I really like google golang but could some one explain what the rationale is for the implementors having left out a basic data structure such as sets from the standard library?
-
8The language is actually called Go, not golang– daniel gratzerNov 28, 2012 at 2:44
-
103But "golang" is more searchable– MattMay 29, 2013 at 19:27
-
24Way more searchable. Googling "go set" returns images of a wooden board with black and white pieces.– Doug RichardsonFeb 10, 2014 at 5:55
-
I believe the community advice is to use Golang, not Go, for the above reasons - searchability– occulusDec 13, 2019 at 11:58
3 Answers
One potential reason for this omission is that it's really easy to model sets with a map.
To be honest I think it's a bit of an oversight too, however looking at Perl, the story's exactly the same. In Perl you get lists and hashtables, in Go you get arrays, slices, and maps. In Perl you'd generally use a hashtable for any and all problems relating to a set, the same is applicable to Go.
Example
to imitate a set ints in Go, we define a map:
set := make(map[int]bool)
To add something is as easy as:
i := valueToAdd()
set[i] = true
Deleting something is just
delete(set, i)
And the potential awkwardness of this construct is easily abstracted away:
type IntSet struct {
set map[int]bool
}
func (set *IntSet) Add(i int) bool {
_, found := set.set[i]
set.set[i] = true
return !found //False if it existed already
}
And delete and get can be defined similarly, I have the complete implementation here . The major disatvantage here is the fact that go doesn't have generics. However it is possible to do this with interface{}
in which case you'd have cast the results of get.
-
3Here's my slightly-revised version with Contains and Size methods: play.golang.org/p/tDdutH672-– Rick-777Apr 16, 2013 at 18:07
-
22Instead of
map[int]bool
one can usemap[int]struct{}
instead. I prefer the last.– oblitumDec 11, 2013 at 2:26 -
22
-
4github.com/fatih/set is an implementation of my based on maps and empty structs. It's thread safe and has a simple api. Jan 11, 2014 at 23:50
-
6With
map[int]struct{}
you can't doif mymap["key"] {
to check for membership. Google recommends usingbool
(search for "A set can be implemented").– TimmmmJan 6, 2015 at 13:18
I think this has to do with golang
focus on simplicity. set
s become really useful with difference
, intersection
, union
, issubset
, and so on.. methods. Perhaps golang
team felt that it is too much for one data structure. But otherwise a "dumb set" that only has add
, contains
and remove
can be easily replicated with map
as explained by @jozefg.
-
i disagree. a set is mostly used for membership checks and uniqeness semantics. a set implementation would be perfectly usable without those methods. that being said, they are also trivial to implement. Aug 17, 2018 at 18:23
The previous answer works ONLY IF the key are a built-in type. To complement the previous answer, here is a way to implement a set whose elements are user-defined types:
package math
// types
type IntPoint struct {
X, Y int
}
// set implementation for small number of items
type IntPointSet struct {
slice []IntPoint
}
// functions
func (p1 IntPoint) Equals(p2 IntPoint) bool {
return (p1.X == p2.X) && (p1.Y == p2.Y)
}
func (set *IntPointSet) Add(p IntPoint) {
if ! set.Contains(p) {
set.slice = append(set.slice, p)
}
}
func (set IntPointSet) Contains(p IntPoint) bool {
for _, v := range set.slice {
if v.Equals(p) {
return true
}
}
return false
}
func (set IntPointSet) NumElements() int {
return len(set.slice)
}
func NewIntPointSet() IntPointSet {
return IntPointSet{(make([]IntPoint, 0, 10))}
}
-
8"works ONLY IF the key are a built-in type" is wrong.
type mySet map[IntPoint]bool
works perfectly well. All that is required of the key type used in a map is that it has==
and!=
. Equality of struct types is well defined, yourEquals
method should be justp1 == p2
.– Dave CMar 27, 2015 at 1:48 -
1It is true that equality for structs is well-defined, but if the struct contains maps or slices as fields, they will be compared by reference, rather than by value. This may not be what you want. Oct 24, 2018 at 19:42
-
1I take a bit of issue with this solution, because
Contains
takes linear time, whileaMap[]
takes constant time, regardless the number of members. A better solution would internally create a unique key based the contents of each member, and leverage the constant-time querying that themap
type provides. Even faster solutions that consider cache behavior, etc. exist as well. Oct 24, 2018 at 19:46