C/C++常量相乘初始化全局变量报Initializer element is not a compile-time constant如何解决

C 语言标准明确要求：全局变量的初始化值必须是编译期可确定的常量表达式，不允许使用运行时才能确定值的变量参与计算。
你代码中P_year、s_year都是普通的全局double变量，即便你给它们赋值了字面量，它们本质上还是变量，不属于编译期常量，因此用二者计算P_s的初始值不符合语法要求，触发报错：

Capacitor.c:11:20: error: initializer element is not a compile-time constant
double P_s = P_year*(1/s_year);

• 方案1：将P_s改为局部变量
  局部变量的初始化没有编译期常量的限制，直接把P_s的定义移动到main函数内部即可正常运行：

#include <stdio.h>
#include <math.h>
//GLOBAL 
//  Power Demand in St John's per year [GJ/yr]
double P_year = 14367865E9;
double s_year = 365*24*60*60;// Seconds in a year [s]
//electric permitivitty of free space [s4A2/m3kg]
double epsilon = 8.85418782E-12;

int main(){
    //  Power Demand in St John's per second [GJ/yr]
    double P_s = P_year*(1/s_year);
    // Energy Required for 6hrs demand [J]
    double E = P_s * 60*60*6;
    // Voltage Difference accross capacitor [V]
    double V = 230E3;

    // Capacitance
    double C = 2*E/pow(V,2);
    
    printf("%f %f \n", E, C);
}

• 方案2：将全局常量改为宏定义
  宏是预编译期直接替换的内容，属于编译期常量，用宏定义替代变量存储固定参数即可正常初始化全局P_s：

#include <stdio.h>
#include <math.h>
#define P_YEAR 14367865E9
#define S_YEAR (365*24*60*60)
//  Power Demand in St John's per second [GJ/yr]
double P_s = P_YEAR*(1.0/S_YEAR);
//electric permitivitty of free space [s4A2/m3kg]
double epsilon = 8.85418782E-12;

int main(){
    // Energy Required for 6hrs demand [J]
    double E = P_s * 60*60*6;
    // Voltage Difference accross capacitor [V]
    double V = 230E3;

    // Capacitance
    double C = 2*E/pow(V,2);
    
    printf("%f %f \n", E, C);
}

• 方案3：延后全局变量赋值
  保留P_s的全局变量属性，不在定义时初始化，改为在main函数最开头完成赋值：

#include <stdio.h>
#include <math.h>
//GLOBAL 
//  Power Demand in St John's per year [GJ/yr]
double P_year = 14367865E9;
double s_year = 365*24*60*60;// Seconds in a year [s]
//  Power Demand in St John's per second [GJ/yr]
double P_s;
//electric permitivitty of free space [s4A2/m3kg]
double epsilon = 8.85418782E-12;

int main(){
    P_s = P_year*(1/s_year);
    // Energy Required for 6hrs demand [J]
    double E = P_s * 60*60*6;
    // Voltage Difference accross capacitor [V]
    double V = 230E3;

    // Capacitance
    double C = 2*E/pow(V,2);
    
    printf("%f %f \n", E, C);
}

代码中使用了pow函数和printf函数，需要在文件最开头引入<stdio.h>和<math.h>两个头文件，否则会出现编译警告或错误。

内容的提问来源于stack exchange，提问作者STOI