- OR -
Shell Scripting for Pedantry's Sake.
Today's "That can't be true!" moment hit me when I started seeing this making the rounds (in various different paraphrased versions) on Teh Intarwebs:
"This month has 5 Fridays, 5 Saturdays and 5 sundays-Only happens every 823 years!"
Truth be known, I don't really care about how many weekends are in a month except for the fact that I get three paychecks this month. That happens about twice per year, and that's always welcome! Once in a while, though, I just can't help it. I have to disprove something. I figured the easiest way to disprove this particular claim would be to write a shell script that used the "cal" tool, found in every unix variant known to mankind.
For there to be 5 Fridays, Saturdays and Sundays in a single month, there is a basic requirement for a 31-day month that begins on a Friday, and only then will the 31st fall on a Sunday to complete 5 "whole weekends" in one month.
Initially, I was thinking of ways to see what months started on a Friday. That would get me close. It would give me months such as February 2013, which have only 28 days. Then it hit me: Look for any month with a 31st day that falls on Sunday. Using "cal," I can simply roll through the calendar year looking for a line that begins with "31" and guarantee that the month will satisfy the requirements of having five Fridays, Saturdays and Sundays.
So here we go!
#!/bin/sh
ye=2010
mo=1
while true
do
until [ $mo -gt 12 ]
do
cal=`cal $mo $ye | grep ^31`
if [ -z "$cal" ]
then
echo -n ""
else
echo
cal $mo $ye
fi
mo=`expr $mo + 1`
done
mo=1
ye=`expr $ye + 1`
done
Output:
January 2010
Su Mo Tu We Th Fr Sa
1 2
3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31
October 2010
Su Mo Tu We Th Fr Sa
1 2
3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31
July 2011
Su Mo Tu We Th Fr Sa
1 2
3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31
March 2013
Su Mo Tu We Th Fr Sa
1 2
3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31
August 2014
Su Mo Tu We Th Fr Sa
1 2
3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31
May 2015
Su Mo Tu We Th Fr Sa
1 2
3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31
I don't know. It looks like these things happen more than every 823 years. You can rest easy knowing that it will actually happen a total of 825 times in the next 823 years. Yep, I counted them.
One of the derivatives mentioned October specifically, though. Perhaps this only happens once every 823 Octobers?
Slightly modified, we make the script check Octobers...
#!/bin/sh
ye=2010
mo=10
while true
do
cal=`cal $mo $ye | grep ^31`
if [ -z "$cal" ]
then
echo -n ""
else
echo
cal $mo $ye
fi
ye=`expr $ye + 1`
done
Output:
October 2010
Su Mo Tu We Th Fr Sa
1 2
3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31
October 2021
Su Mo Tu We Th Fr Sa
1 2
3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31
October 2027
Su Mo Tu We Th Fr Sa
1 2
3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31
October 2032
Su Mo Tu We Th Fr Sa
1 2
3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
31
Nope. More than a decade at times, but not 823 years.
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