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Feb
16
comment Memory limit hit: optimize code for finding twin primes
You should be able to use testPrime in parallel, e.g. `ParallelTable[testPrime[Prime[j], 1], {j, 4, 1000}]
Feb
15
answered Memory limit hit: optimize code for finding twin primes
Feb
15
comment Brackets around each item in matrix
I get similar results, to within a scaling factor for my slow CPU. Interesting that Join is much worse on packed arrays!
Feb
15
awarded  Guru
Feb
14
comment Role of Accuracy in numerical evaluations, SplitBy vs GatherBy
A simpler formulation of the same fundamental question: Given list = SetAccuracy[{1, 1, 1.01, 1.01, 1.02, 1.02}, 2] why does Gather[list] give a different result from Split[list]
Feb
14
comment GatherBy/SplitBy and Sort a list
IMO the best thing would be to remove both updates from the question, and accept an answer which best addresses the problem as originally asked. Then create a new question about why two low-accuracy numbers can give True for a==b but False for N[a]==N[b].
Feb
14
comment GatherBy/SplitBy and Sort a list
The issue of how N affects Accuracy is a quite different problem to the original question regarding efficient sort & gather algorithms. It should be a separate question. People took time and trouble to answer the original question, and it's rather unfair to claim that their (perfectly good) answers don't work because of special features in your data that you didn't reveal until several hours later. It will also be more useful to future visitors looking for efficient sort & gather algorithms if the question is unencumbered by the requirement to avoid using N.
Feb
13
comment GatherBy/SplitBy and Sort a list
Check those results with a calculator, they are wrong!
Feb
13
comment GatherBy/SplitBy and Sort a list
No, the norms are {0., 1.64012, 0.632456, 0.632456, 1.64012, 1.58114, 0.640312, 0.6, 1.6, 0.640312, 1.58114}
Feb
13
comment GatherBy/SplitBy and Sort a list
f7 is lovely. One of those bits of code that seems obvious, once someone else has thought of it...
Feb
13
comment GatherBy/SplitBy and Sort a list
I'm not sure why that's what you expect. For example {-0.5, 0.4} and {0.2, -0.6} have different norms, so why are you expecting them in the same sublist?
Feb
13
revised GatherBy/SplitBy and Sort a list
fixed typo
Feb
13
comment GatherBy/SplitBy and Sort a list
For the first one you need to take into account that GatherBy returns lists of points, so you need to apply Norm to the first element not the whole list, e.g. SortBy[GatherBy[list2, Norm], N[Norm[#[[1]]]] &]
Feb
13
comment GatherBy/SplitBy and Sort a list
Well GatherBy[SortBy[list2, N[Norm[#]] &], Norm] gives the correct answer unlike the typo version.
Feb
13
comment GatherBy/SplitBy and Sort a list
The difference between using Norm and #.#& accounts for a lot of the speed differences. It would be interesting to compare the different approaches using the same norm function throughout.
Feb
13
comment GatherBy/SplitBy and Sort a list
Regarding the update, you have a misplaced bracket. N[Norm[#] &] should be N[Norm[#]] &
Feb
13
comment How to mix symbols and text for labels in manipulate?
You have multiplied the subscript and the string, instead use Row to format expressions in a row, e.g. Row[{Subscript[φ, 0], "(degrees)"}]
Feb
13
comment GatherBy/SplitBy and Sort a list
I suggest using #.#& in place of N@*Norm for integers - it omits the square root step but that doesn't change the ordering.
Feb
13
comment GatherBy/SplitBy and Sort a list
I had result = With[{n = #.# &}, list~GatherBy~n~SortBy~n@*First] - same principle but faster if list is integers.
Feb
13
comment How to count proportion of two phase in a electron microscope picture
Do you have a version of the image without the text and arrows overlay?