32
votes
Accepted
Parallelize Map and ParallelMap
I did a bit of debugging to find the cause of the problem. After I found it, the problem no longer seems as outrageous as it looked at first sight.
The root cause of the problem is the following ...
- 236k
25
votes
Accepted
Metropolis-Hastings Algorithm - Significantly slower than Python
Here a bit of inspiration for you. The following code should be approximately as fast as your Python code.
Preparation:
...
- 110k
17
votes
Accepted
Map vs. Table for index-specific operations on 2D arrays
Many index-specific operations can be implemented via MapIndexed with a level specificaton. Your Power example can be written as:...
- 7,886
16
votes
Accepted
Map every second element of a list
Edit
Ok, so an incorrect answer producing arrays like:
{{"0.", RGBColor["0.00000000000", "0.00000000000", "0.00000000000"]}}
received +12 and an accept. Let's ...
- 138k
16
votes
Simplify the code
☺lookMaNoLetters☺ = 1 ## & @@@ # & /@ # &;
☺lookMaNoLetters☺ @ mylist
{{y1 y2 y3, y3 y4 y5}, {w1 w2 w3, w4 w5 w6}}
Further variations:
...
- 401k
16
votes
Monitoring Apply / @@@ over large lists
You can always transform an expression of the form Apply[f,list,level] to the (almost) equivalent Map[Apply[f],list,level]. You ...
- 34.4k
15
votes
Accepted
Determining moving ratios over a list
list = {5040, 1460, 280, 76, 16, 2, 1};
Ratios:
1 / Ratios @ list
{252/73, 73/14, 70/...
- 401k
14
votes
Accepted
Map a two argument function to every element on list
Since you're looking to learn more about functional style, I'll elaborate rather than just provide an answer.
We start with our table:
...
- 34.1k
14
votes
Accepted
13
votes
Accepted
How can I create an atomic head or treat some head as atomic during Map?
Following this tutorial,
tmp = atomicHead[1, 2, 3];
System`Private`SetNoEntry@tmp
Map[g, {0, {tmp}}, {-1}]
...
- 62.3k
13
votes
Accepted
How to create a matrix of paired values from two matrices?
MapThread[List, {m1, m2}, 2]
{{{a, w}, {b, x}}, {{c, y}, {d, z}}}
More alternatives that yield the same answer:
...
- 17k
13
votes
Accepted
How can I "multiply" nested lists?
Times @@@ Tuples @ {alist, blist}
{{a r, b s, c t}, {a x, b y, c z}, {d r, e s, f t}, {d x, e y, f z}}
- 401k
12
votes
Accepted
ParallelMap vs Map performance comparsion
The difference between the parallelized and the serial version is not due to auto-compilation. Auto-compilation will be used on the subkernels too. You can easily check that turning it off there ...
- 236k
12
votes
Accepted
Unexpected behavior with Map
As noted in comments, the standard ways to evaluate such result is to use Composition or pure function.
Using Composition:
<...
12
votes
Accepted
Simplify the code
Since you appear to want to multiply the list only when all of its elements are atomic, how about just
mylist /. {s__?AtomQ} :> Times[s]
It appears to be the ...
- 5,371
12
votes
Accepted
How can I apply a function only to even-index elements of a list?
I changed D to Dd:
MapAt[Dd, A, 2 ;; ;; 2]
- 34.1k
12
votes
How can I apply a function only to even-index elements of a list?
You can use the new ApplyTo:
A = {1, 2, 3, 4, 5};
Dd[P_] := P + 1
A[[2 ;; ;; 2]] //= Dd;
A
(* {1, 3, 3, 5, 5} *)
- 49.8k
12
votes
Accepted
Maps between finite sets
alist = {3, 4, 5};
blist = {"a", "b"};
mapfs = Tuples@Outer[Rule, blist, alist]
$$\begin{array}{l}
\{\text{a}\to 3,\text{b}\to 3\} \\
\{\text{...
- 59.5k
12
votes
How to create a matrix of paired values from two matrices?
Since particular use of Transpose is shown, let me show the particular use of Flatten:
...
- 68.4k
12
votes
Accumulated instance count of each list element
accumulateCounts := Module[{c$}, c$[_] = 0; Map[PreIncrement@*c$]]
accumulateCounts @ {m, i, s, s, i, s, s, i, p, p, i}
...
- 401k
11
votes
Adding dots to an empty region of space
I feel this answer is as sensible as the question.
First, binarise the image:
m = Import["maz1.jpg"]
bin = Opening[ColorNegate@Binarize[m], 6]
Find the biggest ...
- 14.3k
11
votes
Accepted
List of lists elementwise greater/smaller than
Boole@Positive[a-b]
{{0, 1, 0}, {1, 0, 1}, {0, 1, 0}}
1-UnitStep[b-a]
{{0, 1, 0}, {1, 0, 1}, {0, 1, 0}}
- 401k
11
votes
Mapping string into integers
For the specific numbering in OP, you can also use LetterNumber:
LetterNumber[l]
{{2, 3, 4}, {5, 2}, {1, 2, 4, 5}}
- 401k
11
votes
What should mapping over a number or symbol produce?
@lericr has given a nice answer, but there is another aspect of the OP's question I would emphasize. My feeling is that there is an implicit assumption in the question. The question states that a list ...
- 245k
10
votes
Accepted
10
votes
Accepted
Map[Remove[#]] doesn't seem to do anything
As stated in the comments, a is evaluated before being fed into Remove. We can prevent this by using ...
- 36.8k
10
votes
Accepted
10
votes
Map every second element of a list
If we define:
rgb[s_] := RGBColor @@ StringReplace[s, Whitespace~~n:NumberString :> ToExpression[n]]
then we can write any of the following expressions:
<...
- 69.3k
10
votes
Accepted
Parallelize[MapIndexed[...,Association[...]]] returns broken result
Workaround
Wolfram support suggested the following workaround until the issue is resolved:
...
- 34.4k
10
votes
Accepted
Iterate MapThread with matrices
You can use Tuples:
Tuples[f[{a, b, c}, {a, b, c}]]
{f[a, a], f[a, b], f[a, c], f[b, a], f[b, b], f[b, c], f[c, a],
f[c,...
- 401k
Only top scored, non community-wiki answers of a minimum length are eligible