List Comprehension doesn't work as expected

Question

In trying to use list comprehension more often I tried to code golf an assignment. So this works:

if fraction[0] % i == 0 and fraction[1] % i == 0:
            fraction = [fraction[x]/i for x in range(2)]

But this doesn't work:

fraction = [fraction[x]/i for x in range(2) if fraction[0] % i == 0 and fraction[1] % i == 0]

It gives a list index out of range error

Fraction is a two item list and the if statement has a specific indices which should never be out of range... thoughts?

Answer 1

The issue is that in the second case, fraction always gets overridden.

So for example, if fraction = [2, 4] and i = 3 , your list comprehension will result in fraction == [] , while the loop will just do nothing.

UPDATE

Here's the code I tested with:

def a(fraction, i):
    if fraction[0] % i == 0 and fraction[1] % i == 0:
        fraction = [fraction[x] // i for x in range(2)]
    return fraction

def b(fraction, i):
    fraction = [fraction[x] // i for x in range(2) if fraction[0] % i == 0 and fraction[1] % i == 0]
    return fraction

print(a([2, 4], 3)) # Output: [2, 4]
print(b([2, 4], 3)) # Ouptut: []

GOLF UPDATE

I think if you can reverse the order you're testing values in, your list comprehension can work. Eg:

f = next([n//i for n in f] for i in range(max(f),0,-1) if all(n%i==0 for n in f))

Test:

for fraction in ('2/4', '1/5', '4/2', '1/9'):
    f = [int(x) for x in fraction.split('/')]
    f = next([n//i for n in f] for i in range(max(f),0,-1) if all(n%i==0 for n in f))
    print('/'.join(map(str,f)))

# Output:
# 1/2
# 1/5
# 2/1
# 1/9