对于以0开头的数字在不同代码里有无警告出现的问题

Question

#include<stdio.h>

struct date{
    int month;
    int year;
    int day;
};

int main(int argc,char const *argv[])
{
    struct date today = {09,2022,25};
    struct date thismonth = {.month=9,.year=2022};
    
    printf("Today's date is %i-%i-%i.\n",
    today.year,today.month,today.day);
    
    printf("This month is %i-%i-%i.\n",
    thismonth.year,thismonth.month,thismonth.day);
    
    return 0;
}

以上代码会出现 警告 [Error] invalid digit "9" in octal constant，我知道是0开头的数字，比如‘09’表示八进制(oct)数；但是为什么下面代码里的‘07’就没有出现警告？

#include<stdio.h>

struct date {
    int month;
    int day;
    int year;
};

int main(int argc,char const *argv[])
{
    struct date today;
    
    today = (struct date){07,31,2014}; //对today赋值 
    
    struct date day;    //定义结构变量day 
    
    day = today;    //将today的值赋给day 
    
    printf("Today's date is %i-%i-%i.\n",
    today.year,today.month,today.day);
    
    printf("The day's date is %i-%i-%i.\n",
    day.year,day.month,day.day);
    
    return 0;
}

Answer 1

0开头的数字编译器认为是8进制，所以出现9就是错误的
所以不要写09，只写9就行了