Python嵌套循环-无输出

Question

I've just started learning python. I am trying to check if the integer x is palindrome then divide it by a number between range (starting from largest y ie 999 ) y=999,998,...,100. If x/y=z and z is also a 3 digit integer then finish. Otherwise subtract 1 from x and do the same procedure.

def EuQ4():
    x=998001
    p=999
    while 10000 < x:
        x=x-1
        if str(x)== str(x)[::-1]:
            while p>100:
                if x%p==0:
                    Pal=x/p
                    if Pal < 999:
                        print (Pal,p)
                        break
                    else:
                        x=x-1
                else:
                    p=p-1
        else:
            x=x-1
EuQ4()

This is question 4 from Project Euler ie Find the largest palindrome made from the product of two 3-digit numbers.

Answer 1

You have a few logic errors in here. Some cause loops that just never end. For example, what happens when x % p == 0 but Pal is larger 999 ? You would get an infinite loop.

I made a few modifications, but it could still use some work.

def EuQ4():
    x = 998001
    while 10000 < x:
        if str(x) == str(x)[::-1]:
            print("{} is a pali!".format(x))
            # Move it here so each time it stats at the right
            # number or else it will just skip it after it does it once.
            p = 999
            while p > 100:
                if x % p == 0:
                    pali = int(x / p)
                    if pali < 999:
                        print(pali, p)
                        return
                p -= 1
        x -= 1

EuQ4()

Edit:

I found these mistakes by using the debugger in my IDE. You could have easily done the same thing by going through the code line by line a few times.

Answer 2

I am sorry but it was hurting my head to read your question. If you are trying to learn Python while attempting these questions then I would propose this alternate answer - it does not answer your question but it does lead to the solution and I think it is more Pythonic. The question asks to find the largest palindrone made from the product of two 3 digit numbers. So the inputs should be 3 digit numbers. This code will allow you to specify the number of digits, max and min (as integers).

I am not proposing that this be the best solution the the Euler Problem posed rather it is a solution that gives you exposure to a range of features in Python.

def min_value(integer):
    min_val = '1'
    for n in range(0,integer-1):
       min_val+='0'
    return int(min_val)

def max_value(integer):
    max_val = '9'
    for n in range(0,integer-1):
       max_val += '9'
   return int(max_val) +1

def find_max_palindrones(x,y):
    minimum_value = min_value(x)
    maximum_value = max_value(y)
    palindrones = []
    working_range = [number for number in range(minimum_value,maximum_value,1)]
    for x_value in working_range:
        for y_value in working_range:
            product = x_value * y_value
            orig_order = [item for item in str(product)]
            rev_order = [item for item in str(product)[::-1]]
            if orig_order == rev_order:
                palindrones.append(product)
    max_p = max(palindrones)
    return max_p

>>>find_max_palindrones(3,3)
906609

Answer 3

Put p=999 befor while p > 100 or use for p in range(999, 100, -1) .

p = 999
while p > 100

And I think you call x=x-1 too many times.