Calculating the number of days in a month without Calendar

Question

I'm trying to calculate the number of days in a month given a year and month number. Here is what I have so far.

 public static int daysInMonth(int m, int y) {
    int d;
    if (m == 1 || m == 3 || m == 5 || m == 7 
             || m == 8 || m == 10 || m == 12) { 
      d = 31;
    } else if (m == 4 || m == 6 || m == 9 || m == 11) { 
      d = 30;
    } else if (y % 4 == 0 && m == 2) {  
      d = 29;
    } else if (m == 2) {
      d = 28;   
    } else {    
      d = 0;    
    }   
    return d;   
  }

Unfortunately, it doesn't work properly for the second month of leap years.

How can I fix it without using the Calendar class?

So what exactly is your question? If you just want your code to look better or run more efficiently, you should probably ask this on http://codereview.stackexchange.com/ — Cory Klein, Oct 01 '14 at 17:07
If you just need to find the answer of how many days there are in any given month, there is already a solution [here](http://stackoverflow.com/questions/8940438/number-of-days-in-particular-month-of-particular-year) — Cory Klein, Oct 01 '14 at 17:09
My test of the month 2 in a leap year e.g (2016) failing the test for some reason — Sholyhit, Oct 01 '14 at 17:10
So the question is not about how to test, but how to calculate the days of the month properly? — Cory Klein, Oct 01 '14 at 17:13
[This](http://codegolf.stackexchange.com/questions/7293/calculate-days-of-a-month) question has several examples of how to do this. — Cory Klein, Oct 01 '14 at 17:14
y % 4 is not sufficient to test for a leap year - e.g. 1900 was not a leap year — Peregrine, Oct 01 '14 at 17:16

score 2 · Answer 1 · edited May 23 '17 at 11:49

You need to properly calculate leap years:

public static int daysInMonth(int m, int y) {
    int d;
    if (m == 1 || m == 3 || m == 5 || m == 7
             || m == 8 || m == 10 || m == 12) {
      d = 31;
    } else if (m == 4 || m == 6 || m == 9 || m == 11) {
      d = 30;
    } else if (isLeapYear(y) && m == 2) {
      d = 29;
    } else if (m == 2) {
      d = 28;
    } else {
      d = 0;
    }
    return d;
}

public static boolean isLeapYear(int year) {
  if (year % 4 != 0) {
    return false;
  } else if (year % 400 == 0) {
    return true;
  } else if (year % 100 == 0) {
    return false;
  } else {
    return true;
  }
}

See here.

Here is a complete and tested solution with an Ideone as well.

import java.util.HashSet;
import java.util.Arrays;
import java.util.List;

class MyClass {

  public static void main(String[] args) {
    List<Integer> leapYears = Arrays.asList(1804, 1808, 1812, 1816, 1820, 1824, 1828, 1832, 1836, 1840, 1844, 1848, 1852, 1856, 1860, 1864, 1868, 1872, 1876, 1880, 1884, 1888, 1892, 1896, 1904, 1908, 1912, 1916, 1920, 1924, 1928, 1932, 1936, 1940, 1944, 1948, 1952, 1956, 1960, 1964, 1968, 1972, 1976, 1980, 1984, 1988, 1992, 1996, 2000, 2004, 2008, 2012, 2016, 2020, 2024, 2028, 2032, 2036, 2040, 2044, 2048, 2052, 2056, 2060, 2064, 2068, 2072, 2076, 2080, 2084, 2088, 2092, 2096, 2104, 2108, 2112, 2116, 2120, 2124, 2128, 2132, 2136, 2140, 2144, 2148, 2152, 2156, 2160, 2164, 2168, 2172, 2176, 2180, 2184, 2188, 2192, 2196, 2204, 2208, 2212, 2216, 2220, 2224, 2228, 2232, 2236, 2240, 2244, 2248, 2252, 2256, 2260, 2264, 2268, 2272, 2276, 2280, 2284, 2288, 2292, 2296, 2304, 2308, 2312, 2316, 2320, 2324, 2328, 2332, 2336, 2340, 2344, 2348, 2352, 2356, 2360, 2364, 2368, 2372, 2376, 2380, 2384, 2388, 2392, 2396, 2400);
    for(Integer year = 1804; year < 2400; year++) {
      for(Integer month : Arrays.asList(1,3,5,7,8,10,12)) {
        assert daysInMonth(month, year) == 31;
      }
      for(Integer month : Arrays.asList(4,6,9,11)) {
        assert daysInMonth(month, year) == 30;
      }
      if(leapYears.contains(year)) {
        assert daysInMonth(2, year) == 29;
      }
      else {
        assert daysInMonth(2, year) == 28;
      }
    }
  }

  public static int daysInMonth(int m, int y) {
    HashSet<Integer> monthsWith31Days = new HashSet<Integer>(Arrays.asList(1,3,5,7,8,10,12));
    HashSet<Integer> monthsWith30Days = new HashSet<Integer>(Arrays.asList(4,6,9,11));

    if(monthsWith31Days.contains(m)) {
      return 31;
    } else if(monthsWith30Days.contains(m)) {
      return 30;
    }
    else if(isLeapYear(y)) {
      return 29;
    }
    else {
      return 28;
    }
  }

  public static boolean isLeapYear(int year) {
    if (year % 4 != 0) {
      return false;
    } else if (year % 400 == 0) {
      return true;
    } else if (year % 100 == 0) {
      return false;
    } else {
      return true;
    }
  }
}

score 1 · Answer 2 · answered Oct 01 '14 at 17:14

http://ideone.com/H4VAvv

With your case of 2,2016 the output comes out as expected.

/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
public static void main (String[] args) throws java.lang.Exception
{
    System.out.println(daysInMonth(2,2016));
}
public static int daysInMonth(int m, int y) {
int d;
if (m == 1 || m == 3 || m == 5 || m == 7 
         || m == 8 || m == 10 || m == 12) { 
  d = 31;
} else if (m == 4 || m == 6 || m == 9 || m == 11) { 
  d = 30;
} else if (y % 4 == 0 && m == 2) {  
  d = 29;
} else if (m == 2) {
  d = 28;   
} else {    
  d = 0;    
}   
return d;   
}
}

Agreed, not seeing a problem with Feb 2016. Your method is returning 29 days, as expected. — rajah9, Oct 01 '14 at 20:03

score 0 · Answer 3 · answered Oct 01 '14 at 17:13

I would store the days of the month in an integer array with the first index as a null value to reduce the conditionals

public static int daysInMonth(int m, int y) {
    int[] days = {0, 31, 28, 31, ... 31};
    if (m == 2 && y % 4 == 0) {
        return 29;

    } else {
        return days[m];
    }

}

You would probably want to add some checking to make sure the m parameter is always less than 13 (number of months plus the null value in the beginning)

score 0 · Answer 4 · answered Oct 01 '14 at 17:24

0

Also, for consistency's sake, re-use Date.isLeapYear in your daysInMonth instead of hardcoding y % 4 == 0

answered Oct 01 '14 at 17:24

Sycomor

1,272
11
15

Calculating the number of days in a month without Calendar

4 Answers4