Parsing a UInt8 Array of chars into a float without allocation

Question

I have a this problem here:

function main()
    b = UInt8[0x35, 0x30, 0x2e, 0x30] # This is "50.0" in bytes
    s = parse(Float64, String(b))
end
@btime main()

250.269 ns (2 allocations: 128 bytes)
50.0

I have a UInt8 array which contains a number in string format e.g. "50.0". I want to parse this number into a float without allocation and as fast as possible (I have millions of these numbers to parse). Is there a better way than the way I've produced above (ignoring the allocation for the UInt8 array since this will not exist).

Cheers guys!

Answer 1

You can use:

julia> using Parsers

julia> b = UInt8[0x35, 0x30, 0x2e, 0x30] # This is "50.0" in bytes
4-element Vector{UInt8}:
 0x35
 0x30
 0x2e
 0x30

julia> @btime Parsers.parse(Float64, $b)
  13.527 ns (0 allocations: 0 bytes)
50.0

Note that your code allocates b inside main so it including the time and memory allocation of creation of b.

Also the difference is that in your code String(b) empties b, while with Parsers.parse the b is left untouched. This is especially useful if b would be a view of a slice of a longer tape of UInt8 values.