Arrays, slices (and strings): The mechanics of 'append'
26 Sep 2013
Tags: array, slice, string, copy, append

Rob Pike

* Introduction

One of the most common features of procedural programming languages is
the concept of an array.
Arrays seem like simple things but there are many questions that must be
answered when adding them to a language, such as:

- fixed-size or variable-size?
- is the size part of the type?
- what do multidimensional arrays look like?
- does the empty array have meaning?

The answers to these questions affect whether arrays are just
a feature of the language or a core part of its design.

In the early development of Go, it took about a year to decide the answers
to these questions before the design felt right.
The key step was the introduction of _slices_, which built on fixed-size
_arrays_ to give a flexible, extensible data structure.
To this day, however, programmers new to Go often stumble over the way slices
work, perhaps because experience from other languages has colored their thinking.

In this post we'll attempt to clear up the confusion.
We'll do so by building up the pieces to explain how the `append` built-in function
works, and why it works the way it does.

* Arrays

Arrays are an important building block in Go, but like the foundation of a building
they are often hidden below more visible components.
We must talk about them briefly before we move on to the more interesting,
powerful, and prominent idea of slices.

Arrays are not often seen in Go programs because
the size of an array is part of its type, which limits its expressive power.

The declaration

.code slices/prog010.go /var buffer/

declares the variable `buffer`, which holds 256 bytes.
The type of `buffer` includes its size, `[256]byte`.
An array with 512 bytes would be of the distinct type `[512]byte`.

The data associated with an array is just that: an array of elements.
Schematically, our buffer looks like this in memory,

	buffer: byte byte byte ... 256 times ... byte byte byte

That is, the variable holds 256 bytes of data and nothing else. We can
access its elements with the familiar indexing syntax, `buffer[0]`, `buffer[1]`,
and so on through `buffer[255]`. (The index range 0 through 255 covers
256 elements.) Attempting to index `buffer` with a value outside this
range will crash the program.

There is a built-in function called `len` that returns the number of elements
of an array or slice and also of a few other data types.
For arrays, it's obvious what `len` returns.
In our example, `len(buffer)` returns the fixed value 256.

Arrays have their place—they are a good representation of a transformation
matrix for instance—but their most common purpose in Go is to hold storage
for a slice.

* Slices: The slice header

Slices are where the action is, but to use them well one must understand
exactly what they are and what they do.

A slice is a data structure describing a contiguous section of an array
stored separately from the slice variable itself.
_A_slice_is_not_an_array_.
A slice _describes_ a piece of an array.

Given our `buffer` array variable from the previous section, we could create
a slice that describes elements 100 through 150 (to be precise, 100 through 149,
inclusive) by _slicing_ the array:

.code slices/prog010.go /var slice/

In that snippet we used the full variable declaration to be explicit.
The variable `slice` has type `[]byte`, pronounced "slice of bytes",
and is initialized from the array, called
`buffer`, by slicing elements 100 (inclusive) through 150 (exclusive).
The more idiomatic syntax would drop the type, which is set by the initializing expression:

	var slice = buffer[100:150]

Inside a function we could use the short declaration form,

	slice := buffer[100:150]

What exactly is this slice variable?
It's not quite the full story, but for now think of a
slice as a little data structure with two elements: a length and a pointer to an element
of an array.
You can think of it as being built like this behind the scenes:

	type sliceHeader struct {
		Length        int
		ZerothElement *byte
	}

	slice := sliceHeader{
		Length:        50,
		ZerothElement: &buffer[100],
	}

Of course, this is just an illustration.
Despite what this snippet says that `sliceHeader` struct is not visible
to the programmer, and the type
of the element pointer depends on the type of the elements,
but this gives the general idea of the mechanics.

So far we've used a slice operation on an array, but we can also slice a slice, like this:

	slice2 := slice[5:10]

Just as before, this operation creates a new slice, in this case with elements
5 through 9 (inclusive) of the original slice, which means elements
105 through 109 of the original array.
The underlying `sliceHeader` struct for the `slice2` variable looks like
this:

	slice2 := sliceHeader{
		Length:        5,
		ZerothElement: &buffer[105],
	}

Notice that this header still points to the same underlying array, stored in
the `buffer` variable.

We can also _reslice_, which is to say slice a slice and store the result back in
the original slice structure. After

	slice = slice[5:10]

the `sliceHeader` structure for the `slice` variable looks just like it did for the `slice2`
variable.
You'll see reslicing used often, for example to truncate a slice. This statement drops
the first and last elements of our slice:

	slice = slice[1:len(slice)-1]

[Exercise: Write out what the `sliceHeader` struct looks like after this assignment.]

You'll often hear experienced Go programmers talk about the "slice header"
because that really is what's stored in a slice variable.
For instance, when you call a function that takes a slice as an argument, such as
[[https://golang.org/pkg/bytes/#IndexRune][bytes.IndexRune]], that header is
what gets passed to the function.
In this call,

	slashPos := bytes.IndexRune(slice, '/')

the `slice` argument that is passed to the `IndexRune` function is, in fact,
a "slice header".

There's one more data item in the slice header, which we talk about below,
but first let's see what the existence of the slice header means when you
program with slices.

* Passing slices to functions

It's important to understand that even though a slice contains a pointer,
it is itself a value.
Under the covers, it is a struct value holding a pointer and a length.
It is _not_ a pointer to a struct.

This matters.

When we called `IndexRune` in the previous example,
it was passed a _copy_ of the slice header.
That behavior has important ramifications.

Consider this simple function:

.code slices/prog010.go /^func/,/^}/

It does just what its name implies, iterating over the indices of a slice
(using a `for` `range` loop), incrementing its elements.

Try it:

.play -edit slices/prog010.go /^func main/,/^}/

(You can edit and re-execute these runnable snippets if you want to explore.)

Even though the slice _header_ is passed by value, the header includes
a pointer to elements of an array, so both the original slice header
and the copy of the header passed to the function describe the same
array.
Therefore, when the function returns, the modified elements can
be seen through the original slice variable.

The argument to the function really is a copy, as this example shows:

.play -edit slices/prog020.go /^func/,$

Here we see that the _contents_ of a slice argument can be modified by a function,
but its _header_ cannot.
The length stored in the `slice` variable is not modified by the call to the function,
since the function is passed a copy of the slice header, not the original.
Thus if we want to write a function that modifies the header, we must return it as a result
parameter, just as we have done here.
The `slice` variable is unchanged but the returned value has the new length,
which is then stored in `newSlice`,

* Pointers to slices: Method receivers

Another way to have a function modify the slice header is to pass a pointer to it.
Here's a variant of our previous example that does this:

.play -edit slices/prog030.go /^func/,$

It seems clumsy in that example, especially dealing with the extra level of indirection
(a temporary variable helps),
but there is one common case where you see pointers to slices.
It is idiomatic to use a pointer receiver for a method that modifies a slice.

Let's say we wanted to have a method on a slice that truncates it at the final slash.
We could write it like this:

.play -edit slices/prog040.go /^type/,$

If you run this example you'll see that it works properly, updating the slice in the caller.

[Exercise: Change the type of the receiver to be a value rather
than a pointer and run it again. Explain what happens.]

On the other hand, if we wanted to write a method for `path` that upper-cases
the ASCII letters in the path (parochially ignoring non-English names), the method could
be a value because the value receiver will still point to the same underlying array.

.play -edit slices/prog050.go /^type/,$

Here the `ToUpper` method uses two variables in the `for` `range` construct
to capture the index and slice element.
This form of loop avoids writing `p[i]` multiple times in the body.

[Exercise: Convert the `ToUpper` method to use a pointer receiver and see if its behavior changes.]

[Advanced exercise: Convert the `ToUpper` method to handle Unicode letters, not just ASCII.]

* Capacity

Look at the following function that extends its argument slice of `ints` by one element:

.code slices/prog060.go /^func Extend/,/^}/

(Why does it need to return the modified slice?) Now run it:

.play -edit slices/prog060.go /^func main/,/^}/

See how the slice grows until... it doesn't.

It's time to talk about the third component of the slice header: its _capacity_.
Besides the array pointer and length, the slice header also stores its capacity:

	type sliceHeader struct {
		Length        int
		Capacity      int
		ZerothElement *byte
	}

The `Capacity` field records how much space the underlying array actually has; it is the maximum
value the `Length` can reach.
Trying to grow the slice beyond its capacity will step beyond the limits of the array and will trigger a panic.

After our example slice is created by

	slice := iBuffer[0:0]

its header looks like this:

	slice := sliceHeader{
		Length:        0,
		Capacity:      10,
		ZerothElement: &iBuffer[0],
	}

The `Capacity` field is equal to the length of the underlying array,
minus the index in the array of the first element of the slice (zero in this case).
If you want to inquire what the capacity is for a slice, use the built-in function `cap`:

	if cap(slice) == len(slice) {
		fmt.Println("slice is full!")
	}

* Make

What if we want to grow the slice beyond its capacity?
You can't!
By definition, the capacity is the limit to growth.
But you can achieve an equivalent result by allocating a new array, copying the data over, and modifying
the slice to describe the new array.

Let's start with allocation.
We could use the `new` built-in function to allocate a bigger array
and then slice the result,
but it is simpler to use the `make` built-in function instead.
It allocates a new array and
creates a slice header to describe it, all at once.
The `make` function takes three arguments: the type of the slice, its initial length, and its capacity, which is the
length of the array that `make` allocates to hold the slice data.
This call creates a slice of length 10 with room for 5 more (15-10), as you can see by running it:

.play -edit slices/prog070.go /slice/,/fmt/

This snippet doubles the capacity of our `int` slice but keeps its length the same:

.play -edit slices/prog080.go /slice/,/OMIT/

After running this code the slice has much more room to grow before needing another reallocation.

When creating slices, it's often true that the length and capacity will be same.
The `make` built-in has a shorthand for this common case.
The length argument defaults to the capacity, so you can leave it out
to set them both to the same value.
After

	gophers := make([]Gopher, 10)

the `gophers` slice has both its length and capacity set to 10.

* Copy

When we doubled the capacity of our slice in the previous section,
we wrote a loop to copy the old data to the new slice.
Go has a built-in function, `copy`, to make this easier.
Its arguments are two slices, and it copies the data from the right-hand argument to the left-hand argument.
Here's our example rewritten to use `copy`:

.play -edit slices/prog090.go /newSlice/,/newSlice/

The `copy` function is smart.
It only copies what it can, paying attention to the lengths of both arguments.
In other words, the number of elements it copies is the minimum of the lengths of the two slices.
This can save a little bookkeeping.
Also, `copy` returns an integer value, the number of elements it copied, although it's not always worth checking.

The `copy` function also gets things right when source and destination overlap, which means it can be used to shift
items around in a single slice.
Here's how to use `copy` to insert a value into the middle of a slice.

.code slices/prog100.go /Insert/,/^}/

There are a couple of things to notice in this function.
First, of course, it must return the updated slice because its length has changed.
Second, it uses a convenient shorthand.
The expression

	slice[i:]

means exactly the same as

	slice[i:len(slice)]

Also, although we haven't used the trick yet, we can leave out the first element of a slice expression too;
it defaults to zero. Thus

	slice[:]

just means the slice itself, which is useful when slicing an array.
This expression is the shortest way to say "a slice describing all the elements of the array":

	array[:]

Now that's out of the way, let's run our `Insert` function.

.play -edit slices/prog100.go /make/,/OMIT/

* Append: An example

A few sections back, we wrote an `Extend` function that extends a slice by one element.
It was buggy, though, because if the slice's capacity was too small, the function would
crash.
(Our `Insert` example has the same problem.)
Now we have the pieces in place to fix that, so let's write a robust implementation of
`Extend` for integer slices.

.code slices/prog110.go /func Extend/,/^}/

In this case it's especially important to return the slice, since when it reallocates
the resulting slice describes a completely different array.
Here's a little snippet to demonstrate what happens as the slice fills up:

.play -edit slices/prog110.go /START/,/END/

Notice the reallocation when the initial array of size 5 is filled up.
Both the capacity and the address of the zeroth element change when the new array is allocated.

With the robust `Extend` function as a guide we can write an even nicer function that lets
us extend the slice by multiple elements.
To do this, we use Go's ability to turn a list of function arguments into a slice when the
function is called.
That is, we use Go's variadic function facility.

Let's call the function `Append`.
For the first version, we can just call `Extend` repeatedly so the mechanism of the variadic function is clear.
The signature of `Append` is this:

	func Append(slice []int, items ...int) []int

What that says is that `Append` takes one argument, a slice, followed by zero or more
`int` arguments.
Those arguments are exactly a slice of `int` as far as the implementation
of `Append` is concerned, as you can see:

.code slices/prog120.go /Append/,/^}/

Notice the `for` `range` loop iterating over the elements of the `items` argument, which has implied type `[]int`.
Also notice the use of the blank identifier `_` to discard the index in the loop, which we don't need in this case.

Try it:

.play -edit slices/prog120.go /START/,/END/

Another new technique in this example is that we initialize the slice by writing a composite literal,
which consists of the type of the slice followed by its elements in braces:

.code slices/prog120.go /slice := /

The `Append` function is interesting for another reason.
Not only can we append elements, we can append a whole second slice
by "exploding" the slice into arguments using the `...` notation at the call site:

.play -edit slices/prog130.go /START/,/END/

Of course, we can make `Append` more efficient by allocating no more than once,
building on the innards of `Extend`:

.code slices/prog140.go /Append/,/^}/

Here, notice how we use `copy` twice, once to move the slice data to the newly
allocated memory, and then to copy the appending items to the end of the old data.

Try it; the behavior is the same as before:

.play -edit slices/prog140.go /START/,/END/

* Append: The built-in function

And so we arrive at the motivation for the design of the `append` built-in function.
It does exactly what our `Append` example does, with equivalent efficiency, but it
works for any slice type.

A weakness of Go is that any generic-type operations must be provided by the
run-time. Some day that may change, but for now, to make working with slices
easier, Go provides a built-in generic `append` function.
It works the same as our `int` slice version, but for _any_ slice type.

Remember, since the slice header is always updated by a call to `append`, you need
to save the returned slice after the call.
In fact, the compiler won't let you call append without saving the result.

Here are some one-liners intermingled with print statements. Try them, edit them and explore:

.play -edit slices/prog150.go /START/,/END/

It's worth taking a moment to think about the final one-liner of that example in detail to understand
how the design of slices makes it possible for this simple call to work correctly.

There are lots more examples of `append`, `copy`, and other ways to use slices
on the community-built
[[https://golang.org/wiki/SliceTricks]["Slice Tricks" Wiki page]].

* Nil

As an aside, with our newfound knowledge we can see what the representation of a `nil` slice is.
Naturally, it is the zero value of the slice header:

	sliceHeader{
		Length:        0,
		Capacity:      0,
		ZerothElement: nil,
	}

or just

	sliceHeader{}

The key detail is that the element pointer is `nil` too. The slice created by

	array[0:0]

has length zero (and maybe even capacity zero) but its pointer is not `nil`, so
it is not a nil slice.

As should be clear, an empty slice can grow (assuming it has non-zero capacity), but a `nil`
slice has no array to put values in and can never grow to hold even one element.

That said, a `nil` slice is functionally equivalent to a zero-length slice, even though it points
to nothing.
It has length zero and can be appended to, with allocation.
As an example, look at the one-liner above that copies a slice by appending
to a `nil` slice.

* Strings

Now a brief section about strings in Go in the context of slices.

Strings are actually very simple: they are just read-only slices of bytes with a bit
of extra syntactic support from the language.

Because they are read-only, there is no need for a capacity (you can't grow them),
but otherwise for most purposes you can treat them just like read-only slices
of bytes.

For starters, we can index them to access individual bytes:

	slash := "/usr/ken"[0] // yields the byte value '/'.

We can slice a string to grab a substring:

	usr := "/usr/ken"[0:4] // yields the string "/usr"

It should be obvious now what's going on behind the scenes when we slice a string.

We can also take a normal slice of bytes and create a string from it with the simple conversion:

	str := string(slice)

and go in the reverse direction as well:

	slice := []byte(usr)

The array underlying a string is hidden from view; there is no way to access its contents
except through the string. That means that when we do either of these conversions, a
copy of the array must be made.
Go takes care of this, of course, so you don't have to.
After either of these conversions, modifications to
the array underlying the byte slice don't affect the corresponding string.

An important consequence of this slice-like design for strings is that
creating a substring is very efficient.
All that needs to happen
is the creation of a two-word string header. Since the string is read-only, the original
string and the string resulting from the slice operation can share the same array safely.

A historical note: The earliest implementation of strings always allocated, but when slices
were added to the language, they provided a model for efficient string handling. Some of
the benchmarks saw huge speedups as a result.

There's much more to strings, of course, and a
[[https://blog.golang.org/strings][separate blog post]] covers them in greater depth.

* Conclusion

To understand how slices work, it helps to understand how they are implemented.
There is a little data structure, the slice header, that is the item associated with the slice
variable, and that header describes a section of a separately allocated array.
When we pass slice values around, the header gets copied but the array it points
to is always shared.

Once you appreciate how they work, slices become not only easy to use, but
powerful and expressive, especially with the help of the `copy` and `append`
built-in functions.

* More reading

There's lots to find around the intertubes about slices in Go.
As mentioned earlier,
the [[https://golang.org/wiki/SliceTricks]["Slice Tricks" Wiki page]]
has many examples.
The [[https://blog.golang.org/go-slices-usage-and-internals][Go Slices]] blog post
describes the memory layout details with clear diagrams.
Russ Cox's [[https://research.swtch.com/godata][Go Data Structures]] article includes
a discussion of slices along with some of Go's other internal data structures.

There is much more material available, but the best way to learn about slices is to use them.