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I have two lists:

lst1 = {4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2}; lst2 = Range[0.1, 10, 0.1];

How can find the interval from lst2 for each integer number in lst1?

For example, lst1 starts with integer 4 corresponding to 0.1 (first element in lst2), and it repeats 30 times. Then the 30th element in lst2 (which is 3) should match with the last 4 in lst1. So, the interval of interest for this example is {4, [0.1, 3]}, meaning that 4 remains constant in the range [0.1, 3]. I have a very long list about 10000 of monotonically increasing positive numbers in lst2 and a limited number of integers, positive but not necessarily monotonically increasing.

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6 Answers 6

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Map[{#[[1, 1]], MinMax @ #[[All, 2]]} &] @ SplitBy[Transpose[{##}], First] &[lst1, lst2] 
{{4,{0.1, 3.}}, {5, {3.1, 4.8}}, {4, {4.9, 7.5}}, {3, {7.6, 8.5}}, {2, {8.6, 10.}}}
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  • $\begingroup$ Is there any problem with Transpose? I receive an error relating to Transpose. $\endgroup$ Commented Feb 11, 2024 at 16:00
  • $\begingroup$ can you share the error/warning message? $\endgroup$ Commented Feb 11, 2024 at 16:10
  • $\begingroup$ Transpose::nmtx: The first two levels of {{4,4,4,4,4,4,4,4,4,4,<<4990>>},{0.0001,0.0002,0.0003,0.0004,0.0005,0.0006,0.0007,0.0008,0.0009,0.001,<<9990>>}} cannot be transposed. is the error. $\endgroup$ Commented Feb 11, 2024 at 16:14
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    $\begingroup$ lst1 and lst2 should have the same length. If not, you can try Map[{#[[1, 1]], MinMax @ #[[All, 2]]} &] @ SplitBy[Transpose[{##}], First] &[lst1,Take[ lst2, Length@lst1]. $\endgroup$ Commented Feb 11, 2024 at 16:16
  • $\begingroup$ It works now! Thanks. $\endgroup$ Commented Feb 11, 2024 at 16:23
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Transpose[{First /@ #, MinMax /@ TakeList[lst2, Length /@ #]}] & [Split @ lst1]

{{4, {0.1, 3.}}, {5, {3.1, 4.8}}, {4, {4.9, 7.5}}, {3, {7.6, 8.5}}, {2, {8.6, 10.}}}

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Clear["Global`*"]; lst1 = {4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2}; lst2 = Range[0.1, 10, 0.1] (Transpose[{lst1, lst2}] // SplitBy[#, First] & // Map[Through[{First, Last}[#]] &]) /. { {a_, b_}, {c_, d_}} :> {a, {b, d}}

I wouldn't run it on a very long list, but for variety:

Using SequenceReplace:

SequenceReplace[Transpose[{lst1, lst2}], k : {{a_, b_}, Shortest[{a_, _} ..], {a_, d_}} :> {a, {Last@First@k, Last@Last@k}}]

Result:

{{4, {0.1, 3.}}, {5, {3.1, 4.8}}, {4, {4.9, 7.5}}, {3, {7.6, 8.5}}, {2, {8.6, 10.}}}

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lst1 = {4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2}; lst2 = Range[0.1, 10, 0.1];

A variant of @kglr's answer:

{#1[[1]], #2[[{1, -1}]]} & @@@ Map[Thread]@SplitBy[Thread[{lst1, lst2}], First]

Result:

{{4, {0.1, 3.}}, {5, {3.1, 4.8}}, {4, {4.9, 7.5}}, {3, {7.6, 8.5}}, {2, {8.6, 10.}}}

Or using SequenceCases:

patt = k : {Shortest[{a_, _} ..]} :> {a, {k[[1, -1]], k[[-1, 2]]}}; SequenceCases[Thread[{lst1, lst2}], patt]

Result:

{{4, {0.1, 3.}}, {5, {3.1, 4.8}}, {4, {4.9, 7.5}}, {3, {7.6, 8.5}}, {2, {8.6, 10.}}}

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FoldPairList[ TakeDrop, lst2, Map[Length] @ Split[lst1], {lst1[[-1 - Length @ Last @ #]], #[[1, {1, -1}]]} &] 
{{4, {0.1, 3.}}, {5, {3.1, 4.8}}, {4, {4.9, 7.5}}, {3, {7.6, 8.5}}, {2, {8.6, 10.}}}
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MapThread[ {First @ #1, MinMax @ #2} &, {#, Internal`CopyListStructure[#, lst2]}] & [Split @ lst1]

{{4, {0.1, 3.}}, {5, {3.1, 4.8}}, {4, {4.9, 7.5}}, {3, {7.6, 8.5}}, {2, {8.6, 10.}}}

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