英文:
golang array referencing eg. b[1:4] references elements 1,2,3
问题
Golang博客中提到：
“切片也可以通过“切片”现有的切片或数组来形成。切片是通过用两个索引用冒号分隔的半开范围来完成的。例如，表达式b[1:4]创建一个包含b的第1到第3个元素的切片（结果切片的索引将是0到2）。”
有人可以解释一下上述逻辑吗？也就是说，为什么b[1:4]不引用第1到第4个元素？这与其他数组引用一致吗？
英文:
The golang blog states :
"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)."
Can someone please explain to me the logic in the above. IE. Why doesn't b[1:4] reference elements 1 through 4? Is this consistent with other array referencing?
答案1
得分: 9
索引指向元素的“开始”。所有使用从零开始索引的语言都共享这一点：
| 0 | 第一 | 1 | 第二 | 2 | 第三 | 3 | 第四 | 4 | 第五 | 5 |
[0] = ^
[0:1] = ^ --------> ^
[1:4] = ^-------------------------------------> ^
[0:5] = ^ ----------------------------------------------------------> ^
支持负索引也很常见，尽管Go不允许这样做：
|-6 | |-5 | |-4 | |-3 | |-2 | |-1 |
| 0 | 第一 | 1 | 第二 | 2 | 第三 | 3 | 第四 | 4 | 第五 | 5 |
英文:
Indexes point to the "start" of the element. This is shared by all languages using zero-based indexing:
| 0 | first | 1 | second | 2 | third | 3 | fourth | 4 | fifth | 5 |
[0] = ^
[0:1] = ^ --------> ^
[1:4] = ^-------------------------------------> ^
[0:5] = ^ ----------------------------------------------------------> ^
It's also common to support negative indexing, although Go doesn't allow this:
|-6 | |-5 | |-4 | |-3 | |-2 | |-1 |
| 0 | first | 1 | second | 2 | third | 3 | fourth | 4 | fifth | 5 |
答案2
得分: 4
半开区间在很多情况下都是有意义的，当你深入了解时会发现。例如，对于这样一个半开区间，元素的数量是：
n = end - start
这是一个相当简单和容易记忆的公式。对于一个闭区间，公式将变为：
n = (end - start) + 1
这个公式（虽然不复杂，但仍然）更加复杂。
这也意味着对于例如字符串，整个字符串是[1, len(s)]，这也是直观的。如果区间是闭合的，要获取整个字符串，你需要[1, len(s) + 1]。
英文:
Half-open intervals make sense for many reasons, when you get down to it. For instance, with a half-open interval like this, the number of elements is:
n = end - start
which is a pretty nice and easy formula. For a closed interval, it would be:
n = (end - start) + 1
which is (not a lot, but still) more complicated.
It also means that for e.g. a string, the entire string is [1, len(s)] which also seems intuitive. If the interval was closed, to get the entire string you would need [1, len(s) + 1].
答案3
得分: 4
原因在Go语言规范的Slices部分中给出。
> 对于字符串、数组或切片a，主要表达式
> a[low : high]
> 构造一个子字符串或切片。索引表达式low和high选择出现在结果中的元素。结果的索引从0开始，长度等于high - low。
> 为了方便起见，任何索引表达式都可以省略。缺少的低索引默认为零；缺少的高索引默认为切片操作数的长度。
计算切片的长度为high - low非常简单和高效。
英文:
The reason is given in the Go Language Specification section on Slices.
> For a string, array, or slice a, the
> primary expression
>
> a[low : high]
>
> constructs a substring or slice. The
> index expressions low and high select
> which elements appear in the result.
> The result has indexes starting at 0
> and length equal to high - low.
>
> For convenience, any of the index
> expressions may be omitted. A missing
> low index defaults to zero; a missing
> high index defaults to the length of
> the sliced operand.
It's easy and efficient to calculate the length of the slice as high - low.
答案4
得分: 2
Go使用半开区间（half-open intervals）来表示切片，就像许多其他语言一样。在更数学化的表示法中，切片b[1:4]表示的是区间[1,4)，不包括上限点。
英文:
Go uses half-open intervals for slices like many other languages. In a more mathematical notation, the slice b[1:4] is the interval [1,4) which excludes the upper endpoint.
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