Getting a slice of keys from a map

Question

Is there any simpler nicer way of getting a slice of keys from a map in Go   Currently I am iterating over the map and copying the keys to a slice   i    0 keys    make   int  len mymap   for k    range mymap       keys i    k     i

User · Answer

Visit  https   play golang org p dx6PTtuBXQW  package main  import        fmt       sort     func main         mapEg    map string string  c   a   a   c   b   b       keys    make   string  0  len mapEg       for k    range mapEg           keys   append keys  k            sort Strings keys      fmt Println keys

User · Answer

A nicer way to do this would be to use append   keys     int   for k    range mymap       keys   append keys  k      Other than that  you   re out of luck   Go isn   t a very expressive language

User · Answer

I made a sketchy benchmark on the three methods described in other responses   Obviously pre-allocating the slice before pulling the keys is faster than appending  but surprisingly  the reflect ValueOf m  MapKeys   method is significantly slower than the latter     go run scratch go populating filling 100000000 slots done in 56 630774791s running prealloc took  9 989049786s running append took  18 948676741s running reflect took  25 50070649s   Here s the code  https   play golang org p Z8O6a2jyfTH  running it in the playground aborts claiming that it takes too long  so  well  run it locally

User · Answer

You also can take an array of keys with type   Value by method MapKeys of struct Value from package  reflect    package main  import        fmt       reflect     func main         abc    map string int           a   1           b   2           c   3             keys    reflect ValueOf abc  MapKeys        fmt Println keys      a b c

User · Answer

For example   package main  func main         mymap    make map int string      keys    make   int  0  len mymap       for k    range mymap           keys   append keys  k            To be efficient in Go  it s important to minimize memory allocations

User · Answer

This is an old question  but here s my two cents  PeterSO s answer is slightly more concise  but slightly less efficient   You already know how big it s going to be so you don t even need to use append   keys    make   int  len mymap    i    0 for k    range mymap       keys i    k     i       In most situations it probably won t make much of a difference  but it s not much more work  and in my tests  using a map with 1 000 000 random int64 keys and then generating the array of keys ten times with each method   it was about 20  faster to assign members of the array directly than to use append   Although setting the capacity eliminates reallocations  append still has to do extra work to check if you ve reached capacity on each append

[go] Getting a slice of keys from a map

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