Questions about applying functions or operators to expressions, especially constructs that take advantage of Map (/@) functionality.

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30
votes
12answers
2k views

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 ...
29
votes
3answers
944 views

Changing Values in an Association using Map

I am trying to change the value of one key in an association based on the value of another key in that association. So if my association starts as: ...
20
votes
2answers
1k views

Scan vs. Map vs. Apply

I cannot understand the difference between Apply, Scan, and Map. I have tried to play with ...
17
votes
2answers
443 views

Can A Plot Be Overlayed With A Map?

Some brief, only slightly important background. I am doing a research project using the data from NASA's GRACE mission. I wrote a short Perl script to take two data files and find the change in ...
11
votes
4answers
400 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. ...
10
votes
6answers
350 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: ...
9
votes
5answers
513 views

Span a function across several consecutive elements in a list

Suppose that I have a list called list. I would like to span an arbitrary function f across specified consecutive elements in ...
9
votes
3answers
354 views

Operate over list of pure functions

I have list of pure functions (All functions are InterpolatingFunction) i.e {{a, b}, {c, d}, {e, f}, ...} and I would like ...
8
votes
8answers
503 views

How to use Map inside MapThread?

Given this input lst1 = {{a, b, c}, {d, e, f}}; lst2 = {1, 2}; and the goal is to generate this output ...
8
votes
3answers
707 views

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 ...
8
votes
3answers
2k views

Mapping a pure function with multiple slots [duplicate]

Here is a pure function with multiple slots: (#1^2 + #2^2)&[3, 4] 25 But how can I map that pure function over lists of values? For example, say I want ...
8
votes
5answers
233 views

How to Map a function with HoldAll attribute

First of all, I'm not sure about the title of this question, so please correct me if there is any title more sensible than mine. The problem is the following: I have a function f that checks if a ...
8
votes
3answers
278 views

finding anagrams

What I'm trying to find are the 4 letter words in the mathematica dictionary that have the most anagrams and the amount of anagrams these have each. But I can't seem to find all the 4 letter words in ...
7
votes
3answers
363 views

Functional equivalent of a For loop referring to consecutive elements in a list

I have implemented this function which smooths the values in a list by, if it encounters a zero, replacing this entry by the average of its two neighbours (apart from the first and last values of the ...
7
votes
2answers
375 views

Using Map with If

I have an If statement I need to carry out where the result should be a single number for each time the if is executed. What I have at the moment returns a list for each time the If is executed. I ...
6
votes
2answers
176 views

Speed and the MapThread function

Can anyone help explain why MapThread is so slow in this simple case? ...
6
votes
2answers
529 views

Map a function over the columns of an M x N array

I have a $N \times M$ list of lists, or rather, a list of records and I want to map a function over its columns (e.g. find the mean temperature, age, etc.). Map ...
6
votes
2answers
346 views

Apply function to every element of list regardless of depth

I want to apply a function to every element of a list without taking braces into account (depth level), but with the same form of output. Here is an example : ...
6
votes
2answers
296 views

Mathematica periodic moving map

I am pleased to have MovingMap in Mathematica 10. But, today, I encountered the problem that needs MovingMap with a periodic boundary condition. To be specific, Consider the following example. ...
6
votes
3answers
199 views

Mapping a function over the parts of a deeply nested Array

I have two tensors: tensorS = Array[Subscript[s, #1, #2, #3, #4] &, {2, 16, 16, 19}]; tensorR = Array[Subscript[r, #1, #2, #3] &, {2, 16, 19}]; ...
6
votes
2answers
787 views

Nested Map and Apply

There are some other questions on this topic but I could not get an answer from reading them. What I want to do is use Apply on some of the arguments of a function, ...
6
votes
2answers
240 views

Variable scoping problem when mapping over delayed replacement

This is something that got me curious while I was playing around with Mathematica. Consider the following (contrived) example: ...
6
votes
1answer
568 views

nested use of Apply/Map/MapThread in pure functions

1. My main question I have a function with two argument slots. I wan't to apply this function to 2 lists with different length's. I thought in this solution: ...
5
votes
6answers
263 views

Indexed Map Thread

I'm attempting to MapThread a function of two lists that requires the index of the list values. For example, MapThread[#1*i+#2*j&,{{a,b,c},{e,f,g}}] Where #1 ...
5
votes
3answers
4k views

Map w/ multiple arguments [duplicate]

Possible Duplicate: How do I use Map for a function with two arguments? If I have a function: F[x_,y_,z_]:=x*y*z and I want to call it with several ...
5
votes
3answers
222 views

how to concatenate a list of a list of strings

I have two lists of lists, La and Lb that I want to concatenate so that every element of a list n in La is concatenated with the corresponding element of list n in Lb. For example: La = {{P, O, O, ...
5
votes
2answers
116 views

A problem with Compile and Map

There is a weird behavior of Compile that I do not understand. Consider the following example: ...
5
votes
2answers
134 views

Is there a built-in function to map a list of heads to a list of data?

Assume that this built-in function is called func. What I want is func[{f,g},{a,b,c}] will return the following result: ...
5
votes
2answers
144 views

Indexed Thread on dataset and list

I currently have a dataset, and I would like to add a column based on another list. For example: ...
5
votes
1answer
140 views

Mapping over an association producing an unexpected result

The documentation for associations says that the keys of an association are transparent for many operations. Map is one such operation. However, I am confronted ...
5
votes
1answer
85 views

MapIndexed stops at levelspec 1 if only the index is passed?

If the function passed to MapIndexed references the first (or data) argument, the following performs as expected: ...
5
votes
1answer
155 views

How can I substitute something using only map?

For example, I have the equation x^2 + y^2 == x*y and I want to apply the rule y -> s*x I can do it easily by ...
4
votes
4answers
123 views

Plot range of a 2-dimensional function as a function depending on one variable

I want to plot a function of 2 variables x and y, for discrete values of y, like: ...
4
votes
2answers
184 views

How to convert a list of 3D points to a list of distances?

How do I apply a function (this case EuclideanDistance) between element i and i+1 on a list ...
4
votes
3answers
154 views

How to map over inner keys of nested Associations?

Considering an association: <|"a" -> <|aa-> "asc", bb-> "asd", cc-> 0, ImageType -> "asd", dd-> "asd"|>|> How can I make the inner ...
4
votes
1answer
521 views

Why is there a huge performance gap using Map with more than 100 List entries

im using Map on a List like this: cube= {{1, 1, 1}, {1, 1, 2}, {1, 1, 3}, {1, 1, 4}, ... , {5, 5, 4}, {5, 5, 5}} Mapping the whole List with 125 entries takes ...
4
votes
1answer
163 views

MovingMap maps to second level unexpectedly

I'm trying to build a financial indicator in mathematica. I import some historical data ...
4
votes
1answer
500 views

Using Thread or MapThread over multi-level, ragged lists [duplicate]

This may be a rather strange question. I will try to create a sort of minimal working example. Suppose that I have two lists, list1 and ...
4
votes
1answer
249 views

MapThread Alternatives

I want to add two matrices, the first one containing a 2D vector at each position the other one a list of 2D vectors. The 2D vector from the first matrix should be added to each of the 2D vectors at ...
4
votes
1answer
132 views

Applying function to cartesian product of two lists

I have two lists X = {1, 2, 3}; Y = {5, 6, 7, 8}; I want to apply function g[x,y_,z_] to all pairs from X*Y, so I need to ...
4
votes
1answer
101 views

How can Map be given access to the keys of a Dataset?

Consider an example Dataset: ...
4
votes
2answers
78 views

Count the number of element repeats at various distances

I have a function of x: f[x_] := Abs[Extract[list, x + n] - Extract[list, x]] I then apply this: ...
4
votes
1answer
127 views

Parallelize[Mapindexed[]], level={-1}? Sal Mangano's 《Cookbook》

It is easy to Parallelize[] Map[] & MapIndexed[], as having been shown in MMA9 Help for ParallelMap[] and Parallelize[]. For MapIndexed[], the default levelspec is {1}. However there are many ...
4
votes
2answers
131 views

Can one use Map during Expand?

Can one Map a simplifying function to the summands of an Expand before fully expanding the expression in question first? ...
4
votes
1answer
173 views

MapThread and Compile

After discovering that MapIndexed cannot be used with compile (see MapIndexed and Compile) I am now trying do implement similar functionality using a combination of ...
3
votes
4answers
395 views

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: ...
3
votes
4answers
372 views

How to apply a function of several arguments to a list?

I have a list of triplets: list = {{1, 3, 5}, {4, 5, 1}, {4, 9, 2}} How can I apply a function f to the last element of each triplet when f the last and ...
3
votes
4answers
160 views

Correct slot assignment when mapping from different lists using pure function

Given expr = u == a1 + a2 x + a3 y; and uRep = {u1, u2, u3}; xyRep = {{x1, y1}, {x2, y2}, {x3, y3}}; I'd like to generate ...
3
votes
3answers
499 views

Efficiently compute double sum

Is there a "Mathematica Way", like Map or Apply to compute the following double sum? $\sum_{i=1}^{N_1}\sum_{j=1}^{N_2} m_i n_j \, f(\tau_{i} \gamma_{j})$ I have already stored the lists ...