Interesting Math Problem

[Rüdiger Werner]

"BEES INC" <bees.inc at gmail.com> schrieb im Newsbeitrag 
news:mailman.105.1212653740.1044.python-list at python.org...
...

Problem: Star Ratings

People can rate cheeseburgers on my website with a star rating of 0-5
stars (whole stars only), 5 being mighty tasty and 0 being disgusting.
I would like to show the average of everyone's ratings of a particular
cheeseburger to the nearest half star. I have already calculated the
average rating as a float (star_sum) and the total number of people
that rated the particular cheeseburger (num_raters). The result should
be stored as a float in a variable named "stars."
My Solution (in Python):


Well seems this is a typical half even rounding problem.

See

http://mail.python.org/pipermail/python-list/2008-April/485889.html

for a long discussion

However since your problem looks like a typical homework problem i made my 
example
buggy by intention.

However the result should be correct.

have fun!

def myround(x):
    d, m = divmod(x, 2)
    return 2*d + round(m - 1) + 1

num = 1.5
for _i in xrange(30):
    print num, ' -> ', myround(num * 2)/2
    num -= 0.1

>pythonw -u "test18.py"
1.5  ->  1.5
1.4  ->  1.5
1.3  ->  1.5
1.2  ->  1.0
1.1  ->  1.0
1.0  ->  1.0
0.9  ->  1.0
0.8  ->  1.0
0.7  ->  0.5
0.6  ->  0.5
0.5  ->  0.5
0.4  ->  0.5
0.3  ->  0.5
0.2  ->  0.0
0.1  ->  0.0
-1.94289029309e-016  ->  0.0    < --- hint for bug
-0.1  ->  0.0
-0.2  ->  0.0
-0.3  ->  -0.5
-0.4  ->  -0.5
-0.5  ->  -0.5
-0.6  ->  -0.5
-0.7  ->  -0.5
-0.8  ->  -1.0
-0.9  ->  -1.0
-1.0  ->  -1.0
-1.1  ->  -1.0
-1.2  ->  -1.0
-1.3  ->  -1.5
-1.4  ->  -1.5
>Exit code: 0












to avoid the bug have a look at:

http://mail.python.org/pipermail/python-list/2008-April/485934.html

and try this example:


num = 3
for _i in xrange(num * 10,-num *10, -1):
    if myround(float(_i)/10) != myround(num):
        print num, " -> ", myround(num ), " --- ",  myround(float(_i)/10)
        print "-----"
    num -= 0.1