why we not use array for "candidates" ?

[Bgumel]

Because candidates has attributes (id, vote count)

[tanpv]

I mean array of candidate object.

[iridescent995]

I found this in solidity docs "Mappings can be seen as hash tables which are virtually initialized such that every possible key exists and is mapped to a value whose byte-representation is all zeros: a type’s default value. The similarity ends here, though: The key data is not actually stored in a mapping, only its hash used to look up the value.". I think this is the reason.

and we get a free getter method in mapping, so why not use it :)