Go Slices: usage and internals

Introduction

Go's slice type provides a convenient and efficient means of working with sequences of typed data. Slices are analogous to arrays in other languages, but have some unusual properties. This article will look at what slices are and how they are used.

Arrays

The slice type is an abstraction built on top of Go's array type, and so to understand slices we must first understand arrays.

An array type definition specifies a length and an element type. For example, the type [4]int represents an array of four integers. An array's size is fixed; its length is part of its type ([4]int and [5]int are distinct, incompatible types). Arrays can be indexed in the usual way, so the expression s[n] accesses the nth element, starting from zero.

var a [4]int
a[0] = 1
i := a[0]
// i == 1

Arrays do not need to be initialized explicitly; the zero value of an array is a ready-to-use array whose elements are themselves zeroed:

// a[2] == 0, the zero value of the int type

The in-memory representation of [4]int is just four integer values laid out sequentially:

Go's arrays are values. An array variable denotes the entire array; it is not a pointer to the first array element (as would be the case in C). This means that when you assign or pass around an array value you will make a copy of its contents. (To avoid the copy you could pass a pointer to the array, but then that's a pointer to an array, not an array.) One way to think about arrays is as a sort of struct but with indexed rather than named fields: a fixed-size composite value.

An array literal can be specified like so:

b := [2]string{"Penn", "Teller"}

Or, you can have the compiler count the array elements for you:

b := [...]string{"Penn", "Teller"}

In both cases, the type of b is [2]string.

Slices

Arrays have their place, but they're a bit inflexible, so you don't see them too often in Go code. Slices, though, are everywhere. They build on arrays to provide great power and convenience.

The type specification for a slice is []T, where T is the type of the elements of the slice. Unlike an array type, a slice type has no specified length.

A slice literal is declared just like an array literal, except you leave out the element count:

letters := []string{"a", "b", "c", "d"}

A slice can be created with the built-in function called make, which has the signature,

func make([]T, len, cap) []T

where T stands for the element type of the slice to be created. The make function takes a type, a length, and an optional capacity. When called, make allocates an array and returns a slice that refers to that array.

var s []byte
s = make([]byte, 5, 5)
// s == []byte{0, 0, 0, 0, 0}

When the capacity argument is omitted, it defaults to the specified length. Here's a more succinct version of the same code:

s := make([]byte, 5)

The length and capacity of a slice can be inspected using the built-in len and cap functions.

len(s) == 5
cap(s) == 5

The next two sections discuss the relationship between length and capacity.

The zero value of a slice is nil. The len and cap functions will both return 0 for a nil slice.

A slice can also be formed by "slicing" an existing slice or array. Slicing is done by specifying a half-open range with two indices separated by a colon. For example, the expression b[1:4] creates a slice including elements 1 through 3 of b (the indices of the resulting slice will be 0 through 2).

b := []byte{'g', 'o', 'l', 'a', 'n', 'g'}
// b[1:4] == []byte{'o', 'l', 'a'}, sharing the same storage as b

The start and end indices of a slice expression are optional; they default to zero and the slice's length respectively:

// b[:2] == []byte{'g', 'o'}
// b[2:] == []byte{'l', 'a', 'n', 'g'}
// b[:] == b

This is also the syntax to create a slice given an array:

x := [3]string{"Лайка", "Белка", "Стрелка"}
s := x[:] // a slice referencing the storage of x

Slice internals

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).

Our variable s, created earlier by make([]byte, 5), is structured like this:

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). The distinction between length and capacity will be made clear as we walk through the next few examples.

As we slice s, observe the changes in the slice data structure and their relation to the underlying array:

s = s[2:4]

Slicing does not copy the slice's data. It creates a new slice value that points to the original array. This makes slice operations as efficient as manipulating array indices. Therefore, modifying the elements (not the slice itself) of a re-slice modifies the elements of the original slice:

d := []byte{'r', 'o', 'a', 'd'}
e := d[2:]
// e == []byte{'a', 'd'}
e[1] = 'm'
// e == []byte{'a', 'm'}
// d == []byte{'r', 'o', 'a', 'm'}

Earlier we sliced s to a length shorter than its capacity. We can grow s to its capacity by slicing it again:

s = s[:cap(s)]

A slice cannot be grown beyond its capacity. Attempting to do so will cause a runtime panic, just as when indexing outside the bounds of a slice or array. Similarly, slices cannot be re-sliced below zero to access earlier elements in the array.

Growing slices (the copy and append functions)

To increase the capacity of a slice one must create a new, larger slice and copy the contents of the original slice into it. This technique is how dynamic array implementations from other languages work behind the scenes. The next example doubles the capacity of s by making a new slice, t, copying the contents of s into t, and then assigning the slice value t to s:

t := make([]byte, len(s), (cap(s)+1)*2) // +1 in case cap(s) == 0
for i := range s {
t[i] = s[i]
}
s = t

The looping piece of this common operation is made easier by the built-in copy function. As the name suggests, copy copies data from a source slice to a destination slice. It returns the number of elements copied.

func copy(dst, src []T) int

The copy function supports copying between slices of different lengths (it will copy only up to the smaller number of elements). In addition, copy can handle source and destination slices that share the same underlying array, handling overlapping slices correctly.

Using copy, we can simplify the code snippet above:

t := make([]byte, len(s), (cap(s)+1)*2)
copy(t, s)
s = t

A common operation is to append data to the end of a slice. This function appends byte elements to a slice of bytes, growing the slice if necessary, and returns the updated slice value:

func AppendByte(slice []byte, data ...byte) []byte {
m := len(slice)
n := m + len(data)
if n > 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
}

One could use AppendByte like this:

p := []byte{2, 3, 5}
p = AppendByte(p, 7, 11, 13)
// p == []byte{2, 3, 5, 7, 11, 13}

Functions like AppendByte are useful because they offer complete control over the way the slice is grown. Depending on the characteristics of the program, it may be desirable to allocate in smaller or larger chunks, or to put a ceiling on the size of a reallocation.

But most programs don't need complete control, so Go provides a built-in append function that's good for most purposes; it has the signature

func append(s []T, x ...T) []T

The append function appends the elements x to the end of the slice s, and grows the slice if a greater capacity is needed.

a := make([]int, 1)
// a == []int{0}
a = append(a, 1, 2, 3)
// a == []int{0, 1, 2, 3}

To append one slice to another, use ... to expand the second argument to a list of arguments.

a := []string{"John", "Paul"}
b := []string{"George", "Ringo", "Pete"}
a = append(a, b...) // equivalent to "append(a, b[0], b[1], b[2])"
// a == []string{"John", "Paul", "George", "Ringo", "Pete"}

Since the zero value of a slice (nil) acts like a zero-length slice, you can declare a slice variable and then append to it in a loop:

// Filter returns a new slice holding only
// the elements of s that satisfy fn()
func Filter(s []int, fn func(int) bool) []int {
var p []int // == nil
for _, v := range s {
if fn(v) {
p = append(p, v)
}
}
return p
}

A possible "gotcha"

As mentioned earlier, re-slicing a slice doesn't make a copy of the underlying array. The full array will be kept in memory until it is no longer referenced. Occasionally this can cause the program to hold all the data in memory when only a small piece of it is needed.

For example, this FindDigits function loads a file into memory and searches it for the first group of consecutive numeric digits, returning them as a new slice.

var digitRegexp = regexp.MustCompile("[0-9]+")

func FindDigits(filename string) []byte {
b, _ := ioutil.ReadFile(filename)
return digitRegexp.Find(b)
}

This code behaves as advertised, but the returned []byte points into an array containing the entire file. Since the slice references the original array, as long as the slice is kept around the garbage collector can't release the array; the few useful bytes of the file keep the entire contents in memory.

To fix this problem one can copy the interesting data to a new slice before returning it:

func CopyDigits(filename string) []byte {
b, _ := ioutil.ReadFile(filename)
b = digitRegexp.Find(b)
c := make([]byte, len(b))
copy(c, b)
return c
}

A more concise version of this function could be constructed by using append. This is left as an exercise for the reader.

Further Reading

Effective Go contains an in-depth treatment of slices and arrays, and the Go language specification defines slices and their associated helper functions.

golang ----array and slice的更多相关文章

  1. Golang高效实践之array、slice、map

    前言 Golang的slice类型为连续同类型数据提供了一个方便并且高效的实现方式.slice的实现是基于array,slice和map一样是类似于指针语义,传递slice和map并不涉及底层数据结构 ...

  2. 【javascript 技巧】Array.prototype.slice的妙用

    Array.prototype.slice的妙用 开门见山,关于Array 的slice的用法可以参考这里 http://www.w3school.com.cn/js/jsref_slice_arra ...

  3. Array.prototype.slice.call(arguments)

    Array.prototype.slice.call(arguments)能够将具有length属性的对象转化为数组, 可以理解为将arguments转化成一个数组对象,让它具有slice方法 如: ...

  4. IE下Array.prototype.slice.call(params,0)

    i8 不支持 Array.prototype.slice.call(params,0) params可以是 HTMLCollection.类数组.string字符串

  5. (转)Array.prototype.slice.call自解

    很多框架或者库里面都会有这句的使用,最多的还是通过Array.prototype.slice.call(arguments,0)把arguments这个伪数组转换为真正的数组.但为什么可以这么做,却一 ...

  6. 详解 Array.prototype.slice.call(arguments)

    首先,slice有两个用法,一个是String.slice,一个是Array.slice,第一个返回的是字符串,第二个返回的是数组 在这里我们看第二个方法 1.在JS里Array是一个类 slice是 ...

  7. Array.prototype.slice && Array.prototype.splice 用法阐述

    目的 对于这两个数组操作接口,由于不理解, 往往被误用, 或者不知道如何使用.本文尝试给出容易理解的阐述. 数组 什么是数组? 数组是一个基本的数据结构, 是一个在内存中依照线性方式组织元素的方式, ...

  8. Array.prototype.slice.call(document.querySelectorAll('a'), 0)

    Array.prototype.slice.call(document.querySelectorAll('a'), 0)的作用就是将一个DOM NodeList 转换成一个数组. slice()方法 ...

  9. Array.prototype.slice.call

    Array.prototype.slice.call(arguments)能将具有length属性的对象转成数组 ,::'age'}; Array.prototype.slice.call(arr); ...

随机推荐

  1. ptrace函数深入分析

    ptrace函数:进程跟踪. 形式:#include<sys/ptrace.h> Int ptrace(int request,int pid,int addr,int data); 概述 ...

  2. flink 实现三角枚举EnumTriangles算法详解

    1.三角枚举,从所有无向边对中找到相互连接的三角形 /** * @Author: xu.dm * @Date: 2019/7/4 21:31 * @Description: 三角枚举算法 * 三角枚举 ...

  3. DjangoForm 提交验证

    用户提交数据的验证 1.创建模版 -- class LoginForm(forms.Form):.... 2.将请求交给模版,创建一个对象 -- obj = LoginForm(request.POS ...

  4. SpringBoot使用Swagger2构建API文档

    后端开发中经常需要对移动客户端提供RESTful API接口,在后期版本快速迭代的过程中,修改接口实现的时候都必须同步修改接口文档,而文档与代码又处于两个不同的媒介,除非有严格的管理机制,不然很容易导 ...

  5. 6. [mmc subsystem] mmc core(第六章)——mmc core主模块

    一.说明 1.mmc core概述 mmc core主模块是mmc core的实现核心.也是本章的重点内容. 对应代码位置drivers/mmc/core/core.c. 其主要负责如下功能: mmc ...

  6. odoo10学习笔记五:高级视图

    转载请注明原文地址:https://www.cnblogs.com/ygj0930/p/11189279.html 树视图 tree视图表现出来是列表视图,列表中一行一纪录.可以根据每行纪录的某字段值 ...

  7. Linux下CPU利用率和负载的关系

    1.CPU利用率和负载 CPU利用率显示的是程序在运行期间实时占用的CPU百分比:cpu使用率反映的是当前cpu的繁忙程度,忽高忽低的原因在于占用cpu处理时间的进程可能处于io等待状态但却还未释放进 ...

  8. AssetBundleMaster_Introduce_EN

    This is an integrated solution for building AssetBundles and loading Assets. what it can do is about ...

  9. conan使用(二)--创建私有仓库

    前面我们已经能够使用conan来从公共服务器上拉取C/C++包来集成进我的工程中,但是在实际开发中,我们可能需要自己封装或使用非公开的库,那么自己搭建一个私服是个很现实的需求. 搭建conan私服有几 ...

  10. Go语言goroutine调度器初始化(12)

    本文是<Go语言调度器源代码情景分析>系列的第12篇,也是第二章的第2小节. 本章将以下面这个简单的Hello World程序为例,通过跟踪其从启动到退出这一完整的运行流程来分析Go语言调 ...