How to copy a map

Question

I m trying to copy the contents of a map   amap   inside another one  aSuperMap  and then clear  amap so that it can take  new values on the next iteration loop     The issue is that you can t clear the map without to clear its reference in the supermap as well   Here is some pseudo code   for something    range fruits          aMap    make map string aStruct          aSuperMap    make map string map string aStruct           for x    range something              aMap x    aData             aSuperMap y    aMap             delete aMap  x          save aSuperMap   saveASuperMap something       I ve also tried some dynamic stuff but obviously it throws an error  cannot assign to nil   aSuperMap y  x    aData   The question is how can I create an associative map   In PHP I simply use  aSuperMap y  x    aData  It seems that golang doesn t have any obvious method  If I delete  delete aMap  x  its reference from the super map is deleted as well  If I don t delete it the supermap ends up with duplicate data  Basically on each loop it gets aMap with the new value plus all the old values

User · Answer

Individual element copy  it seems to work for me with just a simple example    maps    map string int        alice  12       jimmy  15     maps2    make map string int  for k2 v2    range maps       maps2 k2    v2    maps2  miki   rand Intn 100   fmt Println  maps    maps   vs     maps2    maps2

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As stated in seong s comment      Also see http   golang org doc effective go html maps  The important part is really the  reference to underlying data structure   This also applies to slices    However  none of the solutions here seem to offer a solution for a proper deep copy that also covers slices   I ve slightly altered Francesco Casula s answer to accommodate for both maps and slices     This should cover both copying your map itself  as well as copying any child maps or slices  Both of which are affected by the same  underlying data structure  issue  It also includes a utility function for performing the same type of Deep Copy on a slice directly    Keep in mind that the slices in the resulting map will be of type   interface    so when using them  you will need to use type assertion to retrieve the value in the expected type   Example Usage  copy    CopyableMap originalMap  DeepCopy     Source File  util go   package utils  type CopyableMap   map string interface   type CopyableSlice   interface       DeepCopy will create a deep copy of this map  The depth of this    copy is all inclusive  Both maps and slices will be considered when    making the copy  func  m CopyableMap  DeepCopy   map string interface         result    map string interface          for k v    range m              Handle maps         mapvalue isMap    v  map string interface            if isMap               result k    CopyableMap mapvalue  DeepCopy               continue                       Handle slices         slicevalue isSlice    v    interface            if isSlice               result k    CopyableSlice slicevalue  DeepCopy               continue                    result k    v            return result       DeepCopy will create a deep copy of this slice  The depth of this    copy is all inclusive  Both maps and slices will be considered when    making the copy  func  s CopyableSlice  DeepCopy     interface         result      interface          for   v    range s              Handle maps         mapvalue isMap    v  map string interface            if isMap               result   append result  CopyableMap mapvalue  DeepCopy                continue                       Handle slices         slicevalue isSlice    v    interface            if isSlice               result   append result  CopyableSlice slicevalue  DeepCopy                continue                    result   append result  v             return result     Test File  util tests go   package utils  import        testing        github com stretchr testify require     func TestCopyMap t  testing T        m1    map string interface             a    bbb            b   map string interface                 c   123                      c     interface                  d    e   map string interface                      f    g                                        m2    CopyableMap m1  DeepCopy        m1  a      zzz      delete m1   b       m1  c      interface    1     x      m1  c      interface    2   map string interface     f      h       require Equal t  map string interface             a    zzz             c     interface                  d    x   map string interface                      f    h                                    m1      require Equal t  map string interface             a    bbb            b   map string interface                 c   123                      c     interface                  d    e   map string interface                      f    g                                    m2

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You have to manually copy each key value pair to a new map  This is a loop that people have to reprogram any time they want a deep copy of a map   You can automatically generate the function for this by installing mapper from the maps package using  go get -u github com drgrib maps cmd mapper   and running  mapper -types string aStruct   which will generate the file map float astruct go containing not only a  deep  Copy for your map but also other  missing  map functions ContainsKey  ContainsValue  GetKeys  and GetValues   func ContainsKeyStringAStruct m map string aStruct  k string  bool          ok    m k      return ok    func ContainsValueStringAStruct m map string aStruct  v aStruct  bool       for    mValue    range m           if mValue    v               return true                      return false    func GetKeysStringAStruct m map string aStruct    string       keys      string        for k       range m           keys   append keys  k             return keys    func GetValuesStringAStruct m map string aStruct    aStruct       values      aStruct        for    v    range m           values   append values  v             return values    func CopyStringAStruct m map string aStruct  map string aStruct       copyMap    map string aStruct        for k  v    range m           copyMap k    v            return copyMap     Full disclosure  I am the creator of this tool  I created it and its containing package because I found myself constantly rewriting these algorithms for the Go map for different type combinations

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I d use recursion just in case so you can deep copy the map and avoid bad surprises in case you were to change a map element that is a map itself   Here s an example in a utils go   package utils  func CopyMap m map string interface    map string interface         cp    make map string interface        for k  v    range m           vm  ok    v  map string interface            if ok               cp k    CopyMap vm            else               cp k    v                      return cp     And its test file  i e  utils test go    package utils  import        testing        github com stretchr testify require     func TestCopyMap t  testing T        m1    map string interface             a    bbb            b   map string interface                 c   123                        m2    CopyMap m1       m1  a      zzz      delete m1   b        require Equal t  map string interface    a    zzz    m1      require Equal t  map string interface             a    bbb            b   map string interface                 c   123                    m2      It should easy enough to adapt if you need the map key to be something else instead of a string

User · Answer

You are not copying the map  but the reference to the map  Your delete thus modifies the values in both your original map and the super map  To copy a map  you have to use a for loop like this   for k v    range originalMap     newMap k    v     Here s an example from the now-retired SO documentation      Create the original map originalMap    make map string int  originalMap  one     1 originalMap  two     2     Create the target map targetMap    make map string int      Copy from the original map to the target map for key  value    range originalMap     targetMap key    value        Excerpted from Maps - Copy a Map  The original author was JepZ  Attribution details can be found on the contributor page  The source is licenced under CC BY-SA 3 0 and may be found in the Documentation archive  Reference topic ID  732 and example ID  9834

[go] How to copy a map?

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