How to convert from char to string in Julia?

Question

I'm trying to convert a word to an array of one-hot encoded arrays using a simple vocabulary. The dictionary I've constructed is keyed off of characters.

vocab = "abc"
char_id = Dict([ (index, char) for (char, index) in enumerate(vocab) ])

# Dict{Char,Int64} with 3 entries:
#   'a' => 1
#   'c' => 3
#   'b' => 2

function char_to_one_hot(char, char_id, max_length)
  one_hot = zeros(max_length)
  setindex!(one_hot, 1.0, char_id[char])
end

function word_to_one_hot(word, char_id, max_length)
  map((char) -> char_to_one_hot(char, char_id, max_length), split(word, ""))
end

word_to_one_hot(word, char_id, max_length)

Unfortunately, this returns an error because the char_id Dict is uses char keys instead of strings. How can I convert either the dictionary to use string values as keys, or chars to strings so the comparison matches?

ERROR: KeyError: key "a" not found
Stacktrace:
 [1] getindex at ./dict.jl:467 [inlined]
 [2] char_to_one_hot(::SubString{String}, ::Dict{Char,Int64}, ::Int64) at ./REPL[456]:3
 [3] #78 at ./REPL[457]:2 [inlined]
 [4] iterate at ./generator.jl:47 [inlined]
 [5] _collect(::Array{SubString{String},1}, ::Base.Generator{Array{SubString{String},1},var"#78#79"{Dict{Char,Int64},Int64}}, ::Base.EltypeUnknown, ::Base.HasShape{1}) at ./array.jl:699
 [6] collect_similar at ./array.jl:628 [inlined]
 [7] map at ./abstractarray.jl:2162 [inlined]
 [8] word_to_one_hot(::String, ::Dict{Char,Int64}, ::Int64) at ./REPL[457]:2
 [9] top-level scope at REPL[458]:1

François Févotte François Févotte · Accepted Answer · 2020-11-06T17:34:45

A string can already be seen as a collection of characters, so you shouldn't need to split the word.

However, map is specialized in such a way that on strings you can only map functions which return chars. And strings are also treated as scalars by the broadcasting system. This leaves us with a few options: a simple for loop or maybe a generator/comprehension.

I think in this case I'd go with the comprehension:

function char_to_one_hot(char, char_id, max_length)
    one_hot = zeros(max_length)
    setindex!(one_hot, 1.0, char_id[char])
end

function word_to_one_hot(word, char_id, max_length)
    [char_to_one_hot(char, char_id, max_length) for char in word]
end

which I think gives what you'd expect:

julia> vocab = "abc"
"abc"

julia> char_id = Dict([ (index, char) for (char, index) in enumerate(vocab) ])
Dict{Char,Int64} with 3 entries:
  'a' => 1
  'c' => 3
  'b' => 2

julia> word_to_one_hot("acb", char_id, 5)
3-element Array{Array{Float64,1},1}:
 [1.0, 0.0, 0.0, 0.0, 0.0]
 [0.0, 0.0, 1.0, 0.0, 0.0]
 [0.0, 1.0, 0.0, 0.0, 0.0]

If you still want to convert between 1-character strings and characters, you can do it this way:

julia> str="a"; first(str)
'a': ASCII/Unicode U+0061 (category Ll: Letter, lowercase)

julia> chr='a'; string(chr)
"a"

How to convert from char to string in Julia?

2 Answers