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How to use an ARRAYFORMULA to count how many rows there are untill I get a specific value at multiple times?

I’m a beginner with google sheets/exel formulas so i’m having trouble to count how many rows there are until I get a specific value, and this, multiple times in a same column.

Example bellow, I would like to set an ARRAYFORMULA in B1 so I can get those results:

Row\Column A B
1 Value Result
2 0 1
3 1 0
4 0 1
5 1 0
6 0 4
7 0 3
8 0 2
9 0 1
10 1 0

The purpose is to count raws untill I get the value "1" in column A and put the result in column B:

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So obviously I could use something like :

B2=IF(A2=0;IF(ISNUMBER(B1);B1+1;1);0)

And deploy this formula to every cells in the column B

But I really wish I could get an ARRAYFORMULA in B1 to do so.

I tried to use:

B1=ARRAYFORMULA({"Result";IF(A2:A=0; COUNTIFS(A2:A; 0; ROW(A2:A);"<="&ROW(A2:A)); 0)})

But as you can presume, this won’t work because A2 stay the value from where the count is done. Besides, the counting is reversed.

We would get something like , which is not what I want:

Row\Column A B
1 Value Result
2 0 1
3 1 0
4 0 2
5 1 0
6 0 3
7 0 4
8 0 5
9 0 6
10 1 0

I really struggle to increment values with arrayformulas so if anyone has done something similar, help would be appreciated.

>Solution :

You may try:

=let(Γ;index(A:A;index(match(2;1/(A:A<>""))));
   map(A2:Γ;lambda(Σ;if(Σ=1;0;xmatch(1;offset(Σ;1;):Γ)))))

enter image description here

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