Thanks Stephan, that helps quite a bit. Looks like another one of those API changes that I'll be struggling with for a little bit.
On Thu, Nov 19, 2015 at 10:40 AM, Stephan Ewen <se...@apache.org> wrote: > Hi Ron! > > You are right, there is a copy/paste error in the docs, it should be a > FoldFunction that is passed to fold(), not a ReduceFunction. > > In Flink-0.10, the FoldFunction is only available on > > - KeyedStream ( > https://ci.apache.org/projects/flink/flink-docs-release-0.10/api/java/org/apache/flink/streaming/api/datastream/KeyedStream.html#fold(R,%20org.apache.flink.api.common.functions.FoldFunction) > ) > > - WindowedStream ( > https://ci.apache.org/projects/flink/flink-docs-release-0.10/api/java/org/apache/flink/streaming/api/datastream/WindowedStream.html#fold(R,%20org.apache.flink.api.common.functions.FoldFunction,%20org.apache.flink.api.common.typeinfo.TypeInformation) > ) > > In most cases, you probably want the variant on the WindowedStream, if you > aggregate values over time. > > -------------------------------------------------------- > > To the difference between fold() and reduce(): It is very subtle. The fold > function can also convert to another type whenever it integrates a new > element. > > Here is an example (with lists, not streams, but same principle). > > -------------------------------------------------------- > > ReduceFunction<Integer> { > > public Integer reduce(Integer a, Integer b) { return a + b; } > } > > [1, 2, 3, 4, 5] -> reduce() means: ((((1 + 2) + 3) + 4) + 5) = 15 > > -------------------------------------------------------- > > FoldFunction<String, Integer> { > > public String fold(String current, Integer i) { return current + > String.valueOf(i); } > } > > [1, 2, 3, 4, 5] -> fold("start-") means: ((((("start-" + 1) + 2) + 3) + > 4) + 5) = "start-12345" (as a String) > > > I hope that example illustrates the difference. > > > Greetings, > Stephan > > > On Thu, Nov 19, 2015 at 7:00 PM, Ron Crocker <rcroc...@newrelic.com> > wrote: > >> Hi Fabian - >> >> Thanks Fabian, that is a helpful description. >> >> That document WAS my source of information and it seems to also be the >> source of my confusion. Further, it appears to be wrong - there is a >> FoldFunction<O,T> ( >> https://ci.apache.org/projects/flink/flink-docs-release-0.10/api/java/org/apache/flink/api/common/functions/FoldFunction.html) >> that should be passed into fold()? >> >> Separate note: fold() doesn't appear in the javadocs for 0.10.0 >> DataStream (see >> https://ci.apache.org/projects/flink/flink-docs-release-0.10/api/java/org/apache/flink/streaming/api/datastream/DataStream.html). >> So this made me look in the freshly-downloaded flink-streaming-java:0.10.0 >> and fold() does not appear in org >> .apache.flink.streaming.api.datastream.DataStream either. Am I looking >> in the wrong place for it? In 0.9.1, it's located in that same class with >> this signature: fold(R initialValue, FoldFunction<OUT, R> folder). >> >> Ron >> >> On Wed, Nov 18, 2015 at 9:39 AM, Fabian Hueske <fhue...@gmail.com> wrote: >> >>> Hi Ron, >>> >>> Have you checked: >>> https://ci.apache.org/projects/flink/flink-docs-release-0.10/apis/streaming_guide.html#transformations >>> ? >>> >>> Fold is like reduce, except that you define a start element (of a >>> different type than the input type) and the result type is the type of the >>> initial value. In reduce, the result type must be identical to the input >>> type. >>> >>> Best, Fabian >>> >>> 2015-11-18 18:32 GMT+01:00 Ron Crocker <rcroc...@newrelic.com>: >>> >>>> Is there a succinct description of the distinction between these >>>> transforms? >>>> >>> >> -- >> Ron Crocker >> Principal Software Engineer >> ( ( •)) New Relic >> rcroc...@newrelic.com >> M: +1 630 363 8835 >> > > -- Ron Crocker Principal Software Engineer ( ( •)) New Relic rcroc...@newrelic.com M: +1 630 363 8835
