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首先你要理解什么是格式符，C语言中常用的格式符有哪些?

例：%s字符串 %d十进制数字 %\p地址 %f浮点数

在printf里格式化输出是要一个变量替换的作用，举例如下

int main(){
    int num = 20;
    float flo = 13.5; 
    char str[30] = "my name is frind";
    printf("%d,%f,%s",num,flo,str); 
}
输出为:20 13.5 my name is frind

不一定啊，比如：

    #include<stdio.h>
    {
        void main()
        {
            printf("Hello World!!");
        }
    }

这里就没有用格式符。