Accumulator
An accumulator is a value, storage location, or combining function that carries or updates an intermediate result as inputs are processed.
Clojure: The Essential Reference
An accumulator is the value representing the result of a reduction after processing some prefix of the input.
- How it is used
- Carries partial results through Clojure reductions.
- How deep it goes
- Defines the accumulator and explains update order, initial values, and empty-input behavior.
- Examples
- Shows running sums of squares, word counts, and a running average with a vector accumulator.
- What you can do after
- Apply it via Clojure reduce and reductions to compute final or running results.
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100 Java Mistakes and How to Avoid Them
An accumulator function combines two values during a reduction operation to produce another value.
- How it is used
- Validates reduction functions used with Java Stream API operations.
- How deep it goes
- Explains associativity, identity, sequential behavior, and parallel failure, but stops short of general reduction implementation details.
- Examples
- Uses List.hashCode and a Java reduce expression whose multiplication-based accumulator fails under parallel evaluation.
- What you can do after
- Evaluate reduction accumulators against identity and associativity requirements before using sequential or parallel streams.
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Grokking Simplicity
A function passed to a reduction operation that combines the accumulated result with the current sequence element.
- How it is used
- Supports reduce operations that combine sequence elements into totals, strings, maps, or other accumulated results.
- How deep it goes
- Defines the two arguments, return rule, initialization, and iteration behavior, but does not descend into implementation internals beyond a loop.
- Examples
- Uses purchase counting, string concatenation, numeric sum and product, and minimum or maximum exercises.
- What you can do after
- Apply it via reduce to build accumulation functions with appropriate initial values.
The Well-Grounded Rubyist, Fourth Edition
An accumulator is a value carried across iterations and replaced by each iteration's calculated result.
- How it is used
- Carries iteration results forward when reducing a collection or collecting block results into a new array.
- How deep it goes
- Traces initialization, replacement, and final return in inject, then shows accumulator-array construction for map.
- Examples
- Uses a four-number sum and a name-uppercasing map, including a map block whose puts calls yield nil results.
- What you can do after
- Build it from scratch by accumulating block results in a my_map method, or apply it with inject to reduce values.
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Related concepts
- maximumincludes · A maximum is the greatest value or element in a specified set under a relevant ordering, or the upper bound imposed on an allowed range.
- reduction operationused by · A reduction operation combines multiple input values or partial results according to a combining rule to produce fewer aggregate results, often a single result.
- aggregation pipelineuses · An aggregation pipeline is an ordered sequence of data-processing stages that filters, transforms, combines, or summarizes records.
- data aggregationpart of · Data aggregation is the combination of multiple values, records, observations, or results into a consolidated or summarized output.
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