Stream: helpdesk (published)

Topic: Appending empty string to `String[]`

mbaz (Jun 01 2021 at 02:00):

Is this a bug?

julia> s=String[]
String[]

julia> append!(s,"")
String[]

push! seems to work:

push!(s,"")
1-element Vector{String}:
 ""

mbaz (Jun 01 2021 at 02:07):

Nevermind, I figured it out.

julia> append!(String[], [""])
1-element Vector{String}:
 ""

Andrey Oskin (Jun 01 2021 at 03:13):

Guess it's the same thing as in
https://discourse.julialang.org/t/tutorial-efficient-and-safe-approaches-to-mutation-in-data-parallelism/62105/4

Probably append!(String[], ("", )) should work too.

Fredrik Ekre (Jun 01 2021 at 05:44):

Are you looking for push!? That is for adding elements whereas append! iterates the argument and adds the elements in the iterator.

Gunnar Farnebäck (Jun 01 2021 at 07:09):

The reason why append!(s, "") does nothing is that it iterates over the characters in the string, but there are none so nothing happens. If you have a non-empty string it fails since the element type is wrong:

julia> s = String[]
String[]

julia> append!(s, "abc")
ERROR: MethodError: Cannot `convert` an object of type Char to an object of type String
Closest candidates are:
  convert(::Type{String}, ::String) at essentials.jl:210
  convert(::Type{T}, ::T) where T<:AbstractString at strings/basic.jl:231
  convert(::Type{T}, ::AbstractString) where T<:AbstractString at strings/basic.jl:232
  ...
Stacktrace:
 [1] setindex!(A::Vector{String}, x::Char, i1::Int64)
   @ Base ./array.jl:839
 [2] _append!(a::Vector{String}, #unused#::Base.HasLength, iter::String)
   @ Base ./array.jl:991
 [3] append!(a::Vector{String}, iter::String)
   @ Base ./array.jl:981
 [4] top-level scope
   @ REPL[2]:1

Not sure I like that it doesn't clean up the pre-allocation when it errors though:

julia> s
3-element Vector{String}:
 #undef
 #undef
 #undef

mbaz (Jun 01 2021 at 13:05):

Thanks to all! I was indeed looking for push!. What happened is that, by mistake, I read append!(String[], "") as "append the empty string to String[]" instead of "append the elements of the empty string to String[]".

I don't work often on strings and I don't automatically think of them as iterators (yet).

Last updated: Oct 02 2023 at 04:34 UTC