Contains method for a slice

Question

Is there anything similar to a slice contains object  method in Go without having to do a search through each element in a slice

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If the slice is sorted  there is a binary search implemented in the sort package

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No  such method does not exist  but is trivial to write   func contains s   int  e int  bool       for    a    range s           if a    e               return true                     return false     You can use a map if that lookup is an important part of your code  but maps have cost too

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Currently there s Contains function in slice package  You can read the docs here  Sample usage   if  slice Contains sliceVar  valueToFind          code here

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Instead of using a slice  map may be a better solution   simple example   package main  import  fmt    func contains slice   string  item string  bool       set    make map string struct    len slice       for    s    range slice           set s    struct                   ok    set item       return ok    func main          s      string  a    b       s1     a      fmt Println contains s  s1        http   play golang org p CEG6cu4JTf

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I think map x bool is more useful than map x struct    Indexing the map for an item that isn t present will return false so instead of    ok    m X   you can just say m X   This makes it easy to nest inclusion tests in expressions

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It might be considered a bit  hacky  but depending the size and contents of the slice  you can join the slice together and do a string search   For example you have a slice containing single word values  e g   yes    no    maybe    These results are appended to a slice  If you want to check if this slice contains any  maybe  results  you may use  exSlice      yes    no    yes    maybe   if strings Contains strings Join exSlice         maybe       fmt Println  We have a maybe        How suitable this is really depends on the size of the slice and length of its members  There may be performance or suitability issues for large slices or long values  but for smaller slices of finite size and simple values it is a valid one-liner to achieve the desired result

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The sort package provides the building blocks if your slice is sorted or you are willing to sort it   input      string  bird    apple    ocean    fork    anchor   sort Strings input   fmt Println contains input   apple       true fmt Println contains input   grow        false       func contains s   string  searchterm string  bool       i    sort SearchStrings s  searchterm      return i  lt  len s   amp  amp  s i     searchterm     SearchString promises to return the index to insert x if x is not present  it could be len a    so a check of that reveals whether the string is contained the sorted slice

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In other thread I commented a solution for this issue in two ways  First method  func Find slice interface    f func value interface    bool  int       s    reflect ValueOf slice      if s Kind      reflect Slice           for index    0  index  lt  s Len    index                 if f s Index index  Interface                      return index                                   return -1    Use example  type UserInfo struct       UserId          int    func main         var           destinationList   UserInfo         userId      int   123                destinationList   append destinationList  UserInfo            UserId            23              destinationList   append destinationList  UserInfo            UserId            12                   idx    Find destinationList  func value interface    bool           return value  UserInfo  UserId    userId                 if idx  lt  0           fmt Println  quot not found quot         else           fmt Println idx               Second method with less computational cost  func Search length int  f func index int  bool  int       for index    0  index  lt  length  index             if f index                return index                     return -1    Use example  type UserInfo struct       UserId          int    func main         var           destinationList   UserInfo         userId      int   123                destinationList   append destinationList  UserInfo            UserId            23              destinationList   append destinationList  UserInfo            UserId            123                   idx    Search len destinationList   func index int  bool           return destinationList index  UserId    userId                 if  idx  lt  0           fmt Println  quot not found quot         else           fmt Println idx

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Not sure generics are needed here  You just need a contract for your desired behavior  Doing the following is no more than what you would have to do in other languages if you wanted your own objects to behave themselves in collections  by overriding Equals   and GetHashCode   for instance   type Identifiable interface      GetIdentity   string    func IsIdentical this Identifiable  that Identifiable  bool      return   amp this     amp that      this GetIdentity      that GetIdentity       func contains s   Identifiable  e Identifiable  bool       for    a    range s           if IsIdentical a e                return true                     return false

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I created the following Contains function using reflect package  This function can be used for various types like int32 or struct etc     Contains returns true if an element is present in a slice func Contains list interface    elem interface    bool       listV    reflect ValueOf list       if listV Kind      reflect Slice           for i    0  i  lt  listV Len    i                 item    listV Index i  Interface                target    reflect ValueOf elem  Convert reflect TypeOf item   Interface               if ok    reflect DeepEqual item  target   ok                   return true                                   return false    Usage of contains function  is below    slice of int32 containsInt32    Contains   int32 1  2  3  4  5   3  fmt Println  quot contains int32  quot   containsInt32      slice of float64 containsFloat64    Contains   float64 1 1  2 2  3 3  4 4  5 5   4 4  fmt Println  quot contains float64  quot   containsFloat64       slice of struct type item struct       ID   string     Name string   list      item      item          ID     quot 1 quot           Name   quot test1 quot              item          ID     quot 2 quot           Name   quot test2 quot              item          ID     quot 3 quot           Name   quot test3 quot            target    item      ID     quot 2 quot       Name   quot test2 quot     containsStruct    Contains list  target  fmt Println  quot contains struct  quot   containsStruct      Output     contains int32  true    contains float64  true    contains struct  true  Please see here for more details  https   github com glassonion1 xgo blob main contains go

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I created a very simple benchmark with the solutions from these answers   https   gist github com NorbertFenk 7bed6760198800207e84f141c41d93c7  It isn t a real benchmark because initially  I haven t inserted too many elements but feel free to fork and change it

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Mostafa has already pointed out that such a method is trivial to write  and mkb gave you a hint to use the binary search from the sort package  But if you are going to do a lot of such contains checks  you might also consider using a map instead   It s trivial to check if a specific map key exists by using the value  ok    yourmap key  idiom  Since you aren t interested in the value  you might also create a map string struct   for example  Using an empty struct   here has the advantage that it doesn t require any additional space and Go s internal map type is optimized for that kind of values  Therefore  map string  struct   is a popular choice for sets in the Go world

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You can use the reflect package to iterate over an interface whose concrete type is a slice   func HasElem s interface    elem interface    bool       arrV    reflect ValueOf s       if arrV Kind      reflect Slice           for i    0  i  lt  arrV Len    i                     XXX - panics if slice element points to an unexported struct field                see https   golang org pkg reflect  Value Interface             if arrV Index i  Interface      elem                   return true                                    return false     https   play golang org p jL5UD7yCNq

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If it is not feasable to use a map for finding items based on a key  you can consider the goderive tool  Goderive generates a type specific implementation of a contains method  making your code both readable and efficient    Example   type Foo struct       Field1 string     Field2 int     func Test m Foo  bool        var allItems   Foo      return deriveContainsFoo allItems  m      To generate the deriveContainsFoo method    Install goderive with go get -u github com awalterschulze goderive Run goderive       in your workspace folder   This method will be generated for deriveContains   func deriveContainsFoo list   Foo  item Foo  bool       for    v    range list           if v    item               return true                     return false     Goderive has support for quite some other useful helper methods to apply a functional programming style in go

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func Contain target interface    list interface     bool  int        if reflect TypeOf list  Kind      reflect Slice    reflect TypeOf list  Kind      reflect Array           listvalue    reflect ValueOf list          for i    0  i  lt  listvalue Len    i                 if target    listvalue Index i  Interface                     return true  i                                   if reflect TypeOf target  Kind      reflect String  amp  amp  reflect TypeOf list  Kind      reflect String           return strings Contains list  string   target  string    strings Index list  string   target  string             return false  -1

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The go style   func Contains n int  match func i int  bool  bool       for i    0  i  lt  n  i             if match i                return true                     return false     s      string  a    b    c    o      test if s contains  o  ok    Contains len s   func i int  bool       return s i      o

[go] Contains method for a slice

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