Why can t I duplicate a slice with copy

Question

I need to make a copy of a slice in Go and reading the docs there is a copy function at my disposal       The copy built-in function copies elements from a source slice into a   destination slice   As a special case  it also will copy bytes from a   string to a slice of bytes   The source and destination may overlap    Copy returns the number of elements copied  which will be the minimum   of len src  and len dst     But when I do   arr      int 1  2  3  tmp      int   copy tmp  arr  fmt Println tmp  fmt Println arr    My tmp is empty as it was before  I even tried to use arr  tmp        1 2 3    You can check it on go playground  So why can not I copy a slice

User · Answer

Another simple way to do this is by using append which will allocate the slice in the process.

arr := []int{1, 2, 3}
tmp := append([]int(nil), arr...)  // Notice the ... splat
fmt.Println(tmp)
fmt.Println(arr)

Output (as expected):

[1 2 3]
[1 2 3]

So a shorthand for copying array arr would be append([]int(nil), arr...)

https://play.golang.org/p/sr_4ofs5GW

User · Answer

The copy   runs for the least length of dst and src  so you must initialize the dst to the desired length   A      int 1  2  3  B    make   int  3  copy B  A  C    make   int  2  copy C  A  fmt Println A  B  C    Output    1 2 3   1 2 3   1 2    You can initialize and copy all elements in one line using append   to a nil slice   x    append   T             Example   A      int 1  2  3  B    append   int    A     C    append   int    A  2      fmt Println A  B  C        Output    1 2 3   1 2 3   1 2    Comparing with allocation copy    for greater than 1 000 elements  use append  Actually bellow 1 000 the difference may be neglected  make it a go for rule of thumb unless you have many slices   BenchmarkCopy1-4                50000000            27 0 ns op BenchmarkCopy10-4               30000000            53 3 ns op BenchmarkCopy100-4              10000000           229 ns op BenchmarkCopy1000-4              1000000          1942 ns op BenchmarkCopy10000-4              100000         18009 ns op BenchmarkCopy100000-4              10000        220113 ns op BenchmarkCopy1000000-4              1000       2028157 ns op BenchmarkCopy10000000-4              100      15323924 ns op BenchmarkCopy100000000-4               1    1200488116 ns op BenchmarkAppend1-4              50000000            34 2 ns op BenchmarkAppend10-4             20000000            60 0 ns op BenchmarkAppend100-4             5000000           240 ns op BenchmarkAppend1000-4            1000000          1832 ns op BenchmarkAppend10000-4            100000         13378 ns op BenchmarkAppend100000-4            10000        142397 ns op BenchmarkAppend1000000-4            2000       1053891 ns op BenchmarkAppend10000000-4            200       9500541 ns op BenchmarkAppend100000000-4            20     176361861 ns op

User · Answer

If your slices were of the same size  it would work   arr      int 1  2  3  tmp      int 0  0  0  i    copy tmp  arr  fmt Println i  fmt Println tmp  fmt Println arr    Would give   3  1 2 3   1 2 3    From  Go Slices  usage and internals       The copy function supports copying between slices of different lengths  it will copy only up to the smaller number of elements    The usual example is   t    make   byte  len s    cap s  1  2  copy t  s  s   t

User · Answer

The builtin copy dst  src  copies min len dst   len src   elements   So if your dst is empty  len dst     0   nothing will be copied   Try tmp    make   int  len arr    Go Playground    arr      int 1  2  3  tmp    make   int  len arr   copy tmp  arr  fmt Println tmp  fmt Println arr    Output  as expected     1 2 3   1 2 3    Unfortunately this is not documented in the builtin package  but it is documented in the Go Language Specification  Appending to and copying slices      The number of elements copied is the minimum of len src  and len dst     Edit   Finally the documentation of copy   has been updated and it now contains the fact that the minimum length of source and destination will be copied      Copy returns the number of elements copied  which will be the minimum of len src  and len dst

User · Answer

The Go Programming Language Specification      Appending to and copying slices      The function copy copies slice elements from a source src to a   destination dst and returns the number of elements copied  Both   arguments must have identical element type T and must be assignable to   a slice of type   T  The number of elements copied is the minimum of   len src  and len dst   As a special case  copy also accepts a   destination argument assignable to type   byte with a source argument   of a string type  This form copies the bytes from the string into the   byte slice   copy dst  src   T  int copy dst   byte  src string  int    tmp needs enough room for arr  For example   package main  import  fmt   func main         arr      int 1  2  3      tmp    make   int  len arr       copy tmp  arr      fmt Println tmp      fmt Println arr      Output    1 2 3   1 2 3

User · Answer

NOTE  This is an incorrect solution as  benlemasurier proved  Here is a way to copy a slice  I m a bit late  but there is a simpler  and faster answer than  Dave s  This are the instructions generated from a code like  Dave s  These is the instructions generated by mine  As you can see there are far fewer instructions  What is does is it just does append slice   which copies the slice  This code  package main  import  quot fmt quot   func main         var foo     int 1  2  3  4  5      fmt Println  quot foo  quot   foo      var bar   append foo      fmt Println  quot bar  quot   bar      bar   append bar  6      fmt Println  quot foo after  quot   foo      fmt Println  quot bar after  quot   bar     Outputs this  foo   1 2 3 4 5  bar   1 2 3 4 5  foo after   1 2 3 4 5  bar after   1 2 3 4 5 6

[go] Why can't I duplicate a slice with `copy()`?

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