查找给定月份的第一天的星期几

Question

How could be calculated a day of the week if we know the day number of the first day in month? Lets say we have 1..7 days in a week I want to get number of the 4th day in the month if the 1st = 5 (Friday) then result should be 1 (Monday).

1st - 5 Friday
2nd - 6 Saturday
3rd - 7 Sunday
4th - 1 Monday

(a=4, b=5) = 1

I tried to calculate the common formula:

result = (a + b - 1) % 7 

So it works for all cases except the case when a = 3, 10, 17, 24, 31, because result = 0, but should be 7. How can it be fixed to get this formula work for all days?

Answer 1

You need to avoid the result zero. Here is one way:

result = (a + b - 2) % 7 + 1

You subtract one more from your sum, to allow zero and work on the previous day, then you take the remainder modulo 7 to get the day which can include zero, then add one to get to the day wanted and avoid zero. Note that the order of operations will do the modulus before adding one. If you want to make that more explicit, you could use

result = ((a + b - 2) % 7) + 1