Golang append an item to a slice

Question

Why does the slice a remain the same  Does append   generate a new slice   package main  import        fmt     var a   make   int  7  8   func Test slice   int        slice   append slice  100      fmt Println slice     func main         for i    0  i  lt  7  i             a i    i            Test a      fmt Println a      Output    0 1 2 3 4 5 6 100   0 1 2 3 4 5 6

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Typical append usage is  a   append a  x    because append may either modify its argument in-place or return a copy of its argument with an additional entry  depending on the size and capacity of its input  Using a slice that was previously appended to may give unexpected results  e g   a      int 1 2 3  a   append a  4  fmt Println a  append a  3   5  fmt Println a    may print   1 2 3 4   1 2 3 5

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package main  import        fmt     func a         x      int       x   append x  0      x   append x  1      commonTags    labelsToTags app Labels      y    append x  2     Tags  append commonTags  labelsToTags d Labels          z    append x  3     Tags  append commonTags  labelsToTags d Labels          fmt Println y  z     func b         x      int       x   append x  0      x   append x  1      x   append x  2      commonTags    labelsToTags app Labels      y    append x  3     Tags  append commonTags  labelsToTags d Labels          z    append x  4     Tags  append commonTags  labelsToTags d Labels          fmt Println y  z     func main         a       b      First guess could be   0  1  2   0  1  3   0  1  2  3   0  1  2  4   but in fact it results in   0  1  2   0  1  3   0  1  2  4   0  1  2  4        More details see https   allegro tech 2017 07 golang-slices-gotcha html

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NOTICE that append generates a new slice if cap is not sufficient   kostix s answer is correct  or you can pass slice argument by pointer

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Try this  which I think makes it clear  the underlying array is changed but our slice is not  print just prints len   chars  by another slice to the cap    you can see the changed array   func main        for i    0  i  lt  7  i           a i    i        Test a     fmt Println a     prints  0  6    fmt Println a  cap a      prints  0  6 100

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I think the original answer is not exactly correct  append   changed both the slices and the underlying array even though the underlying array is changed but still shared by both of the slices   As specified by the Go Doc      A slice does not store any data  it just describes a section of an underlying array   Link    Slices are just wrapper values around arrays  meaning that they contain information about how they slice an underlying array which they use to store a set of data  Therefore  by default  a slice  when passed to another method  is actually passed by value  instead of reference pointer even though they will still be using the same underlying array  Normally  arrays are also passed by value too  so I assume a slice points at an underlying array instead of store it as a value  Regarding your question  when you run passed your slice to the following function   func Test slice   int        slice   append slice  100      fmt Println slice      you actually passed a copy of your slice along with a pointer to the same underlying array That means  the changes you did to the slice didn t affect the one in the main function  It is the slice itself which stores the information regarding how much of an array it slices and exposes to the public  Therefore  when you ran append slice  1000   while expanding the underlying array  you also changed slicing information of slice too  which was kept private in your Test   function    However  if you have changed your code as follows  it might have worked   func main         for i    0  i  lt  7  i             a i    i            Test a      fmt Println a  cap a        The reason is that you expanded a by saying a  cap a   over its changed underlying array  changed by Test   function  As specified here      You can extend a slice s length by re-slicing it  provided it has sufficient capacity   Link

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In order to make your code work without having to return the slice from Test  you can pass a pointer like this   package main  import        fmt     var a   make   int  7  8   func Test slice    int         slice   append  slice  100       fmt Println  slice     func main          for i    0  i  lt  7  i             a i    i            Test  amp a       fmt Println a

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Explanation  read inline comments     package main  import        fmt     var a   make   int  7  8     A slice is a descriptor of an array segment      It consists of a pointer to the array  the length of the segment  and its capacity  the maximum length of the segment      The length is the number of elements referred to by the slice     The capacity is the number of elements in the underlying array  beginning at the element referred to by the slice pointer       - gt  Refer to  https   blog golang org go-slices-usage-and-internals - gt   Slice internals  section  func Test slice   int           slice receives a copy of slice  a  which point to the same array as slice  a      slice 6    10     slice   append slice  100         since  slice  capacity is 8  amp  length is 7  it can add 100 and make the length 8     fmt Println slice  len slice   cap slice      lt  lt  Test 1       slice   append slice  200         since  slice  capacity is 8  amp  length also 8  slice has to make a new slice         - with double of size with point to new array  see Reference 1 below           I m also confused  why not  n 1  2 20   But make a new slice of 16 capacity       slice 6    13    make sure  it s a new slice        fmt Println slice  len slice   cap slice      lt  lt  Test 2      func main         for i    0  i  lt  7  i             a i    i            fmt Println a  len a   cap a       Test a      fmt Println a  len a   cap a       fmt Println a  cap a    len a   cap a          fmt Println a  cap a  1   len a   cap a   - gt  this ll not work      Output    0 1 2 3 4 5 6  7 8  0 1 2 3 4 5 10 100  8 8   lt  lt  Test 1  0 1 2 3 4 5 13 100 200  9 16   lt  lt  Test 2  0 1 2 3 4 5 10  7 8  0 1 2 3 4 5 10 100  7 8   Reference 1  https   blog golang org go-slices-usage-and-internals  func AppendByte slice   byte  data    byte    byte       m    len slice      n    m   len data      if n  gt  cap slice       if necessary  reallocate            allocate double what s needed  for future growth          newSlice    make   byte   n 1  2          copy newSlice  slice          slice   newSlice           slice   slice 0 n      copy slice m n   data      return slice

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Go takes a more lean and lazy approach in doing this  It keeps   modifying the same underlying array until the capacity of a slice is   reached    Ref  http   criticalindirection com 2016 02 17 slice-with-a-pinch-of-salt   Output of the example from the link explains the behavior of slices in Go   Creating slice a   Slice a len 7 cap 7  0 0 0 0 0 0 0    Slice b refers to the 2  3  4 indices in slice a  Hence  the capacity is 5    7-2    b    a 2 5  Slice b len 3 cap 5  0 0 0    Modifying slice b  also modifies a  since they are pointing to the same underlying array   b 0    9 Slice a len 7 cap 7  0 0 9 0 0 0 0  Slice b len 3 cap 5  9 0 0    Appending 1 to slice b  Overwrites a   Slice a len 7 cap 7  0 0 9 0 0 1 0  Slice b len 4 cap 5  9 0 0 1    Appending 2 to slice b  Overwrites a   Slice a len 7 cap 7  0 0 9 0 0 1 2  Slice b len 5 cap 5  9 0 0 1 2    Appending 3 to slice b  Here  a new copy is made as the capacity is overloaded   Slice a len 7 cap 7  0 0 9 0 0 1 2  Slice b len 6 cap 12  9 0 0 1 2 3    Verifying slices a and b point to different underlying arrays after the capacity-overload in the previous step   b 1    8 Slice a len 7 cap 7  0 0 9 0 0 1 2  Slice b len 6 cap 12  9 8 0 1 2 3

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Here is a nice implementation of append for slices  I guess its similar to what is going on under the hood   package main  import  fmt   func main         slice1      int 0  1  2  3  4      slice2      int 55  66  77      fmt Println slice1      slice1   Append slice1  slice2        The       is essential      fmt Println slice1        Append     func Append slice   int  items    int    int       for    item    range items           slice   Extend slice  item            return slice       Extend     func Extend slice   int  element int    int       n    len slice      if n    cap slice               Slice is full  must grow             We double its size and add 1  so if the size is zero we still grow          newSlice    make   int  len slice   2 len slice  1          copy newSlice  slice          slice   newSlice           slice   slice 0   n 1      slice n    element     return slice

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In your example the slice argument of the Test function receives a copy of the variable a in the caller s scope  Since a slice variable holds a  quot slice descriptor quot  which merely references an underlying array  in your Test function you modify the slice descriptor held in the slice variable several times in a row  but this does not affect the caller and its a variable  Inside the Test function  the first append reallocates the backing array under the slice variable  copies its original contents over  appends 100 to it  and that s what you re observing   Upon exiting from Test  the slice variable goes out of scope and so does the  new  underlying array that slice references   Jeff Lee is correct about that it s not what really happens  so the updated version follows  as he correctly states  this answer is correct  if maybe a bit too terse   Outside the Test function  a slice of length 7 and capacity 8 is allocated  and its 7 elements filled  Inside the Test function  the first append sees the that the slice s capacity is still one element larger than its length     in other words  there is room for one more element to add without reallocation  So it  quot eats up quot  that remaining element and places 100 to it  after which it adjusts the length in the copy of the slice descriptor to become equal to the slice s capaticy  This does not affect the slice descriptor s in the caller s scope  And that s what you re observing   Upon exiting from Test  the slice variable goes out of scope and so does the  new  underlying array that slice references  If you want to make Test behave like append  you have to return the new slice from it     just like append does     and require the callers of Test to use it in the same way they would use append  func Test slice   int    int       slice   append slice  100       fmt Println slice       return slice    a   Test a   Please read this article thoroughly as it basically shows you how to implement append by hand  after explaining how slices are working internally   Then read this

[go] Golang append an item to a slice

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