> For the complete documentation index, see [llms.txt](https://eastlakeside.gitbook.io/interpy-zh/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://eastlakeside.gitbook.io/interpy-zh/for_else/else_clause.md).

# else语句

`for` 循环还有一个 `else` 从句，我们大多数人并不熟悉。这个 `else` 从句会在循环正常结束时执行。这意味着，循环没有遇到任何 `break`。一旦你掌握了何时何地使用它，它真的会非常有用。我自己对它真是相见恨晚。

有个常见的构造是跑一个循环，并查找一个元素。如果这个元素被找到了，我们使用 `break` 来中断这个循环。有两个场景会让循环停下来。

* 第一个是当一个元素被找到，`break` 被触发。
* 第二个场景是循环结束。  

现在我们也许想知道其中哪一个，才是导致循环完成的原因。一个方法是先设置一个标记，然后在循环结束时打上标记。另一个是使用 `else` 从句。

这就是 `for/else` 循环的基本结构：

```python
for item in container:
    if search_something(item):
        # Found it!
        process(item)
        break
else:
    # Didn't find anything..
    not_found_in_container()
```

考虑下这个简单的案例，它是我从官方文档里拿来的：

```python
for n in range(2, 10):
    for x in range(2, n):
        if n % x == 0:
            print(n, 'equals', x, '*', n / x)
            break
```

它会找出2到10之间的数字的因子。现在是趣味环节了。我们可以加上一个附加的else语句块，来抓住质数，并且告诉我们：

```python
for n in range(2, 10):
    for x in range(2, n):
        if n % x == 0:
            print(n, 'equals', x, '*', n / x)
            break
    else:
        # loop fell through without finding a factor
        print(n, 'is a prime number')
```


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