How to do a "circular list" in Python

[trilobite]

Hi all -

I'm wanting to do a simple text-based calendar app in Python (it'll be public-domain). ( Btw, I'm running Feisty, with Python 2.5.1).

I've done a bit of "background thinking" on this, as follows -
*** Start of "background notes" ***
Any month can be described with a 3-digit code, where the first two digits are the number of days in the month (28-31), and the last digit is the day of the week that the month starts on.

So, Jan 2007 is shown by 312 - 31 day, starts on Monday (2).
( I'll use 1 for Sunday, through to 7 for Saturday).

So, the month-structure of 2007 is as follows -
Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec
312, 285, 315, 301, 313, 306, 311, 314, 307, 312, 305, 317.

2008 will be as follows -
Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec
313, 296, 317, 303, 315, 301, 313, 316, 302, 314, 307, 312.

We can use "clock arithmetic" (also called "modulo arithmetic") to change the weekdays from year to year.
A "clock" with 7 "hours", representing the days of the week. will be used. This could be done with a "circular list".
*** End of background notes ***

I've already found some very-useful p.d. Python code for doing tables. Now, I just need to do the circular lists and go from there, then merge that code with the table code. However, I've got a bit of a "mental block" as to how to do a circular list.

A good "test-case" is Dec this year (which is a "317") which will then be a "312" next year (as it starts on a Monday).
( Leap-year next year, remember.... ;-) )
So, there's a good example of the "wrap-around" that I want.

So, anyone able to help on doing circular lists? Many thanks in advance.... bye foe now -
- trilobite

[aquavitae]

I can't help with the circular list, but can I ask why you're trying to do it this way - what's the list to be used for? If you're simply trying to get a way of identifying each month I'd have thought that 200712, 200801, etc would be easier? Or better yet use a tuple (2007, 12) (2008, 1) etc.

[steve.horsley]

What do ou want to put in the list? The numbers 0-6? I would think that just using modulo arithmetic would be better suited to that. Frinstance,
6 % 7 = 6
7 % 7 = 0
8 % 7 = 1
etc.

You could do a circular list if you want.

[triptoe]

Edit: whoops. it should be:




if ((week+7) % 7 == 0:
y = '7'

else:
y = str((week+x) % 7



Do you mean you are not allowed or can't use the modulo? I am not sure what you meant but here it is with modulo

if (week+x) > 7:
y = str((week + x) % 7)
else:
y = str(week+x)

you could also use a bunch of if statements but that would be tedious...




# Returns whether year is leap or not
def isLeap(year):
if year % 400 == 0:
return True
elif year % 4 == 0 and year % 100 != 0:
return True
return False
# counts how many times leap year has occurred since 2008 in the year given
def leapCount(year):
counter = 0
i = 1
while i <= (year-2008):
if isLeap(2008 + i):
counter += 1
i += 1
return counter
year2008 = ['313','296','317','303','315','301','313','316','3 02','314','307','312']

#print year2008

year = input("Enter a year greater than 2008: ")

# find how many days should be shifted over
week = (year - 2008) + leapCount(year)
# shift
newYear = []
j = 0
for i in year2008:
x = int(i[-1])
if (week+x) > 7:
y = str((week + x) % 7)
else:
y = str(week+x)
newYear.append(year2008[j].replace(year2008[j], year2008[j][:2] + y))
j += 1

print newYear

[trilobite]

Hi all -

Thanks very much aquavitae, steve.horsley, triptoe for your replies!

I'm just doing this to get into Python a bit more (and yes, the modulo operator will do the trick - no need for "circular lists"... :-) ) I should have looked more closely at the Python docs ;-)

Thanks again - bye for now -
-trilobite

[slavik]

circular lists are easily handled by doubly linked lists.

here's an idea of making a circular list in perl:


sub getitem {
my $i = shift;
return $array[$i%($#array+1)];
}