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I have a list of the following kind:

{{1,0.5},{2,0.6},{3,0.8},{-4,0.9},{-3,0.95}}

The important property is, that somewhere in the list, the first element of the sublists changes sign (above is from + to -, but could be from - to +). How can I most efficiently split this into two lists:

{{1,0.5},{2,0.6},{3,0.8}}

and

{{-4,0.9},{-3,0.95}}

?

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    $\begingroup$ What happens when the sign switches back? A list for each consecutive run of + and -? $\endgroup$
    – Carl Woll
    Commented Feb 3, 2018 at 0:08

2 Answers 2

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list = {{1, 0.5}, {2, 0.6}, {3, 0.8}, {-4, 0.9}, {-3, 0.95}};

SplitBy[#, Sign[First @ #] &] & @ list

{{{1, 0.5}, {2, 0.6}, {3, 0.8}}, {{-4, 0.9}, {-3, 0.95}}}

Or

Split[#,  SameQ @@ Sign [First /@ {##}] &] & @ list

{{{1, 0.5}, {2, 0.6}, {3, 0.8}}, {{-4, 0.9}, {-3, 0.95}}}

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This works on your example data, but it might not be general enough to satisfy you, but it is the best I can do with only one example.

data = {{1, 0.5}, {2, 0.6}, {3, 0.8}, {-4, 0.9}, {-3, 0.95}};
Column[{d1, d2} = SplitBy[data, Sign @* First]]

split

Column, of course, is only used form display.

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