reduce

问题 I\'m using Python 3.2. Tried this: xor = lambda x,y: (x+y)%2 l = reduce(xor, [1,2,3,4]) And got the following error: l = reduce(xor, [1,2,3,4]) NameError: name \'reduce\' is not defined Tried printing reduce into interactive console - got this error: NameError: name \'reduce\' is not defined Is reduce really removed in Python 3.2? If that\'s the case, what\'s the alternative? 回答1: It was moved to functools. 回答2: You can add from functools import reduce before you use the reduce. 回答3: Or if

问题 Some background information: I\'m working with Dataiku DSS, HDFS, and partitioned datasets. I have a particular job running (Hive query) which has two input datasets - one a very large, partitioned dataset, the other a small (~250 rows, 2 columns), non-partitioned dataset. Let\'s call the partitioned table A, and the non-partitioned table B. Question: The query is of the following format, SELECT a.f1, f2, ..., fn FROM A as a LEFT JOIN B as b ON a.f1 = b.f1 WHERE {PARTITION_FILTER} Here is the

问题 Say I want to sum a.x for each element in arr . arr = [{x:1},{x:2},{x:4}] arr.reduce(function(a,b){return a.x + b.x}) >> NaN I have cause to believe that a.x is undefined at some point. The following works fine arr = [1,2,4] arr.reduce(function(a,b){return a + b}) >> 7 What am I doing wrong in the first example? 回答1: After the first iteration your're returning a number and then trying to get property x of it to add to the next object which is undefined and maths involving undefined results in

问题 How can I break the iteration on reduce method? for for (var i = Things.length - 1; i >= 0; i--) { if(Things[i] <= 0){ break; } }; reduce Things.reduce(function(memo, current){ if(current <= 0){ //break ??? //return; <-- this will return undefined to memo, which is not what I want } }, 0) 回答1: UPDATE Some of the commentators make a good point that the original array is being mutated in order to break early inside the .reduce() logic. Therefore, I've modified the answer slightly by adding a

问题 When should I use reduceLeft , reduceRight , foldLeft , foldRight , scanLeft or scanRight ? I want an intuition/overview of their differences - possibly with some simple examples. 回答1: In general, all 6 fold functions apply a binary operator to each element of a collection. The result of each step is passed on to the next step (as input to one of the binary operator's two arguments). This way we can cumulate a result. reduceLeft and reduceRight cumulate a single result. foldLeft and foldRight

问题 In Hadoop when do reduce tasks start? Do they start after a certain percentage (threshold) of mappers complete? If so, is this threshold fixed? What kind of threshold is typically used? 回答1: The reduce phase has 3 steps: shuffle, sort, reduce. Shuffle is where the data is collected by the reducer from each mapper. This can happen while mappers are generating data since it is only a data transfer. On the other hand, sort and reduce can only start once all the mappers are done. You can tell

问题 Why do Scala and frameworks like Spark and Scalding have both reduce and foldLeft ? So then what\'s the difference between reduce and fold ? 回答1: reduce vs foldLeft A big big difference, not mentioned in any other stackoverflow answer relating to this topic clearly, is that reduce should be given a commutative monoid , i.e. an operation that is both commutative and associative. This means the operation can be parallelized. This distinction is very important for Big Data / MPP / distributed

问题 I have to find the average of a list in Python. This is my code so far l = [15, 18, 2, 36, 12, 78, 5, 6, 9] print reduce(lambda x, y: x + y, l) I\'ve got it so it adds together the values in the list, but I don\'t know how to make it divide into them? 回答1: On Python 3.4+ you can use statistics.mean() l = [15, 18, 2, 36, 12, 78, 5, 6, 9] import statistics statistics.mean(l) # 20.11111111111111 On older versions of Python you can do sum(l) / len(l) On Python 2 you need to convert len to a float

问题 How would one take a javascript array of objects such as: my objArr = [ {key:Mon Sep 23 2013 00:00:00 GMT-0400, val:42}, {key:Mon Sep 24 2013 00:00:00 GMT-0400, val:78}, {key:Mon Sep 25 2013 00:00:00 GMT-0400, val:23}, {key:Mon Sep 23 2013 00:00:00 GMT-0400, val:54}] and merge duplicate keys by summing the values. In order to get something like this: my reducedObjArr = [ {key:Mon Sep 23 2013 00:00:00 GMT-0400, val:96}, {key:Mon Sep 24 2013 00:00:00 GMT-0400, val:78}, {key:Mon Sep 25 2013 00

问题 filter , map , and reduce work perfectly in Python 2. Here is an example: >>> def f(x): return x % 2 != 0 and x % 3 != 0 >>> filter(f, range(2, 25)) [5, 7, 11, 13, 17, 19, 23] >>> def cube(x): return x*x*x >>> map(cube, range(1, 11)) [1, 8, 27, 64, 125, 216, 343, 512, 729, 1000] >>> def add(x,y): return x+y >>> reduce(add, range(1, 11)) 55 But in Python 3, I receive the following outputs: >>> filter(f, range(2, 25)) <filter object at 0x0000000002C14908> >>> map(cube, range(1, 11)) <map object