Which is preferable; declaring & setting variable & calling twice, or calculating value on-the-fly twice?

Question

Which is the most efficient way to do this?

Sub pvaSetWeek(Optional weekOffset As Long = 0)
    Dim theDayToday As Long

    theDayToday = Weekday(Now, vbMonday)

    'Set start to Monday
    Range("pvaStartDate") = Int(Now) - (theDayToday - 1) - (weekOffset * 7)

    'Set end to Sunday
    Range("pvaEndDate") = Int(Now) + (7 - theDayToday) - (weekOffset * 7)
End Sub

or

Sub pvaSetWeek(Optional weekOffset As Long = 0)    
    'Set start to Monday
    Range("pvaStartDate") = Int(Now) - (Weekday(Now, vbMonday)- 1) - (weekOffset * 7)

    'Set end to Sunday
    Range("pvaEndDate") = Int(Now) + (7 - Weekday(Now, vbMonday)) - (weekOffset * 7)
End Sub

And why?

Edit to add: I usually go with the first way as it is easier to read/debug and obviously scales better if the same value needs used more than a couple of times, but where the value may only be used a few times I've often wondered if there is any penalty, however small, for doing things one way or another.

Answer 1

The first method is certainly better in terms of readability. I would prefer it.

In my opinion, one variable assignment will not be a noticeable overhead. Plus, the fact that you are writing VBA and not some time / mission critical close-to-the-processor kind of assembly code, means that your biggest worry should be the maintainability of code and not miniscule theoretical computation time efficiencies.

Answer 2

This may seem to be a trite answer... but have you tried timing it? If there is no "significant difference"* then work out which is more readable/maintainable.

** i'll leave that to you.

Answer 3

I think some "refactoring" is available. How about this?

Sub pvaSetWeek(Optional weekOffset As Long = 0)  
  Dim TheDateToday as byte, lngRangeBase as Long, lngRangeStart As Long, lngRangeEnd As Long  
  TheDateToday = Weekday(Now, vbMonday)  
  lngRangeBase = (weekOffset * 7)  

  lngRangeStart =  - ((theDayToday - 1) - lngRangeBase )  
  lngRangeEnd =  (7 - theDayToday) - lngRangeBase  

  'Set start to Monday  
  Range("pvaStartDate") = Int(Now) + lngRangeStart  
  'Set end to Sunday  
  Range("pvaEndDate") = Int(Now) + lngRangeEnd  
End Sub

Answer 4

Other answers are good.

I would just like to point out that SO contains many many questions of the form "Is X more efficient than Y?", like "Is ++i more efficient than i++?" or "Is inlining function calls more efficient than not?". The answer is, most often, as some contributor pointed out, like "getting a haircut to lose weight".

Sure it might actually make a difference, but don't assume it makes a significant difference until you prove that it does in your case.