# 1.4 符号常量

It's bad practice to bury "magic numbers" like 300 and 20 in a program; they convey little information to someone who might have to read the program later, and they are hard to change in a systematic way.

<figure><img src="https://labspc.com/wp-content/uploads/2024/01/1705654402-word-image-258-1.png" alt=""><figcaption></figcaption></figure>

Symbolic constant names are convention-ally written in upper case so they can be readily distinguished from lower case variable names. Notice that there is no semicolon at the end of a # d e fi n e line.

符号常量名 通常用大写字母拼写。

这里 magic number 的叫法，后续还会出现。

{% code title="1.4.1 define-pi.c" lineNumbers="true" %}

```c
#include <stdio.h>

#define PI 3.14159

int main()
{
    double radius = 5.0;
    double area = PI * radius * radius;

    printf("The area of the circle is: %f\n", area);

    return 0;
}
```

{% endcode %}

在这个例子中，我们使用 #define 定义了一个叫做PI的常量，并将其设置为 3.14159。然后在主函数中，我们使用这个常量来计算圆的面积。当编译器遇到PI时，它会自动将其替换为3.14159。

\#define 可以用于定义任何类型的常量，无论是整数、浮点数、字符、字符串，甚至是函数宏。它可以在代码中增加可读性，并让你更方便地管理常用的数值或表达式。

请注意，#define 只是进行简单的文本替换，它在编译前处理阶段就会生效，不会进行类型检查。在使用 #define 定义常量时，应该遵循命名规范，以便代码更易读懂。


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