3.7.37 Types Tutorial Exercise 5

Let's suppose it's January 14, 1991. The easiest thing to do is to keep trying 13ths of months until Calc reports a Friday. We can do this by manually entering dates, or by using t I:

     1:  <Wed Feb 13, 1991>    1:  <Wed Mar 13, 1991>   1:  <Sat Apr 13, 1991>
         .                         .                        .

         ' <2/13> <RET>       <DEL>    ' <3/13> <RET>             t I

(Calc assumes the current year if you don't say otherwise.)

This is getting tedious—we can keep advancing the date by typing t I over and over again, but let's automate the job by using vector mapping. The t I command actually takes a second “how-many-months” argument, which defaults to one. This argument is exactly what we want to map over:

     2:  <Sat Apr 13, 1991>     1:  [<Mon May 13, 1991>, <Thu Jun 13, 1991>,
     1:  [1, 2, 3, 4, 5, 6]          <Sat Jul 13, 1991>, <Tue Aug 13, 1991>,
         .                           <Fri Sep 13, 1991>, <Sun Oct 13, 1991>]
                                    .

         v x 6 <RET>                  V M t I

Et voilà, September 13, 1991 is a Friday.

     1:  242
         .

     ' <sep 13> - <jan 14> <RET>

And the answer to our original question: 242 days to go.