# 2.8 自增运算符与自减运算符

**Increment and Decrement Operators（递增和递减操作符）**:

* `++` : Increment（递增）
* `--` : Decrement（递减）

**前缀（prefix）**使用与**后缀（postfix）**使用，有一些差别。

<pre class="language-c"><code class="lang-c"><strong>++i;
</strong><strong>i++;
</strong></code></pre>

* 前缀，立即自增，其**副作用**是自增 i
* 后缀，先用 i 的原始值，稍后再自增 i

如下是在字符串 s 中删除字符 c

{% code title="squeeze-function.c" lineNumbers="true" %}

```c
void squeeze(char s[], int c)
{
    int i,j;
    for (i=j=0; s[i] != '\0'; i++) 
    {
        if (s[j] != c) 
        {
            s[j++] = s[i];
        }
        s[j] = '\0\;
    }
}
```

{% endcode %}

先拷贝再递增，如下两个写法等价。

```c
if (s[j] != c) 
{
    s[j++] = s[i];
}
```

```c
if (s[j] != c) 
{
    s[j] = s[i];
    j++;
}
```

这里要提到几个术语：

* 由实现定义的行为 implementation-defined，c标准故意对c语言的部分内容未加指定，并认为其细节可以由“实现”来具体定义。
* 未定义行为 undefined behavior，其后果是不可预料的，要尽力避免。
* 副作用 side effect，大多数c语言运算符不会改变操作数的值，但是也有一些会改变。


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