Thread: Working out the day from numeric date (dd/mm/yy format).

Hi All,

Well, I'm back with my Java head on after doing some more logical, straight forward and easier to understand 6502 programming. I have a date class with error checking (ie, valid leap-years or dates, 31-04-02 on DD-MM-YY format would be invalid and reset to a default value after reporting an error).

Here's my Date class:

public class Date
{
  /* Let's do this thing properly and set up a look-up table
   * which will be used in to return month as string bit at
   * the end. We'll store the months in an array of type
   * String and as computers count from zero, our first entry
   * will be monthToString[0], returning January. */
  public static String [] monthToString=
  {
    "January", "February", "March", "April", "May",
    "June", "July", "August", "September", "October",
    "November", "December"
  };
 
  /* Because I'm doing what should be advanced programming,
   * I'm going to use a look-up table for the number of days
   * in each month as follows: */
  public static int [] noOfDays=
  {
    31,28,31,30,31,30,31,31,30,31,30,31
  };
 
  /* Here is a variable of type char that'll force a line-feed
   * when using System.out.print ("text"+ln+"more text"); */
  public static char ln=13;
 
  /* The following variable of type String is also used in
   * the sub-routine that returns the month as string */
  public static String mon="";
 
  /* Okay, here's some variables of type integer (whole numbers)
   * which will be used in the constructor thingy */
  private int day, month, year;
 
  /* Error catching for type date in DD/MM/YY format to
   * check for legal dates, assuming a 19xx or 20xx prefix*/
  public Date (int dd, int mm, int yy)
  {
    day=dd; month=mm; year=yy;
    if (month<1 || month>12)
    {
      System.out.println("?Month not valid error"+ln+
      "Reset to default value.");
      month=01;
    }
 
    if (day<1 || day>31)
    {
      System.out.println("?Day not valid error"+ln+
      "Reset to default value.");
      day=01;
    }
 
    /* As there is no easy way to store a leap-year in a look-up table
     * I'm going to check for the 29th February first and validate it
     * later. Of course, this is advanced programming, isn't it? */
    if (day>29 && month==2)
    {
      System.out.println("?Day not valid error"+ln+
      "Except for leap-years, Feburary"+
      "has only 28 days.");
      day=01;
    }
 
    /* Now, let's check to see if a legal leap-year has been entered
     * by the user, shall we? Oh, we need to do some maths. */
    if (day==29 && month==2 && ((year-2000)%4!=0))
    {
      System.out.println("?Date not valid error"+ln+"Not a leap-year.");
      day=28;
    }
 
    /* So, we've tested for a valid leap-year, the only thing that could
     * trip up our array above for the numbers of days in each month. */
    if (day<1 || day>noOfDays[month-1] && month!=2)
    {
      System.out.println("?Date not valid error"+ln+"Not a legal date.");
      day=01;
    }
    /* That's a whole lot more efficient and advanced than having a
     * 'switch case' or a load of 'if' statements checking for each
     * condition, isn't it? Okay, I'd use a switch case if I thought
     * that the person maintaining the code wasn't very good and needed
     * it to be readable, but if you use comments correctly then you
     * don't necessarily need code to be more readable, or shouldn't */
  }
 
  /* This will initialise the date and then we'll do some stuff
   * wid it */
  public Date()
  {
    day=04;month=05;year=77;
  }
 
  /* This does something, but I can't remember what? */
  public Date (Date other)
  {
    this (other.day, other.month, other.year);
  }
 
  /* This method will return the monthAsString, as suggested by its' name */
  public String monthAsString()
  {
  /* This is very efficient because it has in-built error checking
   * actually without testing for loads of conditions */
  if (month>0 && month<13)
    {
      mon=monthToString[month-1];
      return mon;
    }
 
  /* Just in case any errors have sneaked into validating the month,
   * It will return a Month not set message */
      else
        return "Month not set";
  }
 
  /* I'm probably wasting my time trying to make this method
   * more efficient. Not sure what benefits it has anyway? */
  public boolean earlierThan(Date other)
  {
    if(year<other.year)
    {
      return true;
    }
    else
      if (year==other.year && month<other.month)
      {
        return true;
      }
      else
        if (month==other.month && day<other.day)
          {
            return true;
          }
          else
            return false;
  }
 
  /* This does some sort of test for some reason, again I don't
   * recall what it's actually for? */
  public boolean equals (Date other)
  {
    if (day==other.day && month==other.month && year==other.year)
    {
      return true;
    }
    else
      return false;
  }
 
  public String suffix()
  {
  /* Here is where a switch case is useful, in terms of returning a
   * date: because most days as numbers have a th suffix (ie, 4th),
   * we'll use a switch case for the exceptions to that rule. */
    switch(day)
    {
      case 1: return "st";
      case 2: return "nd";
      case 3: return "rd";
      case 21: return "st";
      case 22: return "nd";
      case 23: return "rd";
      case 31: return "st";
      default: return "th";
    }
  }
 
  public String toString()
  {
  /* This is another attempt at error trapping, just in case
   * anything above has failled. */
  if(month!=0 || day!=0)
    {
      if(year<10)
      {
        /* A nested if statement to do some padding on years less than xx10 */
        return "The "+day+suffix()+" of "+monthAsString()+" in the year 0"+year;
      }
      else
        return "The "+day+suffix()+" of "+monthAsString()+" in the year "+year;
    }
    else
      return "Date not set";
  }
 
  public void copy(Date other)
  /* I think this final thing is the equivalent of a buffer perhaps? */
  {
    day=other.day; month=other.month; year=other.year;
  }
}

As I've said, it works whether the date entered is valid or not.

What I want to do now is work out what day 29-02-00 was. Is there some sort of mathematical trickery that will do this? Ie, if I find out for myself what day 29-02-00 was according to the Gregorian calendar, could I then calculate all other days around that? Something like:

public static String [] daysOfWeek={
  "Monday","Tuesday","Wednesday","Thursday",
  "Friday","Saturday","Sunday"
  };
 
/* Rest of code here */
....
public String dayOfYear()
{
 
  /* This bit does some clever maths */
  {
    return daysOfWeek [answerOfCleverMaths-1];
  }
  else
    return "Day not set";
}

There must be an easy way to solve this? Anyone? Edit: I just need the logic or maths, thought someone may have already come across this problem and solved it, providing me with some useful tips :-)

Regards,

Shaun.

Why don't you just use the standard Date and Calendar classes?