[GitHub] [arrow-rs] alamb opened a new issue #672: Add a way to get the dictionary index and values array reference

[GitBox <gi...@apache.org>]

alamb opened a new issue #672:
URL: https://github.com/apache/arrow-rs/issues/672


   **Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
   To work with the value of a dictionary array at a particular index, the following pattern shows up many times in the arrow-rs and datafusion codebase:
   
   ```rust
       fn do_something <K: ArrowDictionaryKeyType>(
           array: &ArrayRef,
           index: usize,
       ) -> Result<Self> {
           let dict_array = array.as_any().downcast_ref::<DictionaryArray<K>>().unwrap();
   
           // look up the index in the values dictionary
           let keys_col = dict_array.keys();
           let values_index = keys_col.value(index).to_usize().ok_or_else(|| {
               DataFusionError::Internal(format!(
                   "Can not convert index to usize in dictionary of type creating group by value {:?}",
                   keys_col.data_type()
               ))
           })?;
           
           // do actual work with dict_array.values and values_index
           // ...
       }
    }
   ```
   Repeating this code "find the index" code is tedious
   
   **Describe the solution you'd like**
   
   Add a function such as the following on to DictionaryArray (would love suggestions about better names):
   
   ```rust
   impl DictionaryArray<K: ArrowDictionaryKeyType> {
     
     // return the index into the dictionary values for array@index as well
     // as the dictioanry values
     #[inline]
     fn dict_value(
       self: 
       index: usize,
     ) -> Result<(&ArrayRef, usize)>
     {
       // look up the index in the values dictionary
       let keys_col = self.keys();
       let values_index = keys_col.value(index).to_usize().ok_or_else(|| {
           DataFusionError::Internal(format!(
               "Can not convert index to usize in dictionary of type creating group by value {:?}",
               keys_col.data_type()
           ))
       })?;
   
       Ok((self.values(), values_index))
   }
   ```
   
   **Describe alternatives you've considered**
   
   **Additional context**
   


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[GitHub] [arrow-rs] alamb commented on issue #672: Add a way to get the dictionary index and values array reference

[GitBox <gi...@apache.org>]

alamb commented on issue #672:
URL: https://github.com/apache/arrow-rs/issues/672#issuecomment-895198607


   Note the bug in the above code -- it does not handle NULLs correctly!


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