[Tutor] Need help with dates in Python

[Andre Engels]

On Wed, Mar 9, 2011 at 9:21 AM, nookasree ponamala <nookasree at yahoo.com> wrote:
> Hi,
>
> I need help in finding the minimum date and maximum date in a file.
> Here is my test file:
> s.no:   dt1     amt     id1     id2
> 452     2010-02-20      $23.26      059542        06107
> 452     2010-02-05      $20.78      059542        06107
> 451     2010-02-24      $5.99       059542        20151
> 452     2010-02-12      $114.25     839745        98101
> 452     2010-02-06      $28.00      839745        06032
> 451     2010-02-12      $57.00      839745        06269
>
> I want to get the minimum and maximum dt1 for each id1
>
> Required result:
>
> id1 mindate maxdate
> 059542  2010-02-24      2010-02-20
> 839745  2010-02-06      2010-02-12
>
> Code: The code I tried. It doesn't work though.
>
> import sys
> import os
> t = ()
> tot = []
> maxyr = 2012
> minyr = 2008
> maxday = 31
> minday = 1
> maxmon = 12
> minmon = 1
>
> for line in open ('test2.txt','r'):
>        data = line.rstrip().split()
>        a = data[3]
>        b = data[1]
>        (year, month, day) = b.split('-')
>        year = int(year)
>        month = int(month)
>        day = int(day)
> if year > maxyr:
>        maxyr = year
> elif year < minyr:
>        minyr = year
> if month > maxmon:
>        maxmon = month
>        elif month < minmon:
>        minmon = month
>        if day > maxday:
>        maxday = day
>        elif day < minday:
>        minday = day
>        max = (maxyr,maxmon,maxday)
>        min = (minyr,minmon,minday)
>        t = (a,b,max,min)
>        tot.append(t)
>        print t
>
> Could you pls. help me with this.

I see several things go wrong. Here a list, which may well not be complete:

* You want the mindate and maxdate for each id1, but you remember only
a single minyr, maxyr etcetera. There's no way that that is going to
work.
* You initialize minyr etcetera to a date before the first date you
will see, nd maxyr etcetera to a date after the last date. This means
that you will never find an earlier respectively later one, so they
would never be changed. You should do it exactly the other way around
- minyr etcetera should be _later_ than any date that may occur, maxyr
etcetera _earlier_.
* You move "if year > maxyr" back to the left. This means that it is
not part of the loop, but is executed (only) once _after_ the loop has
been gone through
* year < minyear should be "if", not "elif": it is possible that the
new date is both the first _and_ the last date that has been found
(this will be the case with the first date)
* You change maxyear, maxmonth and maxday independently. That is not
what you are trying to do - you want the last date, not the highest
year, highest month and highest day (if the dates were 2001-12-01,
2011-11-03 and 2005-05-30, you want the maximum date to be 2011-11-03,
not 2011-12-30). You should thus find a way to compare the *complete
date* and then if it is later than the maxdate or earlier than the
mindate change the *complete date*
* At the end you show (well, in this case you don't because it is
under "if month > maxmon") a quadruple consisting of id1, current
date, lowest date and highest date - EACH time. You want only the
triple and only after the last date of some value of id1 has been
parsed (best to do that after all lines have been parsed)
* The code as shown will lead to a syntax error anyway because you did
not indent extra after "elif month < minmon:", "if day > maxday:" and
"elif day < minday:".




-- 
André Engels, andreengels at gmail.com