Most elegant way to morph this sequence

Question

I've got the Day of the week stored in a database table (that I do not control), and I need to use it in my code.

Problem is, I want to use the System.DayOfWeek enum for representation for this, and the sequences are not the same.

In the database, it's as follows:

1  2  3  4  5  6  7
S  M  T  W  T  F  S

I need it as follows:

0  1  2  3  4  5  6
M  T  W  T  F  S  S

What's the most elegant way to do this?

for example, I could do:

i = dayOfWeek;
i = i - 2;
if (i < 0) {
    i = 6;
}

but that's a bit inelegant. Any suggestions?

<EDIT>

Ahem. Apparently (.net reflector says) DayOfWeek is 0 indexed starting with Sunday.

Always read the docs before asking daft questions.

However, I'm still interested in an answer, just to satisfy my own curiosity, so go for it.

</EDIT>

Answer 1

The value you want is

(DayOfWeek)((dbDay + 5) % 7)

using the modulo operator % .

Answer 2

Wrap it in a function:

public int DbToDayOfWeek(int dbDay)
{
   if (dbDay == 1)
     return 6;

   return dbDay  -2;

}

Or:

public DayOfWeek DbToDayOfWeek(int dbDay)
{
   if (dbDay == 1)
     return DayOfWeek.Sunday;

   return (DayOfWeek)(dbDay - 2);

}

Answer 3

Although I can't imagine the values changing, you should really avoid assuming that the DayOfWeek enumerated values will stay the same - so code accordingly.

static DayOfWeek[] _toDaysTable = new DayOfWeek[] {
    DayOfWeek.Sunday, DayOfWeek.Monday, DayOfWeek.Tuesday, DayOfWeek.Wednesday,
    DayOfWeek.Thursday, DayOfWeek.Friday, DayOfWeek.Saturday
};

static DayOfWeek ToDayOfWeek(int myDayOfWeek)
{
     int index = myDayOfWeek - 1;
     if (index < 0 || index >= _toDaysTable.Length)
          throw new ArgumentOutOfRangeException("myDayOfWeek");
     return _toDaysTable[index];
}

static int FromDayOfWeek(DayOfWeek day)
{
    int index = Array.IndexOf(_toDaysTable, day);
    if (index < 0)
        throw new ArgumentOutOfRangeException("day");
    return index + 1;
}

Answer 4

You could just create your own enum and map the enum directly to the value in the database

public enum DayOfWeek
{
    Mon = 2,
    Tue = 3,
    Wed = 4,
    Thu = 5,
    Fri = 6,
    Sat = 7,
    Sun = 1
}

Then you could use an extension method of your DayOfWeek type to retrieve the value:

public static int ToInt(this DayOfWeek dow)
{
    return (int)dow;   
}

Unless you are relying on the DayOfWeek for actual comparisons with Dates, otherwise you will have to do the conversion between the offsets.