`luc018.c`¶

Problem Statement

According to Gregorian calender, it was Monday on the date 01/01/01. if any year is input through the keyboard write a program to find out what is the day on 1st January of this year.

Metadata¶

Property	Detail
Author	Amit Dutta (amitdutta4255@gmail.com)
License	MIT
Difficulty	Beginner (index: 1 / 10)

Concepts¶

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Pointers
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Source Code¶

#include <stdio.h>

int is_leap(int year) {
    if ((year % 400 == 0) || (year % 4 == 0 && year % 100 != 0)) {
        return 1;
    }
    return 0;
}

int main() {
    long long year; // long long for year input if years far in the future/past are tested
    int i;
    long long total_days = 0;
    int day_index;

    // Day names array for output
    const char *day_names[] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};

    printf("Enter the year (e.g., 2025): ");
    if (scanf("%lld", &year) != 1 || year < 1) {
        printf("Invalid year input. Please enter a positive integer year (>= 1).\n");
        return 1;
    }
    int years_passed = year - 1;
    long long leap_years = years_passed / 4 - years_passed / 100 + years_passed / 400;
    total_days = years_passed * 365 + leap_years;
    day_index = (total_days + 1) % 7;
    printf("\nOn January 1st, %lld, the day was: %s.\n", year, day_names[day_index]);
    return 0;
}

Explanation¶

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    You are explaining a C programming code to a beginner.

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    - Only use the given code. Do NOT assume anything not present.

    - Do NOT add extra examples.

    - Keep explanation clear and short.

    - If something is unclear, say "Not clear from code".

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    FORMAT:

    [START]

    ## What it does

    (Explain the overall purpose in 1-2 sentences)

    ## Step-by-step

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    ## Key Concepts

    (List concepts like loop, condition, function, etc.)

    ## Notes

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    [END]

    CODE (luc018.c):

    #include <stdio.h>

    int is_leap(int year) {
        if ((year % 400 == 0) || (year % 4 == 0 && year % 100 != 0)) {
            return 1;
        }
        return 0;
    }

    int main() {
        long long year; // long long for year input if years far in the future/past are tested
        int i;
        long long total_days = 0;
        int day_index;

        // Day names array for output
        const char *day_names[] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};

        printf("Enter the year (e.g., 2025): ");
        if (scanf("%lld", &year) != 1 || year < 1) {
            printf("Invalid year input. Please enter a positive integer year (>= 1).\n");
            return 1;
        }
        int years_passed = year - 1;
        long long leap_years = years_passed / 4 - years_passed / 100 + years_passed / 400;
        total_days = years_passed * 365 + leap_years;
        day_index = (total_days + 1) % 7;
        printf("\nOn January 1st, %lld, the day was: %s.\n", year, day_names[day_index]);
        return 0;
    }

luc018.c¶