Count consecutive elements in array

[Indi Kernick]

I hope this is the right place for a question like this.

I have an array of 0s and 1s. I like to count the number of consecutive 0s and 1s in the array and put this information into a new array. I’d like 1s to have a positive count and 0s to have a negative count. Here is an example of what I mean:

[1, 1, 1, 0, 1, 0, 0, 1, 1, 1, 1] -> [3, -1, 1, -2, 4]

I’m not very experienced with functional programming so I can’t figure out how to do this in Jsonnet. If anyone could help me out that would be greatly appreciated.

Love the language by the way!

[Igor Peshansky]

Here's one quick-and-dirty example (loosely based on the std.uniq implementation):

local count_runs(arr) =

  local sign(b) =

    if b < 0 then -1 else if b > 0 then 1 else 0;

  local f(a, b) =

    local runs = a[0];

    local last = a[1];

    local i = if b == 1 then 1 else -1;

    if last == null then

      [runs, i]

    else if sign(last) == i then

      [runs, last + i]

    else

      [runs + [last], i];

    local r = std.foldl(f, arr, [[], null]);

    r[0] + [r[1]];

count_runs([1, 1, 1, 0, 1, 0, 0, 1, 1, 1, 1])

This produces your desired result:

[3, -1, 1, -2, 4]

I'm sure someone with more Jsonnet experience can come up with something shorter / more readable.

FYI, it's very useful to play around with the language interactively on the demo page.

Hope this helps,

        Igor

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[Indi Kernick]

Oh wow!

I didn't know that's what the foldl function was for. The foldl function can be used to iterate an array with seemingly mutable state that changes as the array is iterated. It's a for loop!!!

Thank you so much Igor.

You didn't just give me a snippet that I can copy-and-paste, you taught me something about functional programming!

BTW, is this the right place for questions like "How did I do x in Jsonnet?"?