set-operations

[Update 1: As Matthew Dowle noted, I'm using data.table version 1.6.7 on R-Forge, not CRAN. You won't see the same behavior with an earlier version of data.table .] As background: I am porting some little utility functions to do set operations on rows of a data frame or pairs of data frames (i.e. each row is an element in a set), e.g. unique - to create a set from a list, union, intersection, set difference, etc. These mimic Matlab's intersect(...,'rows') , setdiff(...,'rows') , etc., which don't appear to have counterparts in R (R's set operations are limited to vectors and lists, but not

I am intrigued by the following python expression: d3 = dict(d1, **d2) The task is to merge 2 dictionaries into a third one, and the above expression accomplishes the task just fine. I am interested in the ** operator and what exactly is it doing to the expression. I thought that ** was the power operator and haven't seen it used in the context above yet. The full snippet of code is this: >>> d1 = {'a': 1, 'b': 2} >>> d2 = {'c': 3, 'd': 4} >>> d3 = dict(d1, **d2) >>> print d3 {'a': 1, 'c': 3, 'b': 2, 'd': 4} ** in argument lists has a special meaning, as covered in section 4.7 of the tutorial

This question already has an answer here: How to merge two dictionaries in a single expression? 41 answers How do you calculate the union of two dict objects in Python, where a (key, value) pair is present in the result iff key is in either dict (unless there are duplicates)? For example, the union of {'a' : 0, 'b' : 1} and {'c' : 2} is {'a' : 0, 'b' : 1, 'c' : 2} . Preferably you can do this without modifying either input dict . Example of where this is useful: Get a dict of all variables currently in scope and their values Mechanical snail This question provides an idiom. You use one of the

I have two arrays list1 and list2 which have objects with some properties; userId is the Id or unique property: list1 = [ { userId: 1234, userName: 'XYZ' }, { userId: 1235, userName: 'ABC' }, { userId: 1236, userName: 'IJKL' }, { userId: 1237, userName: 'WXYZ' }, { userId: 1238, userName: 'LMNO' } ] list2 = [ { userId: 1235, userName: 'ABC' }, { userId: 1236, userName: 'IJKL' }, { userId: 1252, userName: 'AAAA' } ] I'm looking for an easy way to execute the following three operations: list1 operation list2 should return the intersection of elements: [ { userId: 1235, userName: 'ABC' }, {

Are there any standard library calls I can use to either perform set operations on two arrays, or implement such logic myself (ideally as functionally and also efficiently as possible)? Yes, Swift has the Set class. let array1 = ["a", "b", "c"] let array2 = ["a", "b", "d"] let set1:Set<String> = Set(array1) let set2:Set<String> = Set(array2) Swift 3.0+ can do operations on sets as: firstSet.union(secondSet)// Union of two sets firstSet.intersection(secondSet)// Intersection of two sets firstSet.symmetricDifference(secondSet)// exclusiveOr Swift 2.0 can calculate on array arguments: set1.union