Avoid allocations

General Usage Performance
1

Hi there,

suppose that I have a Float64 vector x. I wish to identify non-zero elements of this vector. Below are my proposal for that.

f1!(y,x) = y .= (x .!=0 )[:];
function f2!(y,x)
   @inbounds @simd for i ∈ eachindex(x) 
        if x[i]!=0
           y[i] = true
        end
    end
end

q = 43416;
x = Array(sprand(Float64, 1, q, 0.75));
y = falses(q);
@btime f1!($y,$x);
126.599 μs (5 allocations: 15.14 KiB)
@btime f2!($y,$x);
133.500 μs (0 allocations: 0 bytes)

As you can see, f1! is a bit faster than f2!. However, it requires a few allocations. So, my question here is whether it is possible to write a function that is as fast as f1 and does not require allocations? Thank you very much in advance!

2

f1!(y,x) = map!(iszero, y, x)

?

But maybe you need to think about why you needed that [:] :slight_smile: Hint: your x is not a vector.

edit:
oops, yes
f1!(y,x) = map!(!iszero, y, x)

5 Likes
3

Brilliant, this is as fast as fastest loop. I think you meant f1!(y,x) = map!(!iszero, y, x), though.

4 Likes
4

Hi ,

wow, that is awesome! Indeed, x should be a vector. However, in my application, x is obtained from a matrix. Therefore, I used [:] to make it as a vector. Thank you very much for your help! :slight_smile: