for loop within a list

Question

I want to get a ordered list with ascending order, but leave out the first element in the original list. But the first for-loop only runs 4 times. After 4 times loop in first for-loop:

lst=[6, 5, 4]

I think the first for-loop should keep running, but it finishes. I don't know why?

Source code below:

#coding: utf-8
lst = [0, 1, 6, 2, 5, 3, 4]
new_lst = []  # the ordered list, [0, 1, 2, 3, 4, 5, 6]
p0 = lst[0]  # the first element can be arbitrary
lst.remove(p0)
new_lst.append(p0)
for temp in lst:
    lst.remove(temp)
    while 1:
        for pt in lst:
            if temp < pt:
                continue
            else:
                lst.append(temp)
                temp = pt
                lst.remove(pt)
                break
        else:
            new_lst.append(temp)
            break

print("the ordered list:", new_lst)

At last, I only get:

('the ordered list:', [0, 1, 2, 3])

Welcome to StackOverflow. Please read and follow the posting guidelines in the help documentation. Minimal, complete, verifiable example applies here. We cannot effectively help you until you post your MCVE code and accurately describe the problem. We should be able to paste your posted code into a text file and reproduce the problem you described. "cannot be accomplished correctly" is not a problem specification. — Prune
– Prune, Commented Jan 24, 2018 at 1:28
Can you just use sorted? A lot simpler that way (new_lst = sorted(lst[1:]) could replace it all). Either way, you can't safely mutate a list while iterating it. — ShadowRanger
– ShadowRanger, Commented Jan 24, 2018 at 1:31
@fanronghong: As I stated before, the "why" is that it's not safe to mutate a list while iterating it; in practice on CPython, it doesn't error out, but if you remove the item you're currently on, it ends up skipping the following item in the iteration. Click the link in this comment, and view the other linked questions from there; several of them are of the form "I call list.remove in a loop that iterates over the list and it breaks, why?" — ShadowRanger
– ShadowRanger, Commented Jan 24, 2018 at 2:01

rnso · Accepted Answer · 2018-01-24 01:38:03Z

1

To ignore first element in sorting:

newlst = [lst[0]]+sorted(lst[1:])

First element lst[0] has to be enclosed in [ ] to get a list which can be extended using + with rest of the list (lst[1:]).

edited Jan 24, 2018 at 1:38

answered Jan 24, 2018 at 1:36

rnso

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2 Comments

Michael Ilie Over a year ago

Yeah the User isn't very clear as to what he wants. possible flag?

james-see Over a year ago

I think this is exactly what user wanted.

Michael Ilie · Accepted Answer · 2018-01-24 01:35:54Z

0

try new_lst = sorted(lst). This sorts a list by integer value (and ascii value if characters are present). This is a much nicer solution for your problem and is way more pythonic.

edited Jan 24, 2018 at 1:35

answered Jan 24, 2018 at 1:28

Michael Ilie

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Comments

kaifi · Accepted Answer · 2018-01-24 07:59:53Z

0

If you can use sorted, that's the easiest way to do it. If not, this is my solution:

lst = [0, 1, 6, 2, 5, 3, 4]
new = lst[1:]
for num in range(len(new)-1, 0, -1):
    for i in range(num):
        if new[i] > new[i+1]:
            new[i], new[i+1] = new[i+1], new[i]
new.insert(0, lst[0])
print(new)

answered Jan 24, 2018 at 7:59

kaifi

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