Julia: Convert numeric string to float or int

Question

I am trying to write numeric data pulled from a database into a Float64[]. The original data is in ::ASCIIString format, so trying to push it to the array gives the following error:

julia> push!(a, "1")
ERROR: MethodError: `convert` has no method matching convert(::Type{Float64}, ::ASCIIString)
This may have arisen from a call to the constructor Float64(...),
since type constructors fall back to convert methods.
Closest candidates are:
  call{T}(::Type{T}, ::Any)
  convert(::Type{Float64}, ::Int8)
  convert(::Type{Float64}, ::Int16)
  ...
 in push! at array.jl:432

Attempting to convert the data directly unsurprisingly throws the same error:

julia> convert(Float64, "1")
ERROR: MethodError: `convert` has no method matching convert(::Type{Float64}, ::ASCIIString)
This may have arisen from a call to the constructor Float64(...),
since type constructors fall back to convert methods.
Closest candidates are:
  call{T}(::Type{T}, ::Any)
  convert(::Type{Float64}, ::Int8)
  convert(::Type{Float64}, ::Int16)
  ...

Given that I know the data is numeric, is there a way I can convert it before pushing?

p.s. I am using version 0.4.0

Solution 1

You can parse(Float64,"1") from a string. Or in the case of a vector

map(x->parse(Float64,x),stringvec)

will parse the whole vector.

BTW consider using tryparse(Float64,x) instead of parse. It returns a Nullable{Float64} which is null in the case string doesn't parse well. For example:

isnull(tryparse(Float64,"33.2.1")) == true

And usually one would want a default value in case of a parse error:

strvec = ["1.2","NA","-1e3"]
map(x->(v = tryparse(Float64,x); isnull(v) ? 0.0 : get(v)),strvec)
# gives [1.2,0.0,-1000.0]

Solution 2

Use parse(Float64,"1").

See more at: parse specification

Comments

[Reiner Martin]

In the meantime (with Julia 1.0 out), the link to use is docs.julialang.org/en/latest/base/numbers/#Base.parse

[Ricoter]

Now its docs.julialang.org/en/v1/base/numbers/#Base.parse

[Maciek Leks]

@Ricote, I've updated the link. Thank you.

[BallpointBen]

Commenting in 2021: what is Nullable{T}? It looks like it should be Union{Nothing,T}? And should isnull be isnothing (or === nothing) now?