Traverse nested Dict in Julia-lang

问题

While I traverse a nested Dict in Julia, it gives this Error:

ERROR: access to undefined reference
 in next at dict.jl:567

Here is the code where you can reproduce this error:

a = [0,19620,7291,32633,9,32513,42455,10045,31964,42455,11767,54]
b = [14318,16405,19,18913,19,8141,18958,12336,7,16588,17358,30]
d = Dict()
for aa in a
   for bb in b
     if ! haskey(d,aa)
        d[aa]=Dict()
     end
     d[aa][bb] = 0.5
   end
end 
for k1 in keys(d)
   s =0.0               
   for k2 in keys(d[k1])
     s+= d[k1][k2]
   end
   for k2 in keys(d[k1])
     d[k1][k2] = d[k1][k2] / s
   end
end

It's wired, if a = [0,1] b = [0,1], it works fine.

----Update-----

Actually, as long as array b has 11 distinct elements, the error would happen. Also, if

d[k1][k2] = d[k1][k2] / s

become

d[k1][k2] = d[k1][k2] * s

or any other operations, the error disappear.

Any ideas ?

回答1:

This issue has now been fixed in the development branch, and will be available in the prerelease editions as soon as they gets updated.

See: https://github.com/JuliaLang/julia/pull/5894/files

回答2:

The error probably comes from the fact that you modify the contents of d[k1] while you are iterating through it so the key iterator fails after you've modified the contents. (Why it only happens with some operations, I can't say.)

Anyhow, it can be fixed by changing the problematic loop to

for k2 in collect(keys(d[k1]))
    d[k1][k2] = d[k1][k2] / s
end

(I'm not sure that this is the best way to do it.)

来源：https://stackoverflow.com/questions/21926255/traverse-nested-dict-in-julia-lang