, 14.5} by ...
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### Removing an ordered pair from a list of ordered pairs if the second element in the list is Less than a Value

I have a list of ordered pairs from a piezoelectric sensor sampled at 2kH. The first element is time and the second, amplitude. I'm trying to remove all ordered pairs in the list of ordered pairs when ...
### Insert $+$, $-$, $\times$, $/$, $($, $)$ into $123456789$ to make it equal to $100$
Looks like a question for pupils, right? In fact if the available math symbol is limited to $+$, $-$, $\times$, $/$ then it's easy to solve: ...