Why my code doesn't generate the desired result?

Question

What I want the code to do is to print the list containing numbers that are:

product of 2- or 3-digit numbers
palindrome
in the range [101101, 1000000).

There should be no 5-digit numbers and some 6-digit numbers in the final list as they are less than 101101. But there still are some 5-digit numbers after processing. Why is that happening?

list1 = []
for i in range(100, 1000):
    for j in range(100, 1000):
        if str(i*j) == str(i*j)[::-1]:   # checking for palindrome
            list1.append(i*j)


list1 = list(set(list1)) # removing duplicates

print(sorted(list1))
# print(len(list1))

for ii in list1:                      # removing numbers, out of range
    if ii < 101101 or ii >= 1000000:
        list1.remove(ii)


print(sorted(list1))
# print(len(list1))

But when I use sets to remove the elements out of range, it works. The below given code does the job.

set1 = set(range(10000, 101102))
list1 = list(set(list1) - set1)

But I am not understanding, why the previous code fails to print the desired output?

Edit: As 1 of you suggested, yes it is a duplicate of this. My bad for not checking for the question in the existing bank.

Possible duplicate of [How to remove item from a python list in a loop?](https://stackoverflow.com/questions/8312829/how-to-remove-item-from-a-python-list-in-a-loop) — bipll, Aug 28 '18 at 12:45
@Milan I'm going to favorite this question, so that when the phrase becomes popular, I can point people here to see the origins. At least that's the expected outcome ;) — Mad Physicist, Aug 28 '18 at 12:47

score 1 · Answer 1 · answered Aug 28 '18 at 12:52

This is the most simple way to solve your problem;

list1 = []
for i in range(100, 1000):
     for j in range(100, 1000):
        if str(i*j) == str(i*j)[::-1]:   # checking for palindrome
            list1.append(i*j)

# Use list comprehension to filter unwanted values from the list.
list1 = sorted([value for value in set(list1) if 101101 <= value < 1000000])


print(list1)

Mad Physicist · Accepted Answer · 2018-08-28T13:11:59.687

This is a textbook example of why you don't modify a list in-place. The culprit is this loop:

for ii in list1:                      # removing numbers, out of range
     if ii < 101101 or ii >= 1000000:
          list1.remove(ii)

The initial parts of list1 are all going to be 5 digits long, as you already noticed. List iterators step along by index. Let's take a look at what happens when you remove an element from a list of 5-digit numbers:

Start with a list, and ii referencing the first number:
```
  10001, 20002, 30003, 40004, ...
    ^
    ii
```
Remove ii from the list:
```
  20002, 30003, 40004, ...

    ii -> 10001
```
ii is still a valid reference, but not to an item in the list. The list has naturally shifted back by one element.

Continue the loop to the next element:

  20002, 30003, 40004, ...
           ^
           ii

Hopefully you can see how that skips adjacent elements that you want to filter out.

You have a number of viable workarounds. Here are a few to get you started:

Use a list comprehension to make a new list:

  list1 = [x for x in list1 if len(x) > 5]

Get rid of the offending loop entirely, and only append a string to the list on the first loop if it's longer than the retirement.

Iterate backwards, so the shifts don't affect you:

 for ii in reversed(list1):
      if ii < 101101 or ii >= 1000000:
           list1.remove(ii)

OR

 for ii in range(len(list1), -1, -1):
      if list1[ii] < 101101 or list1[ii] >= 1000000:
           del list1[ii]

Personally, I'd recommend option 2 since it's the least hassle in the end. You don't even need to check the length of the string, just whether the number is greater than 99999.

Why my code doesn't generate the desired result?

2 Answers2