c言語時間の出力と比較
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time_t
The most basic representation of a date and time is the type time_t. The value of a time_t variable is the number of seconds since January 1, 1970, sometimes call the Unix epoch . This is the best way to internally represent the start and end times for an event because it is easy to compare these values. Two time_t values a and b can be compared using <:
struct tm
While time_t represents a date and time as a single number, struct tm represents it as a struct with a lot of numbers:
Conversion
You can convert a time_t value to a struct tm value using the localtime function:
You can convert a struct tm value to a time_t value using the mktime function:
How to input a date
Your program will need to input a date. Here is an example of a safe and robust way to input a date:
This function asks the user for a time with the given prompt. If the time is correctly entered, it converts it to type time_t, expressing a time and date of 12:00am on the entered date.
In order to use the strptime function, you will need to #include. However, you must preceed the #include with this code:
Any time you #include in your program, use the line #define __USE_XOPEN before it. Note that it begins with two underlines.
How to input a time
After entering the date, you can enter a time using this safe and robust function:
This function is passed a date expressed as a time_t value, the output of the InputDate function. The time is entered and added to the date to produce a date and time value that is returned. Here is an example of how these functions can be used to input a date, start time, and end time.
How to output a time_t value.
To output a time or date, you convert it to a struct tm:
Once converted, the value in struct tm are:
Now I can get the hour in the example as startTM.tm_hour. There are a couple strange things about this struct. To get the actual year, use startTM.tm_year + 1900. To get the actual month number, use startTM.tm_mon + 1.
The function ctime
The function ctime converts a time_t value into a string of the following form:
This is a handy way to test to see if a time is what you expect. For example, if I have a variable: time_t timeDate and want to know what its value is, I can add this code while debugging:
This is also the easiest way to save the start date/time and end date/time to a file.
Reading the output of ctime
To read the output of ctime from a file and convert it back to a time_t value, use this code:
This inputs the date/time string from a file into the character string buffer. It then used strptime to convert it to a struct tm structure. Then the funciton mktime converts the struct tm value to a time_t value.
The most basic representation of a date and time is the type time_t. The value of a time_t variable is the number of seconds since January 1, 1970, sometimes call the Unix epoch . This is the best way to internally represent the start and end times for an event because it is easy to compare these values. Two time_t values a and b can be compared using <:
if(a < b)
{
printf("Time a is before time b
");
}struct tm
While time_t represents a date and time as a single number, struct tm represents it as a struct with a lot of numbers:
struct tm
{
int tm_sec; /* Seconds. [0-60] (1 leap second) */
int tm_min; /* Minutes. [0-59] */
int tm_hour; /* Hours. [0-23] */
int tm_mday; /* Day. [1-31] */
int tm_mon; /* Month. [0-11] */
int tm_year; /* Year - 1900. */
int tm_wday; /* Day of week. [0-6] */
int tm_yday; /* Days in year.[0-365] */
int tm_isdst; /* DST. [-1/0/1]*/
};Conversion
You can convert a time_t value to a struct tm value using the localtime function:
struct tm startTM;
time_t start;
/* ... */
startTM = *localtime(&start);You can convert a struct tm value to a time_t value using the mktime function:
struct tm startTM;
time_t start;
/* ... */
start = mktime(&startTM);How to input a date
Your program will need to input a date. Here is an example of a safe and robust way to input a date:
time_t InputDate(char *prompt)
{
char buffer[100];
char *result;
struct tm date;
do
{
printf("%s", prompt);
/* Get a line of up to 100 characters */
fgets(buffer, sizeof(buffer), stdin);
/* Remove any
we may have input */
if(strlen(buffer) > 0)
buffer[strlen(buffer)-1] = '\0';
result = strptime(buffer, "%m/%d/%Y", &date);
} while(result == NULL);
/* Convert to time_t format */
date.tm_min = 0;
date.tm_hour = 0;
date.tm_sec = 0;
date.tm_isdst = 1;
return mktime(&date);
}This function asks the user for a time with the given prompt. If the time is correctly entered, it converts it to type time_t, expressing a time and date of 12:00am on the entered date.
In order to use the strptime function, you will need to #include
#define __USE_XOPEN
#include <time.h>Any time you #include
How to input a time
After entering the date, you can enter a time using this safe and robust function:
time_t InputTime(char *prompt, time_t date)
{
char buffer[100];
char *result;
struct tm time;
time = *localtime(&date);
do
{
printf("%s", prompt);
/* Get a line of up to 100 characters */
fgets(buffer, sizeof(buffer), stdin);
/* Remove any
we may have input */
if(strlen(buffer) > 0)
buffer[strlen(buffer)-1] = '\0';
result = strptime(buffer, "%I:%M%p", &time);
} while(result == NULL);
return mktime(&time);
}This function is passed a date expressed as a time_t value, the output of the InputDate function. The time is entered and added to the date to produce a date and time value that is returned. Here is an example of how these functions can be used to input a date, start time, and end time.
time_t date;
time_t start;
time_t end;
/*...*/
date = InputDate("Event date: ");
start = InputTime("Start time: ", date);
end = InputTime("End time: ", date);How to output a time_t value.
To output a time or date, you convert it to a struct tm:
struct tm startTM;
time_t start;
/* ... */
startTM = *localtime(&start);Once converted, the value in struct tm are:
struct tm
{
int tm_sec; /* Seconds. [0-60] (1 leap second) */
int tm_min; /* Minutes. [0-59] */
int tm_hour; /* Hours. [0-23] */
int tm_mday; /* Day. [1-31] */
int tm_mon; /* Month. [0-11] */
int tm_year; /* Year - 1900. */
int tm_wday; /* Day of week. [0-6] */
int tm_yday; /* Days in year.[0-365] */
int tm_isdst; /* DST. [-1/0/1]*/
};Now I can get the hour in the example as startTM.tm_hour. There are a couple strange things about this struct. To get the actual year, use startTM.tm_year + 1900. To get the actual month number, use startTM.tm_mon + 1.
The function ctime
The function ctime converts a time_t value into a string of the following form:
Thu Apr 14 14:00:00 2011This is a handy way to test to see if a time is what you expect. For example, if I have a variable: time_t timeDate and want to know what its value is, I can add this code while debugging:
printf("The date/time is %s
", ctime(&timeDate));This is also the easiest way to save the start date/time and end date/time to a file.
Reading the output of ctime
To read the output of ctime from a file and convert it back to a time_t value, use this code:
struct tm tim;
time_t startTime;
fgets(buffer, sizeof(buffer), file);
strptime(buffer, "%a %b %d %H:%M:%S %Y", &tim);
startTime = mktime(&tim);This inputs the date/time string from a file into the character string buffer. It then used strptime to convert it to a struct tm structure. Then the funciton mktime converts the struct tm value to a time_t value.