找出数字是否为 3 的幂的 Python 程序：给出不正确的输出

Question

def if_powerof_3(n):
    var = None
    power  = 0
    if n % 3 == 0:
        while var == 1:
            var = n / 3
            power = power + 1
            if not var % 3 == 0:
                return "{} is not a power of 3".format(n)
                break
            else:
                n = var
        else:
            return "{} is a power of 3".format(n)
    else:
        return "{} is not a power of 3".format(n)

print if_powerof_3(18)

Output : 18 is a power of 3

It is totally unexpected and a wrong output obviously but not able to figure where is my code going logically wrong and printing a mathematically incorrect output

Answer 1

why are you using so many if else it can be done much simpler like this

def if_powerof_3(n):
    var = n
    power  = 0
    while var>1:
        if var%3 ==0:
            var=var/3
            power=power+1
        else:
             return "{} is not a power of 3".format(n)
    return "{} is a power of 3".format(n)
print (if_powerof_3(27))

Answer 2

https://gist.github.com/bondnotanymore/9afc91cb1730c9b9f637326fafc69371

This is improved piece of code with a little more robustness added.

Output

C:\Python27\python.exe 
C:/Users/kapil/PycharmProjects/KapilsucksatPython/py/O.py
16 is not even a multiple of 3
27 is a number, which is 3th power of 3
21 is not a power of 3, but just a multiple of 3
243 is a number, which is 5th power of 3
729 is a number, which is 6th power of 3

Process finished with exit code 0