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When I input DayName[{2012,12,24}] I get Monday as output. What is the most effective way to convert Monday into 1, Tuesday into 2 etc.?

To do this, I can create a length-7 array to store all the day-name and then do some matching and return the matched position. But that approach is too clumsy. Is there any other lighter and faster way?

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2 Answers 2

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The most efficient way is not to use DayName but this:

DateValue[{2012, 12, 24}, "ISOWeekDay"]

Regarding Robert's comment

Well, that was a correct answer but not on the exact question. What if I don't have a date but just a weekday as e.g. Monday how do I now get the ISOWeekDay number. Without explicitly coding. Is there a a ready to use MMA function f with f[Monday] == 1

the shortest solution I found is

ISODay = System`DateObjectDump`$dowAssociation;

ISODay[Monday]
(* 1 *)
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  • $\begingroup$ Well, that was a correct answer but not on the exact question. What if I don't have a date but just a weekday as e.g. Monday how do I now get the ISOWeekDay number. Without explicitly coding. Is there a a ready to use MMA function f with f[Monday] == 1 $\endgroup$
    – Robert Nowak
    Commented Jul 18, 2019 at 18:14
  • $\begingroup$ @RobertNowak See my edit above. $\endgroup$
    – halirutan
    Commented Jul 18, 2019 at 18:59
  • $\begingroup$ May I ask by which black spell incantation you discovered this dowAssociation? $\endgroup$
    – Roman
    Commented Jul 18, 2019 at 21:05
  • $\begingroup$ I was quite sure that at some point, WL's functions need a conversion from day-name to number, because how else would they implement the "ISOWeekDay". So I started digging with PrintDefinitions[DateValue] and followed the trail... $\endgroup$
    – halirutan
    Commented Jul 19, 2019 at 13:02
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ISODay[day_] := Mod[With[{o = {1, 1, 0}}, DateDifference[
  DayPlus[o, 1, Monday],
  DayPlus[o, 1, day]
  ][[1]]], 7] + 1
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