[GitHub] [arrow-rs] alamb commented on pull request #1296: wip. Implement an iterator for DictionaryArray

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alamb commented on pull request #1296:
URL: https://github.com/apache/arrow-rs/pull/1296#issuecomment-1037228245


   > Thanks @alamb. Yea, that is also I have concern with it. But I don't have better idea for DictionaryIter which supports all types of arrays (primitive, utf8, binary..).
   
   I thought a bit more about this.
   
   What about adding something slightly more general such as:
   
   ```rust
   impl DictionaryArray {
     /// return an iterator over the keys (indexes into the dictionary)
     fn keys_iter() -> impl Iterator<Item = usize>
   }
   ```
   
   Then add an iterator to the various other types of array:
   
   ```rust
   impl PrimitiveArray<T> {
   ...
      /// return an iterator that returns the values of `array.value(i)` for an iterator with each element `i`
      fn take_iter(&self, indexes: impl Iterator<item = usize>) -> impl Iterator<Item = T::NativeType> {
         ..
      }
   }
   ```
   
   So then to iterate over a dictionary by value the user would do something like:
   ```rust
   let keys = dict_array.keys_iter();
   let values = dict_array.values().downcast_ref::<UInt64Array>().unwrap().take_iter(keys);
   // values is an interator over u64!
   ```
   
   


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