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Truthify an array
Jelly has an atom called untruth, which when given indices, creates an array with 1s at those places: [2,4] → [0,1,0,1].
You are required to perform the inverse of this. Given a 2D boolean array, find the indices of the true values in it.
Challenge
You will be given a single 2D boolean array $M$. You may take its dimensions as a separate argument if needed.
The output must consist of all the indices of the true values in $M$.
Output may be 0-indexed or 1-indexed.
It is guaranteed that the input will only consist of two different values.
Test Cases
All test cases are 0-indexed.
I: [[1,0,1,1,1,0]]
O: [[0,0],[0,2],[0,3],[0,4]]
I: [[1,0,1],[1,0,1],[0,1,0]]
O: [[0,0],[0,2],[1,0],[1,2],[2,1]]
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JavaScript (Node.js), 68 bytes
f=(a)=>a.map((x)=>[...x.entries()].filter((e)=>e[1]).map((e)=>e[0]))
Run the function f on your input, e.g. f([[1,0,1],[1,0,1],[0,1,0]]).
The output looks like this:
[[0, 2], [0, 2], [1]]
with one subarray for each input subarray.
Explanation:
f=(a)=> // creates a function to run the input on
a.map((x)=> // for each outer array:
[...x.entries()] // creates things like this: [index, value]
.filter((e)=>e[1]) // keep ones where the value acts like true
.map((e)=>e[0]) // return the indices for each one
)
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