Question

I’m trying to write a date calculator in C program with the range from 1/1/1902 to 12/31/2299, I’ve follow the algorithm from http://en.wikipedia.org/wiki/Calculating_the_day_of_the_week with the century table,month table and day table, but when I tried to print out, this is what I have

Enter Year, Month and Day as YYYY,M,DD
1982 4 24
Saturday1982 ,6, 24, is  6

Instead of saying 1982 4 24 is a Saturday

What is wrong in the program I wrote? The placement of the switch cases?

#include <stdio.h>

int main (void)
{
    // insert code here...
    int day,month,year;

    printf("Enter Year, Month and Day as YYYY,M,DD\n");

    scanf("%4d%d%2d", &year, &month, &day);

    if (year >= 1901 && year <= 2299 &&
        month >= 1 && month <= 12 &&
        day >= 0 && day <= 31)
    {
        int century = year/100;

        /* making a century table for calculation*/

        switch(century)
        {
            case 19:
                century=0;
                break;
            case 20:
                century=6;
                break;
            case 21:
                century=4;
                break;
            case 22:
                century=2;
                break;
        }

        int last2_of_year= year % 100;

        /* Last 2 digits of the year entered*/

        int last2_div_4 =  last2_of_year/4;

        switch (month)
        {
            case 1:
                month=0;
                break;

            case 2:
                month=3;
                break;
            case 3:
                month=3;
                break;
            case 4:
                month=6;
                break;
            case 5:
                month=1;
                break;
            case 6:
                month=4;
                break;
            case 7:
                month=6;
                break;
            case 8:
                month=2;
                break;
            case 9:
                month=5;
                break;
            case 10:
                month=0;
                break;
            case 11:
                month=3;
                break;
            case 12:
                month=5;
                break;
         }


        int total_num = (century+ last2_of_year +day +month +last2_div_4)%7;

        switch (total_num) 
        {
            case 0:
            printf("Sunday");
                break;
            case 1:
            printf("Monday");
                break;
            case 2:
            printf("Tuesday");
                break;
            case 3:
            printf("Wednesday");
                break;
            case 4:
            printf("Thursday");
                break;
            case 5:
            printf("Friday");
                break;
            case 6:
            printf("Saturday");
                break;
        }

        printf("%d ,%d, %d, is a %d", year,month,day,total_num);
        }
        else
        {
            printf("invalid\n");
        }



    return 0;

    }

Answer 1

Looks like @QuantumMechanic found the root cause of the problem, but I’d like to suggest a few changes:

    int century = year/100;

    /* making a century table for calculation*/

    switch(century)
    {
        case 19:
            century=0;
            break;

I’m very leery of using a single variable to represent two different things. Here, century represents both the user-input human-readable century and an offset to the first day of the week for the century. Two variables would give clearer code, and allow you to re-use the century information later, should the need arise.

Second, using a case statement to store offsets for the months feels a little … overdone:

    switch (month)
    {
        case 1:
            month=0;
            break;

        case 2:
            month=3;
            break;
        case 3:
            month=3;
            break;

This could instead be handled with an array lookup:

int leap_month[] = [-1, 6, 2, 3, 6, 1, 4, 6, 2, 5, 0, 3, 5];
int norm_month[] = [-1, 0, 3, 3, 6, 1, 4, 6, 2, 5, 0, 3, 5];

if (leap_year)
    month_offset = leap_month[month];
else
    month_offset = norm_month[month];

The -1 is just to allow referring to the table with human-friendly indexes (Jan == 1). Feel free to remove it and use leap_month[month-1] or similar if you find that easier.