13

I am struggling with Python's sort().

My program iterates through every integer combination from 100 to 999 and checks if their products are palindromic. If they are, my program appends the product to a list. I need the list sorted.

Here's my program:

list = [] # list of numbers

for x in xrange(100,1000): # loops for first value of combination
  for y in xrange(x,1000): # and 2nd value
    mult = x*y
    reversed = str(mult)[::-1] # reverses the number
    if (reversed == str(mult)):
      list.append(reversed)

list.sort()
print list[:10]

Which nets:

['101101', '10201', '102201', '102201', '105501', '105501', '106601', '108801',
'108801', '110011']

Clearly, index 0 is larger then 1. Any idea what's going on? I have a feeling it's got something to do with trailing/leading zeroes, but I had a quick look and I can't see the problem.

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  • 2
    It's from Project Euler, I think. Commented Oct 3, 2009 at 14:01

8 Answers 8

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25

You are sorting strings, not numbers. '101101' < '10201' because '1' < '2'. Change list.append(reversed) to list.append(int(reversed)) and it will work (or use a different sorting function).

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Oh man... just when I thought I was graduating from noob, to rookie ;) Thanks!
13

Sort is doing its job. If you intended to store integers in the list, take Lukáš advice. You can also tell sort how to sort, for example by making ints:

list.sort(key=int)

the key parameter takes a function that calculates an item to take the list object's place in all comparisons. An integer will compare numerically as you expect.

(By the way, list is a really bad variable name, as you override the builtin list() type!)

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2

Your list contains strings so it is sorting them alphabetically - try converting the list to integers and then do the sort.

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1

You're sorting strings, not numbers. Strings compare left-to-right.

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1

No need to convert to int. mult already is an int and as you have checked it is a palindrome it will look the same as reversed, so just:

list.append(mult)
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0

You have your numbers stored as strings, so python is sorting them accordingly. So: '101x' comes before '102x' (the same way that 'abcd' will come before 'az').

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0

No, it is sorting properly, just that it is sorting lexographically and you want numeric sorting... so remove the "str()"

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2 Comments

On a seperate topic, your program could e optimized. Hint : generate the numbers..
Yeah I know, and I might compile them later, but it's quick enough for the task at hand.
0

The comparator operator is treating your input as strings instead of integers. In string comparsion 2 as the 3rd letter is lexically greater than 1. reversed = str(mult)[::-1]

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