Julia: broadcasting `findfirst()` across rows of a matrix

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I want to find the index of the first value in each row of a matrix that satisfies some condition. I want to figure out how to do this without using array comprehensions.

This is how I would do it with an array comprehension:

# let's say we want to find the first column index, per row, where a number in that row is below some threshold.

threshold = 0.5;
data = randn(50,100);

first_threshold_crossings = [findfirst(data[i,:]<threshold) for i in 1:size(data,1)];

Yielding a list of indices that tells you where (column-wise) each row has a value that first drops below the threshold, going from left to right.

Any faster way you can imagine doing this?

array-broadcasting julia broadcast multidimensional-array logical-operators
1 Answers

Here's how you can do it:

julia> using Random # For RNG reproducability

julia> A = rand(Random.MersenneTwister(0), 3, 3)
3×3 Array{Float64,2}:
 0.823648  0.177329  0.0423017
 0.910357  0.27888   0.0682693
 0.164566  0.203477  0.361828

julia> findfirst.(x < 0.1, eachrow(A))
3-element Array{Union{Nothing, Int64},1}:
 3
 3
  nothing

Note that findfirst returns nothing if no index satisfies the condition.

user_109926670
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