Leah Culver, well rounded programmer.

Here, [spoiler]/prog/[spoiler], have some memento:

http://web.archive.org/web/20070504110113/www.leahculver.com/2007/04/19/star-ratings/

In case she learns about robots.txt:


Star Ratings

I’ve been awfully busy programming lately. My Django-based side project is coming along well and I hope to have it ready for use in a few weeks. Please don’t ask more about it, that’s really all I can say for now. Anyways, I came across an interesting little math problem today and was hoping some skilled programmers out there could come up with a more elegant solution than mine.

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):
# round to one decimal place and
# separate into whole and fractional parts
parts = str(round(star_sum/num_raters, 1)).split('.')
whole = int(parts[0])
frac = int(parts[1])
if frac < 3:
___frac = 0
elif frac > 7:
___frac = 0
___whole += 1
else:
___frac = 5
# recombine for a star rating rounded to the half
stars = float(str(whole)+’.'+str(frac))

Mmmm… In-N-Out Burgers… Please leave a comment if you’ve got a better solution.

Update (hooray!)
I’ve updated my solution to:
stars = round(star_sum*2/num_raters)/2

I knew there was a way to do this mathematically, I just couldn’t remember how. I suppose the same trick would work for rounding to thirds, fourths, etc. Thanks so much to r, Christopher Finke, and Bob for the better approach.


_____



1. r Says:
April 19th, 2007 at 2:11 pm

might be misunderstanding what you’re trying to do, but

formatting an average value to the nearest half (forgive the lack of python syntax):

round(AvgValue * 2) / 2

should work

---

2. Christopher Finke Says:
April 19th, 2007 at 2:58 pm

If you’re willing to change your boundaries to .75 and .25 (rather than .7 and .3), you could do this:

stars = float(round(star_sum / num_raters * 2) / 2);

---

3. J Taylor Says:
April 19th, 2007 at 5:55 pm

#I don’t program in Python and I’m not sure if this code has the correct syntax
#Calculate rating out of 10
rating = round(star_sum*2/num_raters, 1)
#Calculate number of whole stars by integer division
whole_stars = rating // 2
#Calculate number of half stars by remainder
half_stars = rating % 2
#Combine into one number
stars = whole_stars + (half_stars * 0.5)

---

4. Bob Ippolito Says:
April 19th, 2007 at 6:02 pm

In general you should never have to convert a number to a string. If you needed to get the fractional and whole parts of a float there’s math.modf(someFloat) which returns (frac, whole). Anyway, I would probably just double the sum, round, then half it.

stars = 0.5 * round((2.0 * star_sum) / num_raters)

---

5. Alexander Repty Says:
April 20th, 2007 at 12:13 am

I gave this to some of my co-workers (system administrators and software developers) as a challenge. Only 2 out of 7 could get it right (both software developers - one of them an apprentice).

I immediately knew the solution when I finished reading the problem at hand, and I would have done it exactly like Bob did. I guess it’s just the experience in solving problems like this one that enables you to see this solution.

---

6. Richard Kiss Says:
April 20th, 2007 at 10:11 am

Here’s an idea:

stars = float(str(round(float(star_sum)/num_raters,1)).split(’.')[0]) + {’0′:0, ‘1′:0, ‘2′:0, ‘3′:0.5, ‘4′:0.5, ‘5′:0.5, ‘6′:0.5, ‘7′:0.5, ‘8′:1.0,’9′:1.0}[str(round(float(star_sum)/num_raters,1)).split(’.')[1]]

or more concisely,

stars = round(float(star_sum+star_sum)/num_raters)/2

---

7. leah.culver Says:
April 20th, 2007 at 10:52 am

Alexander - seems to me like something people would remember if they had used it before. I’d never had to round like that before and I kept thinking that there was some way to do it mathematically, but I couldn’t remember how. I hope it’s not as important to know the best solution as it is to know a better solution exists.

Richard and Christopher - I suppose I was being too generous with .7. I don’t mind the rounding difference and I doubt the cheeseburgers will either.

Everybody - Thanks so much for the help! I suppose I could have looked around more for the solution but now I feel like my blog was actually useful.

---

8. Sam Purtill Says:
April 21st, 2007 at 1:48 pm

Hey Leah… Why Django over Ruby on Rails? I picked Django too, and we all have our reasons. Write a post about it!!!

---

9. Tom Says:
April 23rd, 2007 at 8:07 am

A perfect example of why everyone’s first course in programming should be Fortran.