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

0 votes

One to Many Lists Merge

(* combine[expr_, l_] := With[{matches = Select[l, First[#] == First[expr] &]}, If[Length[matches] == 0, {Append[expr, Missing[]]}, Map[Join[expr, Rest[#]] &, match …
Igor Rivin's user avatar
  • 5,134
1 vote

Find maxima and minima of data like {time,value}

To find minima, run the same command with your data negated. To find the times and values, do Transpose[{data[[PeakDetect[data]], 1]], FindPeaks[data[[All, 2]]]}]
Igor Rivin's user avatar
  • 5,134
2 votes

Short way to combine the elements of a list with an operation

For the particular question: If[Or @@ (# > 1& /@ a), 1, 0] but there is a problem you should be aware of: Or returns immediately after a True clause, so if your list is $10^{10}$ long, the version …
Igor Rivin's user avatar
  • 5,134
4 votes
Accepted

Swapping of elements

Assuming I understand the question correctly, Permute[xp, Cycles[{{1, 21}}] will do the needful. Notice that this works to effect an arbitrary permutation of the elements.
Igor Rivin's user avatar
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1 vote

Issue with very large lists in Mathematica

This looks like a bug, but presumably a workaround is to use a hash table. That is, in @Szabolcs's code, change the first line to: Table[arr[i] = RandomInteger[10, 5].x^Range[5], {i, 1, 1000000}]; …
Igor Rivin's user avatar
  • 5,134
4 votes

Sublists of Random Permutations

mrandomperms[n_, m_]:= Table[PermutationList[RandomPermutation[n]], {m}] Was my initial answer, but, as pointed out by Mr. Wizard, PermutationList should be given $n$ as a second argument, since othe …
Igor Rivin's user avatar
  • 5,134
0 votes

Working with calculations that depend on the previous value in a list

I am not sure what it is you are trying to do, but presumably list = foo /@ RotateRight[list] is something like what you want?! The most general solution involves the function ListConvolve[] whic …
Igor Rivin's user avatar
  • 5,134
0 votes

Why does Arrays interpret lists as Cycles?

I don't understand the question: Mathematica likes to represent permutations as products of cycles (do a help on "Permutations"), so it seems natural that it would represent the symmetry group of the …
Igor Rivin's user avatar
  • 5,134
0 votes

Easy way to covert a Range or set of vales to an Interval object

Well, here is the simplest I can think of: foo[l_, x_] := Module[{ll = l[[-1, -1]]}, If[ll + 1 == x, Append[l[[;; -2]], Append[l[[-1]], x]], Append[l, {x}]]] z[l_] := Fold[foo, {{l[[1]]}}, Rest …
Igor Rivin's user avatar
  • 5,134
5 votes

Better way than using multiple flattens?

This will certainly be less efficient, but is slightly shorter: ReactionPower = Tr[RF1.Transpose[RF1vel] + RF2.Transpose[RF2vel]];
Igor Rivin's user avatar
  • 5,134
1 vote
2 answers
179 views

quantilization (if that is a word)

Suppose I have a list of data, and a list of quantiles ({0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9} is a perennial favorite), and I want to assign to each element the quantile it is in (so for Range …
Igor Rivin's user avatar
  • 5,134
4 votes

To overcome iteration limit and trying to speed up

To answer the specific question, I think that instead of Apply[Dot, l], Fold[Dot, l] will not run into any problems. Of course, all the other comments are valid.
Igor Rivin's user avatar
  • 5,134
9 votes
3 answers
4k views

Generating random symmetric matrix

Suppose first that I want to generate a matrix whose elements are independent and identically distributed (i.i.d.) with distribution dist. This is easy: randMat[dist_, n_, m_] := RandomVariate[dist, …
Igor Rivin's user avatar
  • 5,134
3 votes
Accepted

Replace $z$ with interpolated function and plotting

I am not sure what you are trying to do exactly, but this works fine: ContourPlot[w[x, y, z] /. z -> int[x, y], {x, 0, 8}, {y, 0, 8}]
Igor Rivin's user avatar
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