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3

$begingroup$

I have built a solution to swap the lowest values with the highest values in a list.

With

SeedRandom[987]
test = RandomSample@*Join @@ Range @@@ 6, 10, 56, 60, 1, 5, -5, -1

-1, 2, 7, 8, 60, 57, 58, 10, 9, 4, -5, -3, 3, 59, 1, 5, -4, 6, -2, 56

Then

swapPositions =
 PermutationReplace[
 Ordering@Ordering@test,
 With[len = Length@test,
 Cycles@
 Transpose@Range @@ 1, Floor[len/2], Reverse@*Range @@ Ceiling[len/2] + 1, len
 ]
 ];

Sort[test][[swapPositions]]

56, 9, 4, 3, -5, -2, -3, 1, 2, 7, 60, 58, 8, -4, 10, 6, 59, 5, 57, -1

The largest half of the numbers have had their positions swapped with lowest half of the numbers.

However, it feels too verbose and I think Sort might be expensive in this case. Is there a built-in function or more terse method to achieve this. Of course with no loss in speed. The actual case is for list of length 100000 and more.

list-manipulation performance-tuning sorting

share|improve this question

edited 14 mins ago

m_goldberg

87.7k872198

asked 51 mins ago

EdmundEdmund

26.6k330102

$endgroup$

add a comment | 

3

$begingroup$

I have built a solution to swap the lowest values with the highest values in a list.

With

SeedRandom[987]
test = RandomSample@*Join @@ Range @@@ 6, 10, 56, 60, 1, 5, -5, -1

-1, 2, 7, 8, 60, 57, 58, 10, 9, 4, -5, -3, 3, 59, 1, 5, -4, 6, -2, 56

Then

swapPositions =
 PermutationReplace[
 Ordering@Ordering@test,
 With[len = Length@test,
 Cycles@
 Transpose@Range @@ 1, Floor[len/2], Reverse@*Range @@ Ceiling[len/2] + 1, len
 ]
 ];

Sort[test][[swapPositions]]

56, 9, 4, 3, -5, -2, -3, 1, 2, 7, 60, 58, 8, -4, 10, 6, 59, 5, 57, -1

The largest half of the numbers have had their positions swapped with lowest half of the numbers.

However, it feels too verbose and I think Sort might be expensive in this case. Is there a built-in function or more terse method to achieve this. Of course with no loss in speed. The actual case is for list of length 100000 and more.

list-manipulation performance-tuning sorting

share|improve this question

edited 14 mins ago

m_goldberg

87.7k872198

asked 51 mins ago

EdmundEdmund

26.6k330102

$endgroup$

add a comment | 

$begingroup$

I have built a solution to swap the lowest values with the highest values in a list.

With

SeedRandom[987]
test = RandomSample@*Join @@ Range @@@ 6, 10, 56, 60, 1, 5, -5, -1

-1, 2, 7, 8, 60, 57, 58, 10, 9, 4, -5, -3, 3, 59, 1, 5, -4, 6, -2, 56

Then

swapPositions =
 PermutationReplace[
 Ordering@Ordering@test,
 With[len = Length@test,
 Cycles@
 Transpose@Range @@ 1, Floor[len/2], Reverse@*Range @@ Ceiling[len/2] + 1, len
 ]
 ];

Sort[test][[swapPositions]]

56, 9, 4, 3, -5, -2, -3, 1, 2, 7, 60, 58, 8, -4, 10, 6, 59, 5, 57, -1

The largest half of the numbers have had their positions swapped with lowest half of the numbers.

However, it feels too verbose and I think Sort might be expensive in this case. Is there a built-in function or more terse method to achieve this. Of course with no loss in speed. The actual case is for list of length 100000 and more.

list-manipulation performance-tuning sorting

share|improve this question

edited 14 mins ago

m_goldberg

87.7k872198

asked 51 mins ago

EdmundEdmund

26.6k330102

$endgroup$

SeedRandom[987]
test = RandomSample@*Join @@ Range @@@ 6, 10, 56, 60, 1, 5, -5, -1

swapPositions =
 PermutationReplace[
 Ordering@Ordering@test,
 With[len = Length@test,
 Cycles@
 Transpose@Range @@ 1, Floor[len/2], Reverse@*Range @@ Ceiling[len/2] + 1, len
 ]
 ];

Sort[test][[swapPositions]]

share|improve this question

edited 14 mins ago

m_goldberg

87.7k872198

asked 51 mins ago

EdmundEdmund

26.6k330102

share|improve this question

edited 14 mins ago

m_goldberg

87.7k872198

asked 51 mins ago

EdmundEdmund

26.6k330102

edited 14 mins ago

m_goldberg

87.7k872198

edited 14 mins ago

m_goldberg

87.7k872198

asked 51 mins ago

EdmundEdmund

26.6k330102

asked 51 mins ago

EdmundEdmund

26.6k330102

1 Answer
1

active

oldest

votes

5

$begingroup$

How about:

Module[tmp = test,
 With[ord=Ordering[tmp],
 tmp[[ord]] = Reverse @ tmp[[ord]]];
 tmp
]

56, 9, 4, 3, -5, -2, -3, 1, 2, 7, 60, 58, 8, -4, 10, 6, 59, 5, 57, -1

share|improve this answer

answered 37 mins ago

Carl WollCarl Woll

71.2k394185

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  That is so obvious I want to cry. Thanks (+1).
  $endgroup$
  – Edmund
  33 mins ago
  

  
  
  
  

add a comment | 

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5

$begingroup$

How about:

Module[tmp = test,
 With[ord=Ordering[tmp],
 tmp[[ord]] = Reverse @ tmp[[ord]]];
 tmp
]

56, 9, 4, 3, -5, -2, -3, 1, 2, 7, 60, 58, 8, -4, 10, 6, 59, 5, 57, -1

share|improve this answer

answered 37 mins ago

Carl WollCarl Woll

71.2k394185

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  That is so obvious I want to cry. Thanks (+1).
  $endgroup$
  – Edmund
  33 mins ago
  

  
  
  
  

add a comment | 

5

$begingroup$

How about:

Module[tmp = test,
 With[ord=Ordering[tmp],
 tmp[[ord]] = Reverse @ tmp[[ord]]];
 tmp
]

56, 9, 4, 3, -5, -2, -3, 1, 2, 7, 60, 58, 8, -4, 10, 6, 59, 5, 57, -1

share|improve this answer

answered 37 mins ago

Carl WollCarl Woll

71.2k394185

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  That is so obvious I want to cry. Thanks (+1).
  $endgroup$
  – Edmund
  33 mins ago
  

  
  
  
  

add a comment | 

$begingroup$

How about:

Module[tmp = test,
 With[ord=Ordering[tmp],
 tmp[[ord]] = Reverse @ tmp[[ord]]];
 tmp
]

56, 9, 4, 3, -5, -2, -3, 1, 2, 7, 60, 58, 8, -4, 10, 6, 59, 5, 57, -1

share|improve this answer

answered 37 mins ago

Carl WollCarl Woll

71.2k394185

$endgroup$

Module[tmp = test,
 With[ord=Ordering[tmp],
 tmp[[ord]] = Reverse @ tmp[[ord]]];
 tmp
]

share|improve this answer

answered 37 mins ago

Carl WollCarl Woll

71.2k394185

answered 37 mins ago

Carl WollCarl Woll

71.2k394185

answered 37 mins ago

Carl WollCarl Woll

71.2k394185