How to efficiently determine if a column of a matrix is made of integers

General Usage
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1

Which is the most efficient way to determine if the columns of a matrix are all integers, e.g. a = [1 1.5 2; 10 3.1 3] columns 1 and 3 ?

Note that if I would have defined a = Any[1 1.5 2; 10 3.1 3] I could have used

julia> identity.(a[:,1])
2-element Vector{Int64}:
  1
 10

and then use eltype, but it doesn’t work if a has been already cast to a matrix of Floats and I don’t know its content a priori.

I can use try/catch with convert, but I feel it would be even more inefficient…

2 
julia> isinteger.(Any[1 1.5 2; 10 3.1 3])
2×3 BitMatrix:
 1  0  1
 1  0  1
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3

Thanks, I wasn’t aware of the specific isinteger function. Contrary to identity it works also for Float converted matrices:

julia> a = [1 1.5 2; 10 3.1 3]
2×3 Matrix{Float64}:
  1.0  1.5  2.0
 10.0  3.1  3.0

julia> all(isinteger.(a[:,1]))
true
4

Better try to avoid slicing and broadcasting, by using the all(func, arr) version, with a dimension keyword:

julia> a = [1 1.5 2; 10 3.1 3]
2×3 Matrix{Float64}:
  1.0  1.5  2.0
 10.0  3.1  3.0

julia> all(isinteger, a; dims=1)
1×3 Matrix{Bool}:
 1  0  1
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5

Actually, this didn’t directly work for me, as in some cases I have string.

I solved with:

isinteger_bml(_)          = false
isinteger_bml(_::Integer) = true
isinteger_bml(x::AbstractFloat)   = isinteger(x)

I don’t know why there isn’t an isinteger() defaulting to false for all non integer types (except Nothing and Missing)

6

Because we have no way of know that’s correct for novel numeric types. It would arguably be fine for non-numeric types, but then again why are you calling isinteger on values that aren’t even numbers?

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7

You can skip the variable entirely, like this:

isinteger_bml(::Integer) = true

You mean non-numeric? I think it makes more sense to assume that asking if e.g. a Dict is an integer is a bug, rather than a just returning false.

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