如何在python中找到由两个三位数组成的最大回文？

Question

I was trying Problem 4 on Project Euler when I ran into a problem. Here is my code.

def Problem4():
    x = 100
    y = 909
    a = []
    b = []
    x1 = []
    y1 = []
    while y < 1000:
        while x < 1000:
            z = x*y
            if str(z) == str(z)[::-1]:
                a.append(z)
                x1.append(x)
                y1.append(y)
            else:
                b.append(z)
            x = x + 1
        y = y + 1       
    print(a)
    print(x1)
    print(y1)
Problem4()

This code runs okay, but the value of y remains constant, even after the y = y + 1 . Why is it doing that? Is there a better way of doing this.

Answer 1

I got the solution. The reason why y didn't increase was because the value of x didn't get reset from 1000 . It just automatically skipped that chunk of code because the value of x was already 1000. This is my improved code which also sorts the array in order.

def Problem4():

    y = 100
    a = []
    x1 = []
    y1 = []
    while y < 1000:
        y = y + 1
        x = 100
        while x < 1000:
            z = x*y
            if str(z) == str(z)[::-1]:
                a.append(z)
            x = x + 1

    a.sort()
    print(a)
Problem4()

Answer 2

move the x = 100 line to the position between first while loop and the second while loop would solve your problem.

def Problem4():
    y = 909
    a = []
    b = []
    x1 = []
    y1 = []
    while y < 1000:
        x = 100
        while x < 1000:
            z = x*y
            if str(z) == str(z)[::-1]:
                a.append(z)
                x1.append(x)
                y1.append(y)
            else:
                b.append(z)
            x = x + 1
        y = y + 1       
    print(a)
    print(x1)
    print(y1)
Problem4()

Answer 3

As for "better", you might want to use xrange():

a = []
for y in xrange(909, 1000):
  for x in xrange(100, 1000):
    x = x * y
    if str(z) = str(z)[::-1]:
      a.append(z)
a.sort()
print a