Program to find the largest odd number among three variables

Question

def greatest(a,b):
    if a>b:
        return a
    return b
def odd_check(a):
    return not(a%2 == 0)
def go(x,y,z):
    a = odd_check(x)
    b = odd_check(y)
    c = odd_check(z)
    if a and b and not c:
        print greatest(x,y)
    elif a and not b and c:
        print greatest(x,z)
    elif not a and b and c:
        print greatest(y,z)
    elif not a and not b and c:
        print z
    elif not a and b and not c:
        print y
    elif a and not b and not c:
        print x
    elif a and b and c:
        print greatest(x,greatest(y,z))
    else:
        print "None of them are odd"
go(int(input()),int(input()),int(input()))

Doing the problem it felt like I am going over all the eight combinations \$2^3\$ for three variables. Is there any way to avoid this?

Answer 1

You could filter out the even numbers, and then get the maximal value:

def odd_check(a):
    return not(a%2 == 0)

def go(x, y, z):
    l = [e for e in [x, y, z] if odd_check(e)]
    try:
        print(max(l))
    except ValueError:
        print("None of them are odd")

Answer 2

And for those who love comprehensions...

def max_odd(*args):
     return max(x for x in args if x%2)

Answer 3

A clearer algorithm would be to filter out the even numbers and then just pick the highest value.

Here's a quick implementation using higher order functions. If you prefer list comprehensions you could probably do it just as cleanly.

def highest_odd(*args):
    odds = filter(lambda x: x % 2 != 0, args)
    if odds:
        return max(odds)
    else:
        return None

highest_odd(5,10,15) # = 15
highest_odd(1,2,4) # = 1

Answer 4

Instead of checking all 3 variables at once you could check two together and then the larger one with the remaining one.

Example: 3, 9, 1

3, 9 -> 9 is larger
9, 1 -> 9 is larger
-> 9 is the largest

Answer 5

Building on @enedil s answer, I would return the max and not print it, allowing re-usability of your function. I would also make it accept a variable number of input parameters:

def odd_check(a):
    return not(a%2 == 0)

def max_odd(*args):
    try:
        return max(e for e in args if odd_check(e))
    except ValueError:
        raise ValueError("None of the variables is odd")

args = [input() for _ in range(3)]
print(max_odd(*args))
print(max_odd(1,2,3))
print(max_odd(1,3,5,7,9))

I also gave the function a more descriptive name.

Answer 6

PiePi, this looks to have been as useful a learning experience for me as asking the question is for you. The approach you took is simply a laborious way round, not wrong, My struck out answer reveals my long-winded approach based on my history of using C and C++(hence it is struck out rather than simply edited away). However a quick look through the python inbuilt function reveals a few useful pieces:

• filter and
• max

These two functions along with a simple checker (which you already have) for the oddness of the number allow for a simple fast and effective approach.

Obliterating the bad code below (damnable habits from C/C++ I spent 5 minutes researching a more pythonic way (Thanks to @Joe-Wallis for politely pointing out!:^) )

now the code sums up to 2 two-line functions:

def IsOdd(intIn):
    return not(intIn%2==0)

def GetLargestOdd(x,y,z):
    return max(filter(IsOdd, (x,y,z)))

Ohh and used like:

print(GetLargestOdd(1,2,3))

No re-invention of the wheel or needless variables:)