Extracting specific data from a list

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Consider the following list

1., -1, 1.5, -1, 2., 1, 2.5, 1, 3., 1, 3.5, 1, 4., 1, 4.5, 1, 5., 1, 5.5, 1, 6., 1, 6.5, 1, 7., -1, 7.5, -1, 8., -1, 8.5, -1, 9., -1, 9.5, -1, 10., -1

How can I extract only the values after (or before) the second element in the 2-element sublists changes sign, i.e, I would like to obtain as a result

2., 1,7., -1

list-manipulation

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asked Aug 19 at 13:32

jarhead

689412

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up vote
2
down vote

favorite

Consider the following list

1., -1, 1.5, -1, 2., 1, 2.5, 1, 3., 1, 3.5, 1, 4., 1, 4.5, 1, 5., 1, 5.5, 1, 6., 1, 6.5, 1, 7., -1, 7.5, -1, 8., -1, 8.5, -1, 9., -1, 9.5, -1, 10., -1

How can I extract only the values after (or before) the second element in the 2-element sublists changes sign, i.e, I would like to obtain as a result

2., 1,7., -1

list-manipulation

share|improve this question

asked Aug 19 at 13:32

jarhead

689412

add a comment | 

up vote
2
down vote

favorite

up vote
2
down vote

favorite

Consider the following list

1., -1, 1.5, -1, 2., 1, 2.5, 1, 3., 1, 3.5, 1, 4., 1, 4.5, 1, 5., 1, 5.5, 1, 6., 1, 6.5, 1, 7., -1, 7.5, -1, 8., -1, 8.5, -1, 9., -1, 9.5, -1, 10., -1

How can I extract only the values after (or before) the second element in the 2-element sublists changes sign, i.e, I would like to obtain as a result

2., 1,7., -1

list-manipulation

share|improve this question

asked Aug 19 at 13:32

jarhead

689412

Consider the following list

1., -1, 1.5, -1, 2., 1, 2.5, 1, 3., 1, 3.5, 1, 4., 1, 4.5, 1, 5., 1, 5.5, 1, 6., 1, 6.5, 1, 7., -1, 7.5, -1, 8., -1, 8.5, -1, 9., -1, 9.5, -1, 10., -1

How can I extract only the values after (or before) the second element in the 2-element sublists changes sign, i.e, I would like to obtain as a result

2., 1,7., -1

list-manipulation

share|improve this question

asked Aug 19 at 13:32

jarhead

689412

share|improve this question

share|improve this question

asked Aug 19 at 13:32

jarhead

689412

asked Aug 19 at 13:32

jarhead

689412

asked Aug 19 at 13:32

jarhead

689412

689412

add a comment | 

add a comment | 

1 Answer
1

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up vote
5
down vote



accepted

First /@ Rest@SplitBy[list, Sign[#[[2]]] &]

2., 1, 7., -1

or

First /@ Rest@Split[list, Sign[#[[2]]] == Sign[#2[[2]]] &]

2., 1, 7., -1

Also

Pick[list, Developer`ToPackedArray[Unitize[Join[0, 
 Differences @ UnitStep @ Transpose[list][[2]]]]], 1]

2., 1, 7., -1

share|improve this answer

edited Aug 19 at 13:46

answered Aug 19 at 13:36

kglr

158k8182380

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1 Answer
1

active

oldest

votes

1 Answer
1

active

oldest

votes

active

oldest

votes

active

oldest

votes

up vote
5
down vote



accepted

First /@ Rest@SplitBy[list, Sign[#[[2]]] &]

2., 1, 7., -1

or

First /@ Rest@Split[list, Sign[#[[2]]] == Sign[#2[[2]]] &]

2., 1, 7., -1

Also

Pick[list, Developer`ToPackedArray[Unitize[Join[0, 
 Differences @ UnitStep @ Transpose[list][[2]]]]], 1]

2., 1, 7., -1

share|improve this answer

edited Aug 19 at 13:46

answered Aug 19 at 13:36

kglr

158k8182380

add a comment | 

up vote
5
down vote



accepted

First /@ Rest@SplitBy[list, Sign[#[[2]]] &]

2., 1, 7., -1

or

First /@ Rest@Split[list, Sign[#[[2]]] == Sign[#2[[2]]] &]

2., 1, 7., -1

Also

Pick[list, Developer`ToPackedArray[Unitize[Join[0, 
 Differences @ UnitStep @ Transpose[list][[2]]]]], 1]

2., 1, 7., -1

share|improve this answer

edited Aug 19 at 13:46

answered Aug 19 at 13:36

kglr

158k8182380

add a comment | 

up vote
5
down vote



accepted

up vote
5
down vote



accepted

First /@ Rest@SplitBy[list, Sign[#[[2]]] &]

2., 1, 7., -1

or

First /@ Rest@Split[list, Sign[#[[2]]] == Sign[#2[[2]]] &]

2., 1, 7., -1

Also

Pick[list, Developer`ToPackedArray[Unitize[Join[0, 
 Differences @ UnitStep @ Transpose[list][[2]]]]], 1]

2., 1, 7., -1

share|improve this answer

edited Aug 19 at 13:46

answered Aug 19 at 13:36

kglr

158k8182380

First /@ Rest@SplitBy[list, Sign[#[[2]]] &]

2., 1, 7., -1

or

First /@ Rest@Split[list, Sign[#[[2]]] == Sign[#2[[2]]] &]

2., 1, 7., -1

Also

Pick[list, Developer`ToPackedArray[Unitize[Join[0, 
 Differences @ UnitStep @ Transpose[list][[2]]]]], 1]

2., 1, 7., -1

share|improve this answer

edited Aug 19 at 13:46

answered Aug 19 at 13:36

kglr

158k8182380

share|improve this answer

share|improve this answer

edited Aug 19 at 13:46

edited Aug 19 at 13:46

edited Aug 19 at 13:46

answered Aug 19 at 13:36

kglr

158k8182380

answered Aug 19 at 13:36

kglr

158k8182380

answered Aug 19 at 13:36

kglr

158k8182380

158k8182380

add a comment | 

add a comment | 

 

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