Slices are an incredibly important data structure in Go. They form the basis for how we manage and manipulate data in a flexible, performant and dynamic way. It is incredibly important for all Go programmers to learn how to uses slices.

  • Slices are like dynamic arrays with special and built-in functionality.
  • There is a difference between a slices length and capacity and they each service a purpose.
  • Slices allow for multiple “views” of the same underlying array.
  • Slices can grow through the use of the built-in function append.

Declare and length

slice1 := make([]string, 5)
slice1[0] = "Apple"
slice1[1] = "Orange"
slice1[2] = "Banana"
slice1[3] = "Grape"
slice1[4] = "Plum"

Create a slice with a length of 5 elements.

make is a special built-in function that only works with slice, map and channel. make creates a slice that has an array of 5 strings behind it. We are getting back a 3 word data structure:

  • first word points to the backing array
  • second word is length
  • third word is capacity
// -------     -------------------------------
// |  *  | --> | nil | nil | nil | nil | nil |
// -------     |-----|-----|-----|-----|-----|
// |  5  |     |  0  |  0  |  0  |  0  |  0  |
// -------     -------------------------------
// |  5  |
// -------

Length vs Capacity

Length is the number of elements from this pointer position we have access to (read and write).

Capacity is the total number of elements from this pointer position that exist in the backing array.

Syntactic sugar -> looks like array. It also have the same cost that we’ve seen in array. One thing to be mindful about: there is no value in the bracket []string inside the make function. With that in mind, we can constantly notice that we are dealing with a slice, not array.

Reference types

fruits := make([]string, 5, 8)
fruits[0] = "Apple"
fruits[1] = "Orange"
fruits[2] = "Banana"
fruits[3] = "Grape"
fruits[4] = "Plum"

Create a slice with a length of 5 elements and a capacity of 8.

make allows us to adjust the capacity directly on construction of this initialization. What we end up having now is a 3 word data structure where the first word points to an array of 8 elements, length is 5 and capacity is 8.

// -------     -------------------------------------------------
// |  *  | --> | nil | nil | nil | nil | nil | nil | nil | nil |
// -------     |-----|-----|-----|-----|-----|-----|-----|-----|
// |  5  |     |  0  |  0  |  0  |  0  |  0  |  0  |  0  |  0  |
// -------     -------------------------------------------------
// |  8  |
// -------

It means that I can read and write to the first 5 elements and I have 3 elements of capacity that I can leverage later.

Appending Slices

The idea of appending: making slice a dynamic data structure.

// Declare a nil slice of strings, set to its zero value.
var data []string

This is a 3 word data structure: first one points to nil, second and last are zero.

Empty literal construction for a slice

What if I do data := string{}? Is it the same? No, because data in this case is not set to its zero value.

This is why we always use var for zero value because not every type when we create an empty literal we have its zero value in return. What actually happen here is that we have a slice but it has a pointer (as opposed to nil).

This is consider an empty slice, not a nil slice. There is a semantic between a nil slice and an empty slice.

Remember: when it comes to a reference type, any time a reference type is set to its zero value, we consider it to be nil, interfaces, channels, maps, slices, functions.

If we pass a nil slice to a marshal function, we get back a string that said null but when we pass an empty slice, we get an empty JSON document. But where does that pointer point to? It is an empty struct, which we will review later.

var data []string

// Capture the capacity of the slice.
lastCap := cap(data)

// Append ~100k strings to the slice.
for record := 1; record <= 1e5; record++ {

    // Use the built-in function append to add to the slice.
    value := fmt.Sprintf("Rec: %d", record)
    data = append(data, value)

    // When the capacity of the slice changes, display the changes.
    if lastCap != cap(data) {

        // Calculate the percent of change.
        capChg := float64(cap(data)-lastCap) / float64(lastCap) * 100

        // Save the new values for capacity.
        lastCap = cap(data)

        // Display the results.
        fmt.Printf("Addr[%p]\tIndex[%d]\t\tCap[%d - %2.f%%]\n",

append allows us to add value to a slice, making the data structure dynamic, yet still allows us to use that contiguous block of memory that gives us the predictable access pattern from mechanical sympathy. The append call is working with value semantic. Notice that append does mutate. But we’re not using pointers. We are not sharing this slice but appending to it and returning a new copy of it. The slice gets to stay on the stack, not heap.

What is memory leak in Go

A memory leak in Go is when you maintain a reference to a value in the heap, and that reference never goes away. This is complicated because you can’t instrument for memory leak when it’s reference based. Who is to say that reference is or isn’t supposed to be there at any given time. So, if you think you have a memory leak, which is the only way to look at the GC trace. Is the memory going up on every garbage collection? And if it is, we have our memory leak.

Let’s keep going here. Every time append runs, it checks the length and capacity. If it is the same, it means that we have no room. append creates a new backing array, double its size, copy the old value back in and append the new value. It mutates its copy on its stack frame and return us a copy. We replace our slice with the new copy. If it is not the same, it means that we have extra elements of capacity we can use. Now we can bring these extra capacity into the length and no copy is being made. This is very efficient.

// Outputs:
// Addr[0xc000010200]      Index[1]                Cap[1 - +Inf%]
// Addr[0xc00000c080]      Index[2]                Cap[2 - 100%]
// Addr[0xc000064080]      Index[3]                Cap[4 - 100%]
// Addr[0xc00007e000]      Index[5]                Cap[8 - 100%]
// Addr[0xc000080000]      Index[9]                Cap[16 - 100%]
// Addr[0xc00007c200]      Index[17]               Cap[32 - 100%]
// Addr[0xc000082000]      Index[33]               Cap[64 - 100%]
// Addr[0xc000084000]      Index[65]               Cap[128 - 100%]
// Addr[0xc000079000]      Index[129]              Cap[256 - 100%]
// Addr[0xc000086000]      Index[257]              Cap[512 - 100%]
// Addr[0xc00008a000]      Index[513]              Cap[1024 - 100%]
// Addr[0xc000090000]      Index[1025]             Cap[1280 - 25%]
// Addr[0xc00009a000]      Index[1281]             Cap[1704 - 33%]
// Addr[0xc0000b2000]      Index[1705]             Cap[2560 - 50%]
// Addr[0xc0000c0000]      Index[2561]             Cap[3584 - 40%]
// Addr[0xc0000d4000]      Index[3585]             Cap[4608 - 29%]
// Addr[0xc0000ec000]      Index[4609]             Cap[6144 - 33%]
// Addr[0xc00010e000]      Index[6145]             Cap[7680 - 25%]
// Addr[0xc000134000]      Index[7681]             Cap[9728 - 27%]
// Addr[0xc000166000]      Index[9729]             Cap[12288 - 26%]
// Addr[0xc0001a6000]      Index[12289]            Cap[15360 - 25%]
// Addr[0xc0001f4000]      Index[15361]            Cap[19456 - 27%]
// Addr[0xc000258000]      Index[19457]            Cap[24576 - 26%]
// Addr[0xc0002d6000]      Index[24577]            Cap[30720 - 25%]
// Addr[0xc000372000]      Index[30721]            Cap[38400 - 25%]
// Addr[0xc000434000]      Index[38401]            Cap[48128 - 25%]
// Addr[0xc00053c000]      Index[48129]            Cap[60416 - 26%]
// Addr[0xc000628000]      Index[60417]            Cap[75776 - 25%]
// Addr[0xc000750000]      Index[75777]            Cap[94720 - 25%]
// Addr[0xc0008c2000]      Index[94721]            Cap[118784 - 25%]

Looking at the last column in the output, when the backing array is 1000 elements or less, it doubles the size of the backing array for growth. Once we pass 1000 elements, growth rate moves to 25%.

Taking Slices of Slices

slice2 := slice1[2:4] // slicing syntax

Take a slice of slice1. We want just indexes 2 and 3. The length of slice2 is 2 and capacity is 6. Parameters are [starting_index : (starting_index + length)]

// Outputs:
// Length[5] Capacity[8]
// [0] 0xc00007c000 Apple
// [1] 0xc00007c010 Orange
// [2] 0xc00007c020 Banana
// [3] 0xc00007c030 Grape
// [4] 0xc00007c040 Plum
// Length[2] Capacity[6]
// [0] 0xc00007c020 Banana
// [1] 0xc00007c030 Grape

By looking at the output, we can see that they are sharing the same backing array. Thes slice headers get to stay on the stack when we use these value semantics. Only the backing array that needed to be on the heap.

Side effects

// Change the value of the index 0 of slice2.
slice2[0] = "CHANGED"

When we change the value of the index 0 of slice2, who are going to see this change? The answer is both. We have to always to aware that we are modifying an existing slice. We have to be aware who are using it, who is sharing that backing array.

How about slice2 := append(slice2, "CHANGED")? Similar problem will occur with append if the length and capacity is not the same. Instead of changing slice2 at index 0, we call append on slice2. Since the length of slice2 is 2, capacity is 6 at the moment, we have extra rooms for modification. We go and change the element at index 3 of slice2, which is index 4 of slice2. That is very dangerous.

So, what if the length and capacity is the same? Instead of making slice2 capacity 6, we set it to 2 by adding another parameter to the slicing syntax like this: slice2 := slice1[2:4:4]. When append looks at this slice and see that the length and capacity is the same, it wouldn’t bring in the element at index 4 of slice1. It would detach. slice2 will have a length of 2 and capacity of 2, still share the same backing array. On the call to append, length and capacity will be different. The addresses are also different. This is called 3 index slice. This new slice will get its own backing array and we don’t affect anything at all to our original slice.

Copy a slice

// Make a new slice big enough to hold elements of slice 1 and copy the
// values over using the builtin copy function.
slice3 := make([]string, len(slice1))
copy(slice3, slice1)

copy only works with string and slice only.

Slices and References

Sample program to show how one needs to be careful when appending to a slice when you have a reference to an element.

type user struct {
	likes int

// Declare a slice users with 3 values.
users := make([]user, 3)

// Share the user at index 1.
shareUser := &users[1]

// Add a like for the user that was shared.

// Display the number of likes for all users.
for i := range users {
    fmt.Printf("User: %d Likes: %d\n", i, users[i].likes)

// Append a new value to the slice.
// This line of code raises a red flag.
// users is a slice with length 3, capacity 3. Since the length and capacity is
// the same, we're now going to have to create a new backing array.
// Our new backing array will be doubling in size, and our length and capacities
// are going to change. append then copy values over. users nows points to diffrent
// memory block and has a length of 4, capacity of 6.
users = append(users, user{})

// We continue increment likes for the user that was shared.

// Notice the last like has not been recorded.
// When we change the value of the second element of the slice, it is not change
// because it points to the old slice. Everytime we read it, we will get the
// wrong value.

// By displaying the number of likes for all users, we can see that we are in trouble.
for i := range users {
    fmt.Printf("User: %d Likes: %d\n", i, users[i].likes)

In this case, we kind of have a memory leak in a sense that this memory can’t get released because of that pointer. And you might think this would never happen, and we know some of the best Go developers on the planet who have created bugs like this, not thinking how append, when length and capacity are the same, is going to make a copy of the current data, which is the point of truth, and now make this the point of truth, and yet we have these pointers now working against the old data structures.

This is a side effect, these are the nasty bugs that are so hard to find, and so anytime we’re working with pointer semantics, that’s great, it will give us levels of efficiency, right, we have to be careful there, but we also have to make sure that we’re very clean with data updates, like with slices, and that our mutations are not going to cause problems, or the mutations are happening in the wrong place.

Strings in Go

Strings in Go are UTF-8 based. If we use different encoding scheme, we might have a problem.

Sample program.

What’s interesting about UTF-8 is that it’s a three layer character set. You’ve got bytes at the bottom, in the middle you have what are call code points. And a code point is a 32-bit or 4-byte value. And then, after code points, you have characters.

A code point is anywhere from one to four bytes. A character is anywhere from one to multiple code points. You have this, kind of like, n-tiered type of character set.

// Declare a literal string with both Chinese and English characters.
s := "δΈ–η•Œ means world"

This string actually is going to be 18 bytes. Why is that? For each Chinese character, we need 3 byte for each one. Because UTF-8 is built on 3 layers: bytes, code point and character. From Go perspective, string are just bytes. That is what we are storing.

In our example, the first 3 bytes represents a single code point that represents that single character. We can have anywhere from 1 to 4 bytes representing a code point (a code point is a 32 bit value) and anywhere from 1 to multiple code points can actually represent a character. To keep it simple, we only have 3 bytes representing 1 code point representing 1 character. So we can read s as 3 bytes, 3 bytes, 1 byte, 1 byte, … (since there are only 2 Chinese characters in the first place, the rest are English).

// buf is an array, it's not a slice.
// utf8.UTFMax is a constant, which represents the max number of bytes you need for code point.
// UTFMax is 4 -- up to 4 bytes per encoded rune.
// So buf is an array of four bytes.
var buf [utf8.UTFMax]byte

Maximum number of bytes we need to represent any code point is 4.

// Wait, we can range over string?

for i, r := range s {

    // Capture the number of bytes for this rune.
    rl := utf8.RuneLen(r)

    // Calculate the slice offset for the bytes associated with this rune.
    si := i + rl

    // Copy of rune from the string to our buffer.
    copy(buf[:], s[i:si]) // we're slicing the string, s here.

    // Display the details.
    fmt.Printf("%2d: %q; codepoint: %#6x; encoded bytes: %#v\n", i, r, r, buf[:rl])

When we are ranging over a string, are we doing it byte by byte or code point by code point or character by character? The answer is code point by code point. On the first iteration, i is 0. On the next one, i is 3 because we are moving to the next code point. Then i is 6.

r represents the type rune. Rune really isn’t a type in Go. Rune is its own type. It is an alias for int32 type. In fact, similar to type byte we are using, it is just an alias for uint8.

copy is a built-in function and only works with slices and string.

We want to go through every code point and copy them into our array buf, and display them on the screen.

buf[:]: the syntax in Go allows us to apply by slicing syntax to an array.

Every array is just a slice waiting to happen. β€” favorite sayings in Go

That slicing syntax will create a new slice value using the backing array buf as our storage, and setting length and capacity to 4 in our slice header. All of them are on the stack. There is no allocation here.

Range Mechanics

Sample program to show how the for range has both value and pointer semantics.

Using the value semantic form of the for range.

friends := []string{"Annie", "Betty", "Charley", "Doug", "Edward"}
for _, v := range friends {
    friends = friends[:2]
    fmt.Printf("v[%s]\n", v)

// Outputs:
// v[Annie]
// v[Betty]
// v[Charley]
// v[Doug]
// v[Edward]

Using the pointer semantic form of the for range.

friends = []string{"Annie", "Betty", "Charley", "Doug", "Edward"}
for i := range friends {
    friends = friends[:2]
    fmt.Printf("v[%s]\n", friends[i])

// Outputs:
// v[Annie]
// v[Betty]
// panic: runtime error: index out of range [2] with length 2

// goroutine 1 [running]:
// main.main()
//         /home/cedric/m/dev/work/repo/experiments/go/ultimate-go/language/slices/example8/example8.go:24 +0x1fd
// exit status 2