Hey everyone, I was wondering if there was an efficient ("canonical") way to traverse a dictionary with a JSON like structure. For example, I have a dictionary with an array of dictionaries that sometimes don't have the same keys. I then need to iterate over these dictionaries and check if an specific key has a certain value. For example:
for cell in cells
if cell["key1"]["key2"]["key3"] == true
# do stuff
end
end
The issue is that sometimes the cell won't have either "key1", or "key2" or "key3", so a simple get(cell, key1, false)
won't work. Of course, I could always write a bunch of if statements, but I was wondering if there was a smarter and more direct way of doing this.
Well, it's hardly elegant, but you can chain get
s by putting empty Dict
s in instead of false
:
if get(get(get(cell, "key1", Dict()), "key2", Dict()), "key3", nothing) == "value"
# Do stuff
end
Or, perhaps marginally less inelegant:
if cell |> x -> get(x, "key1", Dict()) |> x -> get(x, "key2", Dict()) |> x -> get(x, "key3", nothing) == "value"
# Do something
end
Richard Reeve said:
Well, it's hardly elegant, but you can chain
get
s by putting emptyDict
s in instead offalse
:if get(get(get(cell, "key1", Dict()), "key2", Dict()), "key3", nothing) == "value" # Do stuff end
Or, perhaps marginally less inelegant:
if cell |> x -> get(x, "key1", Dict()) |> x -> get(x, "key2", Dict()) |> x -> get(x, "key3", nothing) == "value" # Do something end
Thanks, the empty Dict seems like a good idea. Putting it in a function and thus allowing an arbitrary number of keys looks like a nice work around.
You may write your own get
function, which supports nested getting.
Is better then Dict()
, since it will be less allocating and as such giving less pressure to gc.
Can you give an example?
I'm trying to write a get function that parses the nested dict, but I'm not finding a clever way of doing so.
julia> function nested_get(dict::Dict, keys::AbstractVector, default = nothing)
if isempty(keys)
return dict
end
key = popfirst!(keys)
if haskey(dict, key)
return nested_get(dict[key], keys)
end
return default
end
nested_get (generic function with 2 methods)
julia> function nested_get(non_dict, keys::AbstractVector, default = nothing)
if isempty(keys)
return non_dict
end
return default
end
nested_get (generic function with 4 methods)
julia> cells = Dict{String, Any}("a" => 1)
Dict{String, Any} with 1 entry:
"a" => 1
julia> nested_get(cells, ["key1", "key2", "key3"]) === nothing
true
julia> cells["key1"]=Dict("key2" => Dict("key3"=>1))
Dict{String, Dict{String, Int64}} with 1 entry:
"key2" => Dict("key3"=>1)
julia> nested_get(cells, ["key1", "key2", "key3"])
1
So it'll return nothing
if there's no match at any point, and it'll return the value if there's a full match. Something like that?
It's more complicated than what I suggested originally, but Andrey is right that this is a better solution (assuming that's what he had in mind!).
Yes, that was the idea.
Instead of nothing you can provide default value, as in usual get
function.
Oh!
You did :-)
Yes, I just updated it to do that!
You can use splatting instead of vector, but this is just a matter of taste.
Thanks @Richard Reeve , it works beautifully. I actually prefer the vector of keys than the splatting. :)
It gives a sense that the order is important (which it is), while just splatting gives me the sense that they are somehow parallel.
This function should be in Base, it's very neat, and I constantly need something like it.
And it fits perfectly as another multiple dispatch for get
.
If we do some type piracy, then vector gives funny notation
d[["key1", "key2", "key3"]]
, while splatting d["key1", "key2", "key3"]
Honestly, I do not know which one is better.
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Last updated: Nov 06 2024 at 04:40 UTC