[Tutor] Removing duplicates

[Oscar Benjamin]

On 1 August 2018 at 21:38, Roger Lea Scherer <rls4jc at gmail.com> wrote:
>
> I'm trying to get a list of tuples to be a float, a numerator, and a
> denominator for all the fractions: halves, thirds, fourths etc up to
> ninths. 1/2 returns the same float as 2/4, 3/6, 4/8. I would like to keep
> only the 1/2. When I try (line 18) to "pop"  from the list I get a "TypeError:
> integer argument expected, got float". When I try (line 18)  to "remove"
> from the list, nothing happens: nothing is removed and I do not receive an
> error message.
>
> What do you think is a good way to solve this?
>
> Thank you as always.
>
> import math
>
> fractions = [(0, 0, 0)]
>
> for i in range(1, 10):
>     for j in range(1, 10):
>         if i < j:
>             x = i/j
>             if x not in fractions:
>                 fractions.append((x, i, j))
>     sortedFrac =  sorted(fractions)
>
> print(sortedFrac)
>
> for i in range(len(sortedFrac)):
>     try:
>         if sortedFrac[i][0] == sortedFrac[i-1][0]: # so if the float equals
> the previous float
>             sortedFrac.pop(sortedFrac[i][0])           # remove the second
> float
>         else:
>             sortedFrac.append(sortedFrac[i][0])
>     except ValueError:
>         continue

Comparing floats for equality can be flakey. Sometimes two floats that
should be equal will not compare equal e.g.:

>>> 0.01 + 0.1 - 0.1 == 0.01
False

This happens in this case because of intermediate rounding errors. I
don't think that should affect you since you are doing precisely one
floating point operation i/j to calculate each of your floats but
unless you have a very solid understanding of binary floating point I
would recommend that you shouldn't compare floating point values as in
a==b.

For this particular problem I would use integer arithmetic or I would
use the fractions module. Doing this with integers you should
normalise the numerator and denominator by dividing out their GCD. The
fractions module takes care of this for you internally.
https://docs.python.org/3.7/library/fractions.html

Otherwise in general to remove duplicates you would be better of with
a set rather than a list. If you only put x in the set and not i and j
then the set will automatically take care of duplicates:
https://docs.python.org/3.7/tutorial/datastructures.html#sets

--
Oscar