Variable parameter list va_list

The variable parameter list is implemented through macros, which are defined in the stdarg.h header file. The execution steps are as follows:

1. The ellipsis in the parameter list means that the number and type of parameters that may be passed here are not determined
2. First, declare a variable value of type va_list in the function , which is used to access the undetermined part of the parameter list . This variable is a pointer to the parameter
3. Use the macro va_start to initialize the defined variable of type va_list. The first parameter of the macro va_start is the name of the va_list variable value, and the second parameter is the last named parameter before the ellipsis . The initialization process points the variable value of the va_list type to the variable The first parameter of the parameter section
4. Use the macro va_arg to return a variable parameter . The macro va_arg receives two parameters: a variable of type va_list and the type of the next parameter in the parameter list, and makes value point to the next variable parameter.
5. After accessing the last variable parameter, use the macro va_end to end the acquisition of the variable parameter

Note : Variable parameter access must be accessed one by one in order from beginning to end

/*求n个整形参数的平均值，利用可变参数列表来实现
形参列表中必须要有一个固定值，不能只有省略号所代表的未确定参数*/
#include <stdio.h>
#include <stdarg.h>

float average(int n_values,...)
{
    
    
    va_list value;  //va_list类型声明
    int count;
    float sum=0;
    float average;
    
    /*准备访问可变参数*/
  	va_start(value,n_values);  //初始化参数，指向第一个可变参数
    
    /*添加取自可变参数列表的值*/
    for(count=0;count<n_values;count++)
    {
    
    
        sum += va_arg(value,int);  //取当前参数，并指向下一个参数
     }
    
    va_end(value); //完成可变参数的处理
    average=((float)((int)((sum/values+0.005)*100)))/100;  //取小数点后两位，四舍五入
    return average;
}

#define n_values 3
int main()
{
    
    
    float aver;
    aver=average(n_values,9,9,8);
    printf("%.2f",aver);  //保留两位小数打印
    return 0;
}