Why I am not to update String as value in Dictionary if Dictionary is created with Int64 values

General Usage
1

I am working and learning Dictionaries, now here is what is happening,

# Initialize a dictionary
d = Dict("a"=>1, "b" => 2, "c" => 3)
d["c"] = 4 # works 
d["c"] = "Hello" # Fails with error pasted below
d["d"] = "Hello" # Fails with same error
d["d"] = 9 # works

Error is

MethodError: Cannot convert an object of type String to an object of type Int64

Closest candidates are:

convert(::Type{T}, !Matched::T) where T<:Number

@ Base number.jl:6

convert(::Type{T}, !Matched::Number) where T<:Number

@ Base number.jl:7

convert(::Type{T}, !Matched::Base.TwicePrecision) where T<:Number

@ Base twiceprecision.jl:273

…

  1. setindex!(::Dict{String, Int64}, ::String, ::String)@dict.jl:369
  2. top-level scope@Local: 1 [inlined]
2

When you initialize your dict like the above, the Dict created becomes specific for string keys and int values:

julia> d = Dict("a"=>1, "b" => 2, "c" => 3)
Dict{String, Int64} with 3 entries:
  "c" => 3
  "b" => 2
  "a" => 1

(note the Dict{String, Int64}).

If you want the Dict to contain more general values than those used to initialize the dict, you can be explicit about that, for example:

julia> d = Dict{String,Union{String,Int}}("a"=>1, "b" => 2, "c" => 3)
Dict{String, Union{Int64, String}} with 3 entries:
  "c" => 3
  "b" => 2
  "a" => 1

julia> d["c"] = "a"
"a"

julia> d
Dict{String, Union{Int64, String}} with 3 entries:
  "c" => "a"
  "b" => 2
  "a" => 1

or, even more general:

julia> d = Dict{String,Any}("a"=>1, "b" => 2, "c" => 3)
Dict{String, Any} with 3 entries:
  "c" => 3
  "b" => 2
  "a" => 1

julia> d["c"] = [1,2,3]
3-element Vector{Int64}:
 1
 2
 3

julia> d
Dict{String, Any} with 3 entries:
  "c" => [1, 2, 3]
  "b" => 2
  "a" => 1
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