More String Manipulations (Education Vector, July 1990)

Transcribed from the printed issue; not yet reviewed. Read it in the PDF of the issue, page 21

More String Manipulations (Education Vector, July 1990)

In this article, Alan Sykes gives a little function named DAYOFWEEK which, given the month and year, finds the weekday number (starting with Monday). Alan attributes DAYOFWEEK to Jerry Turner and professes to not even trying to understand it. As they say: “Me too, neither!”. Instead, I give below an alternative called WEEKDAY, which I believe is rather easier to understand:

      ∇WD←WEEKDAY;CONST;YRMO;DAYS
[1] ⍝ Get day of the week     Peter B  Util  Apr 91
[2] CONST←0 8 25 5 1 9 5 13 21 17 25 21
[3] YRMO←(⎕TS[1]-1900)+CONST[⎕TS[2]]
[4] WD←7|⎕TS[3]+(7|YRMO)+⌊((28|YRMO)+3)÷4
[5] ⍝
[6] ⍝ To return name of day, instead of no, add next 2 lines
[7] ⍝ DAYS←7 4⍴'SUN MON TUE WED THU FRI SAT '
[8] ⍝ WD←DAYS[WD;]
      ∇

Note the different calling syntax since it doesn’t need any arguments at all. Also there are a suggested couple of fairly obvious extra lines which can be de-commented to give the name of the day, instead of the number, if preferred. Note also that I am a bit more conventional, so Sunday is Day 1 here.

On the same sort of theme, another sometimes-useful utility function, using a similar technique, is JULDAY which gives the Julian Day number:

      ∇JD←JULDAY
[1] ⍝ Get Julian day number  Peter B  Util  Nov 90
[2] JD←0 31,(28+0=4|⎕TS[1]),31 30 31 30 31 31 30 31 30
[3] JD←⎕TS[3]++/⎕TS[2]↑JD
      ∇