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Re: Only one Friday 13th coming in 2016

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On Tue, Dec 22, 2015 at 04:04:16AM +0100, Ángel González wrote:
> Aren't you making things more complex than needed, with so much pipes
> and awk?
> 
> date(1) is your friend:
> 
> For instance:
>  $ for y in {1900..2199} ; do echo -n "$y "; for m in {1..12}; do date +%A -d $y-$m-13; done | grep -c Friday ; done
> 
> shows there are between 1 and 3 Fridays per year.

This is the most obvious approach, but it does a fair amount of forking.
Also, you're relying on GNU date.  There's no portable way to do simple
date lookups in a shell script, unfortunately.

A pure-bash approach that avoids forking might look something like:

t=946702800	# Start at Sat Jan  1 12:00:00 EST 2000
endyear=2036

while true; do
  printf -v year '%(%Y)T' "$t"
  ((year > endyear)) && break
  printf -v day  '%(%d)T' "$t"
  printf -v dow  '%(%w)T' "$t"
  if [[ $day = 13 && $dow = 5 ]]; then
    printf -v month '%(%m)T' "$t"
    echo "$year-$month-$day"
  fi
  ((t += 86400))
done

But just because it doesn't fork, doesn't mean it's *fast*.  Bash is so
slow at everything. :(  Your one-fork-per-month loop (plus one fork per
year) might end up being much faster than my zero-forks-per-day loop.
Mine is portable, though.

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Re: Only one Friday 13th coming in 2016 Greg Wooledge <wooledg@eeg.ccf.org> - 2015-12-22 08:16 -0500
  Re: Only one Friday 13th coming in 2016 Ralf Goertz <me@myprovider.invalid> - 2015-12-22 14:43 +0100

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