Conditional Gathering of lists

Just need a little help with the GatherBy / SplitBy function(s).

I have a list of random numbers here:

{8, 4, 2, 1, 9, 4, 2, 1, 5, 2, 1, 3, 1, 2, 11, 4, 2, 1, 5, 2, 1, 3, \
1, 2, 7, 2, 1, 3, 1, 2, 5, 1, 2, 4, 15, 4, 2, 1, 5, 2, 1, 3, 1, 2, 7, \
2, 1, 3, 1, 2, 5, 1, 2, 4, 11, 2, 1, 3, 1, 2, 5, 1, 2, 4, 9, 1, 2, 4,
  8}

How can I write a function with a look-ahead? I want to gather the numbers so it splits whenever it the next number is larger than the current one? (spaced for clarity):

{{8, 4, 2, 1},

{9, 4, 2, 1},
{5, 2, 1},
{3, 1},
{2},
{11, 4, 2, 1},
...}

Tried and failed:

SplitBy[%, Greater]

Answer

You need Split:

Split[list, Greater]

SplitBy doesn't work here because the specified function is applied to each element separately before doing a normal Split. What you want is a pair-wise comparison with a custom comparator, which is what Split does.

Looking at this again you may want GreaterEqual to group identical elements in the same list:

Split[{2, 1, 1, 7, 5, 5, 5, 6, 0}, GreaterEqual]

{{2, 1, 1}, {7, 5, 5, 5}, {6, 0}}

For fun I tried to do this operation without Split. Since I was having fun I used Do rather than Module to localize symbols i and x.

split = Last @ Reap @ Do[If[n > x, i++]; Sow[x = n, i], {i, 1}, {x, 1}, {n, #}] &;

split @ {2, 1, 1, 7, 5, 5, 5, 6, 0}

{{2, 1, 1}, {7, 5, 5, 5}, {6, 0}}

Comments

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