c语言计算昨天的日期简单

保证输入的日期合法，而且输入的日期和计算结果都在[1000-01-01，9999-12-31]范围内。
输入
1
2019-10-01
输出
2019-09-30

计算昨日就是-1天，很简单的，先获取输入日期的天数是否为1，为1则月-1天数为该年上月天数，若月为1则年-1月为12天为31。https://blog.csdn.net/xianfajushi/article/details/104001986?spm=1001.2014.3001.5502

int n=0,y=0,t=0;char fg;
cin>>n>>fg>>y>>fg>>t;
if(t==1&&y==1)cout<<--n<<"-"<<12<<"-"<<31<<"\n";
else if(t==1&&y>1)cout<<n<<"-"<<--y<<"-"<<返回月的天数(n,y)<<"\n";
else cout<<n<<"-"<<y<<"-"<<--t<<"\n";

我有个计算前天的日期，你要不拿来改改？要的话回我一下我发你

//计算昨天的时间信息
//这个程序又重新认识了指针
//指针定义必须初始化，就是必须对指针赋值地址值
#include<stdio.h>
/*
#include<stdlib.h>
#include<string.h>
#include
using namespace std;
*/

//结构体的定义部分

typedef struct
{
unsigned char year;
unsigned char month;
unsigned char day;
}_SystemTime,_ComputingTime;

/*
typedef struct
{
unsigned char year;
unsigned char month;
unsigned char day;
} _SystemTime;

typedef struct
{
unsigned char year;
unsigned char month;
unsigned char day;
} _ComputingTime;
/
/
#define _SystemTime (struct date);
#define _ComputingTime (struct date);
/
/
struct date _SystemTime;
struct date _ComputingTime;
*/

//计算昨天时间信息的函数
void ComputingLastThreeDaysTime( _SystemTime *SystemTime, _ComputingTime ComputingTime ) //参数1是系统时间，参数2是昨天的时间
{
unsigned char year,yearshi,yearge;
unsigned char month,monthshi,monthge;
unsigned char day,dayshi,dayge;
ComputingTime->year = SystemTime->year;
ComputingTime->month = SystemTime->month;
ComputingTime->day = SystemTime->day;
year = ComputingTime->year;
month = ComputingTime->month;
day = ComputingTime->day;
yearshi = ( year & 0xf0 ) >> 4;
yearge = ( year & 0x0f );
monthshi = ( month & 0xf0 ) >> 4;
monthge = ( month & 0x0f );
dayshi = ( day & 0xf0 ) >> 4;
dayge = ( day & 0x0f );
if( dayge == 1 && dayshi == 0 )
{
if( dayshi == 0 )
{
if( ( monthge == 0 ) && ( monthshi == 1 ) )
{
monthge = 9;
monthshi = 0;
dayshi = 3;
dayge = 0;
}
else if( (monthshi == 0 ) && ( monthge == 1 ) )
{
monthge = 2;
monthshi = 1;
dayshi = 3;
dayge = 1;
if( yearge == 0 )
{
if( yearshi == 0 )
{
year = 9;
}
else
{
yearshi = yearshi - 1;
}
}
else
{
yearge = yearge - 1;
}
}
else
{
monthge = monthge - 1;
if( ( monthshi == 0 ) && ( ( monthge == 1 ) || ( monthge == 3 ) || ( monthge == 5 ) || ( monthge == 7 ) || ( monthge == 8 ) ) )
{
dayshi = 3;
dayge = 1;
}
if( ( monthshi == 1 ) && ( ( monthge == 0 ) || ( monthge == 2 ) ) )
{
dayshi = 3;
dayge = 1;
}
if( ( monthshi == 0 ) && ( ( monthge == 4 ) || ( monthge == 6 ) || ( monthge == 9 ) ) )
{
dayshi = 3;
dayge = 0;
}
if( ( monthshi == 1 ) && ( monthge == 1 ) )
{
dayshi = 3;
dayge = 0;
}
if( ( monthshi == 0 ) && ( monthge == 2 ) )
{
unsigned int years;
years = 2000 + yearshi * 10 + yearge;
if( ( years % 4 == 0 && years % 100 != 0 ) || years % 400 == 0 )
{
dayshi = 2;
dayge = 9;
}
else
{
dayshi = 2;
dayge = 8;
}
}
}
}
else
{
dayshi = dayshi - 1;
}
}
else if( dayge == 0 && dayshi != 0 )
{
dayge = 9;
dayshi = dayshi - 1;
}
else
{
dayge = dayge - 1;
}
year = ( yearshi << 4 ) + yearge;
month = ( monthshi << 4 ) + monthge;
day = ( dayshi << 4 ) + dayge;
ComputingTime->year = year;
ComputingTime->month = month;
ComputingTime->day = day;
}
int main()
{
/
_SystemTime *SystemTime;
_ComputingTime *ComputingTime;
ComputingTime->year = SystemTime->year = 19;
ComputingTime->month = SystemTime->month = 4;
ComputingTime->day = SystemTime->day = 24;
*/
_SystemTime SystemTime;
_ComputingTime ComputingTime;
//此处定义今天的时间信息，可修改
ComputingTime.year = SystemTime.year = 19;
ComputingTime.month = SystemTime.month = 4;
ComputingTime.day = SystemTime.day = 24;
// cout << "asadasd";
printf("田贝贝的程序输出：\n");
printf("今天：20%d年%d月%d日\n",ComputingTime.year,ComputingTime.month,ComputingTime.day);
// printf("今天：20%d年%d月%d日\n",ComputingTime->year,ComputingTime->month,ComputingTime->day);
ComputingLastThreeDaysTime(&SystemTime,&ComputingTime);
printf("昨天：20%d年%d月%d日\n",ComputingTime.year,ComputingTime.month,ComputingTime.day);
//printf("昨天：20%d年%d月%d日\n",ComputingTime->year,ComputingTime->month,ComputingTime.day);
return 0;
}