C Program to convert 24 Hour time to 12 Hour time

In this tutorial, we will write a program that will convert a given time of 24-hour format to a 12-hour format.

The time will be given in the format of - Hours: Minutes: Seconds

For example -

Input : 20:35:20

Output : 8:35:20 PM

Input : 00:15:40

Output : 12:15:40 AM

Algorithm

The midnight in 24-hour clock format is 00:00:00 and in 12-hour format is 12:00:00. Now we need to observe that the minutes and seconds in both the observations are the same. There is a change in hours only when there is a change in the meridian.

To check hours we will first convert the input string of hours into an integer. After changing it into integer we will modulo the hour with 12 and that will be in the 12-hour format.

The case when there is 00 in the hour will be calculated as a separate case.

C code Implementation

#include <stdio.h>
#include <string.h>
void convert12(char* str) // Take the time array
{
    int h1 = (int)str[0] - '0'; // Get the first digit of hour in 24 hour format
    int h2 = (int)str[1] - '0'; // Get the second digit of hour in 24 hour format
    int hh = h1 * 10 + h2; // Caluclate hour as an integer value
    char Meridien[2]; // Make the AM or PM array
    if (hh < 12) { // If the time is less than 12 hour it is AM
        Meridien[0] = 'A';
        Meridien[1] = 'M';
    }
    else {
        Meridien[0] = 'P'; // When the time is 24 hour it is PM
        Meridien[1] = 'M';
    }

    hh %= 12; // Calculate the 12 hour time

    // Handle 00 and 12 case separately
    if (hh == 0) {
        printf("12"); // Base format as 00 in 24-hour is 12 in 12-hour

        // Printing minutes and seconds
        for (int i = 2; i < 8; ++i) {
            printf("%c", str[i]);
        }
    }
    else {
        printf("%d", hh);
        // Printing minutes and seconds
        for (int i = 2; i < 8; ++i) {
            printf("%c", str[i]);
        }
    }

    // After time is printed Meridien
    printf(" %c%c", Meridien[0], Meridien[1]);
}

int main()
{

    char str[8] = "17:35:20"; // array containing the time

    convert12(str); // Call the function

    return 0;
}

Output:

C Program to convert 24 Hour time to 12 Hour time

Code Explanation

Take the char str into the function.
Obtain the first digit and second digit of the hour from the 24-hour format string. The '0' converts the string value to an integer.

int h1 = (int)str[0] - '0'; // Get the first digit of hour in 24 hour format
int h2 = (int)str[1] - '0'; // Get the second digit of hour in 24 hour format

As we have got individual digit of hour we will convert into a number.

Check if the hh is in 12 hour or 24 hour format using if-else and set the Meridian as AM or PM.

if (hh < 12) { // If the time is less than 12 hour it is AM
       Meridien[0] = 'A';
       Meridien[1] = 'M'
}
else {
       Meridien[0] = 'P'; // When the time is 24 hour it is PM
        Meridien[1] = 'M';
          }

Modulo hh with 12 and convert the time into 12-hour format.

We will handle the 00 in 24-hour separately as it is the same as 12 in 12-hour format.
After printing the hour print the minutes and seconds
Print the minutes and seconds if it is not the 00 case
Print the meridian of 12-hour format.

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