Which transformer would you use to aggregate per-feature data into lists by area?

Which transformer would you use to aggregate per-feature data into lists by area?

• A. ListHistogrammer
• B. ListSummer
• C. ListRangeExtractor
• D. ListBuilder

Answer: (B)

Aggregating per-feature data into lists by area is about grouping features that share the same area and collecting their values into a single list on the area feature. The transformer that fits this pattern is ListSummer. It acts as a running accumulator: for each incoming feature, it takes the per-feature value and appends it to a list on the corresponding area feature. As features within the same area pass through, their values are collected, so you end up with one output feature per area that contains the complete list of values for that area. This is why ListSummer is the best choice here. It explicitly handles cross-feature aggregation by area, turning dispersed per-feature data into area-centered lists. The other options don’t align with this goal. ListHistogrammer is about turning a list into a histogram of value frequencies, not about building area-based lists. ListRangeExtractor derives the range (min/max) from a list, which isn’t about aggregating values across features. ListBuilder can create a list, but it doesn’t inherently group and accumulate values across multiple features by area in the way ListSummer does.

Aggregating per-feature data into lists by area is about grouping features that share the same area and collecting their values into a single list on the area feature. The transformer that fits this pattern is ListSummer. It acts as a running accumulator: for each incoming feature, it takes the per-feature value and appends it to a list on the corresponding area feature. As features within the same area pass through, their values are collected, so you end up with one output feature per area that contains the complete list of values for that area.

This is why ListSummer is the best choice here. It explicitly handles cross-feature aggregation by area, turning dispersed per-feature data into area-centered lists.

The other options don’t align with this goal. ListHistogrammer is about turning a list into a histogram of value frequencies, not about building area-based lists. ListRangeExtractor derives the range (min/max) from a list, which isn’t about aggregating values across features. ListBuilder can create a list, but it doesn’t inherently group and accumulate values across multiple features by area in the way ListSummer does.