Euler problem number #4

Question

Using Python, I am trying to solve problem #4 of the Project Euler problems. Can someone please tell me what I am doing incorrectly? The problem is to Find the larg

Answer 1

If your program is running slow, and you have nested loops like this:

for z in range(100, 1000):
    for y in range(100, 1000):
        for x in range(1, 1000000):

Then a question you should ask yourself is: "How many times will the body of the innermost loop execute?" (the body of your innermost loop is the code that starts with: x = str(x))

In this case, it's easy to figure out. The outer loop will execute 900 times. For each iteration the middle loop will also execute 900 times – that makes 900×900, or 810,000, times. Then, for each of those 810,000 iterations, the inner loop will itself execute 999,999 times. I think I need a long to calculate that:

>>> 900*900*999999
809999190000L

In other words, you're doing your palindrome check almost 810 billion times. If you want to make it into the Project Euler recommended limit of 1 minute per problem, you might want to optimise a little :-) (see David's comment)