Sort Go map values by keys

Question

When iterating through the returned map in the code  returned by the topic function  the keys are not appearing in order   How can I get the keys to be in order   sort the map so that the keys are in order and the values correspond   Here is the code

User · Accepted Answer

The Go blog  Go maps in action has an excellent explanation   When iterating over a map with a range loop  the iteration order is not specified and is not guaranteed to be the same from one iteration to the next  Since Go 1 the runtime randomizes map iteration order  as programmers relied on the stable iteration order of the previous implementation  If you require a stable iteration order you must maintain a separate data structure that specifies that order   Here s my modified version of example code  http   play golang org p dvqcGPYy3- package main  import        quot fmt quot       quot sort quot     func main            To create a map as input     m    make map int string      m 1     quot a quot      m 2     quot c quot      m 0     quot b quot          To store the keys in slice in sorted order     keys    make   int  len m       i    0     for k    range m           keys i    k         i             sort Ints keys          To perform the opertion you want     for    k    range keys           fmt Println  quot Key  quot   k   quot Value  quot   m k            Output  Key  0 Value  b Key  1 Value  a Key  2 Value  c

User · Answer

In reply to James Craig Burley s answer  In order to make a clean and re-usable design  one might choose for a more object oriented approach  This way methods can be safely bound to the types of the specified map  To me this approach feels cleaner and organized   Example   package main  import        fmt       sort     type myIntMap map int string  func  m myIntMap  sort    index   int        for k       range m           index   append index  k            sort Ints index      return    func main         m    myIntMap          1    one           11   eleven           3    three             for    k    range m sort             fmt Println m k             Extended playground example with multiple map types   Important note  In all cases  the map and the sorted slice are decoupled from the moment the for loop over the map range is finished  Meaning that  if the map gets modified after the sorting logic  but before you use it  you can get into trouble   Not thread   Go routine safe   If there is a change of parallel Map write access  you ll need to use a mutex around the writes and the sorted for loop   mutex Lock   for    k    range m sort         fmt Println m k     mutex Unlock

User · Answer

All of the answers here now contain the old behavior of maps  In Go 1 12   you can just print a map value and it will be sorted by key automatically  This has been added because it allows the testing of map values easily  func main         m    map int int 3  5  2  4  1  3      fmt Println m          In Go 1 12         Output  map 1 3 2 4 3 5          Before Go 1 12  the order was undefined         map 3 5 2 4 1 3      Maps are now printed in key-sorted order to ease testing  The ordering rules are   When applicable  nil compares low ints  floats  and strings order by  lt  NaN compares less than non-NaN floats bool compares false before true Complex compares real  then imaginary Pointers compare by machine address Channel values compare by machine address Structs compare each field in turn Arrays compare each element in turn Interface values compare first by reflect Type describing the concrete type and then by concrete value as described in the previous rules   When printing maps  non-reflexive key values like NaN were previously displayed as  lt nil gt   As of this release  the correct values are printed   Read more here

User · Answer

This provides you the code example on sorting map  Basically this is what they provide   var keys   int for k    range myMap       keys   append keys  k    sort Ints keys      Benchmark1-8      2863149           374 ns op         152 B op          5 allocs op   and this is what I would suggest using instead   keys    make   int  0  len myMap   for k    range myMap       keys   append keys  k    sort Ints keys      Benchmark2-8      5320446           230 ns op          80 B op          2 allocs op   Full code can be found in this Go Playground

User · Answer

If  like me  you find you want essentially the same sorting code in more than one place  or just want to keep the code complexity down  you can abstract away the sorting itself to a separate function  to which you pass the function that does the actual work you want  which would be different at each call site  of course    Given a map with key type K and value type V  represented as  lt K gt  and  lt V gt  below  the common sort function might look something like this Go-code template  which Go version 1 does not support as-is       Go apparently doesn t support allow  interface    as the value  or    key  of a map such that any arbitrary type can be substituted at    run time  so several of these nearly-identical functions might be    needed for different key value type combinations     func sortedMap lt K gt  lt T gt  m map  lt K gt   lt V gt   f func k  lt K gt   v  lt V gt          var keys    lt K gt      for k       range m           keys   append keys  k            sort Strings keys     or sort Ints keys   sort Sort       etc   per  lt K gt      for    k    range keys           v    m k          f k  v            Then call it with the input map and a function  taking  k  lt K gt   v  lt V gt   as its input arguments  that is called over the map elements in sorted-key order   So  a version of the code in the answer posted by Mingu might look like   package main  import        fmt       sort     func sortedMapIntString m map int string  f func k int  v string         var keys   int     for k       range m           keys   append keys  k            sort Ints keys      for    k    range keys           f k  m k            func main            Create a map for processing     m    make map int string      m 1     a      m 2     c      m 0     b       sortedMapIntString m          func k int  v string    fmt Println  Key    k   Value    v         The sortedMapIntString   function can be re-used for any map int string  assuming the same sort order is desired   keeping each use to just two lines of code   Downsides include    It s harder to read for people unaccustomed to using functions as first-class It might be slower  I haven t done performance comparisons    Other languages have various solutions    If the use of  lt K gt  and  lt V gt   to denote types for the key and value  looks a bit familiar  that code template is not terribly unlike C   templates  Clojure and other languages support sorted maps as fundamental data types  While I don t know of any way Go makes range a first-class type such that it could be substituted with a custom ordered-range  in place of range in the original code   I think some other languages provide iterators that are powerful enough to accomplish the same thing

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According to the Go spec  the order of iteration over a map is undefined  and may vary between runs of the program  In practice  not only is it undefined  it s actually intentionally randomized  This is because it used to be predictable  and the Go language developers didn t want people relying on unspecified behavior  so they intentionally randomized it so that relying on this behavior was impossible   What you ll have to do  then  is pull the keys into a slice  sort them  and then range over the slice like this   var m map keyType valueType keys    sliceOfKeys m     you ll have to implement this for    k    range keys       v    m k         k is the key and v is the value  do your computation here

[go] Sort Go map values by keys

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