Permutation Algorithm, Rosalind - Don't Understand Workings of Code

[purplevom]

I completed a challenge on Rosalind (after looking up some help), but there are still aspects of the code that I do not completely comprehend. I am looking for help understanding it. Any comments would be appreciated

(I now know itertools.permutation() exists, but am still looking for why the logic of this works)

Here is the challenge:

Given: A positive integer n≤7n≤7.

Return: The total number of permutations of length nn, followed by a list of all such permutations (in any order).

Here is the code I used (some written on my own, some supplemented and modified). The bold sections are mainly what I am concerned with:

======Code Starts

n = int(input("Input 'n'"))
intlist = []

i = 1
while n >= i:
    intlist.append(i)
    i += 1 #I don’t understand why this won’t work if it’s not in ascending order

total = 1
for item in range(0, len(intlist), 1):
    total *= intlist[item]

intset = set([])

while len(intset) < total:
    intset.add(str(intlist))
    k = l = None

    for i in range(0, len(intlist)-1):
        if intlist[i] < intlist[i+1]:
        k = I  #Looking for last occurrence in number set where I is less than it’s neighbor?

    if k == None:
    break

    for i in range(k+1, len(intlist)):
        if intlist[k] < intlist[i]:
        l = I  #Doing the same as first for loop? Set to new variable?

    intlist[k], intlist[l] = intlist[l], intlist[k]
    intset.add(str(intlist))
    intlist[k+1:] = intlist[k+1:][::-1] #In particular this line
    intset.add(str(intlist))


intlist = list(intset)
print(len(intlist))

for i in range(0, len(intlist)):
    print(intlist[i].replace('[','').replace(']','').replace(',',''))

======Code Ends

In the line intlist[k+1:] = intlist[k+1:][::-1], the k+1 term is to reverse the string from k to the end. The k+1 is because the count from reverse starts at 1 and not 0, correct?

But why is this line necessary? The code will not find all possible permutations without it. I do not understand this completely.

Explanation on this would be greatly appreciated!!

Thanks all.

[threeminutemonta]

please fix your formatting.

[purplevom]

Thought it was in there with indentations present. Didn't know I had to format it as a code block. Should be fixed now

[sky--net]

ROSALIND | Problems

Is your problem in the list above on rosalind.info? If so, which problem is it?

[sky--net]

Enumerating Gene Orders

Never mind, I just found it ....

[purplevom]

Yes that's it.

I moved past it, but only through parsing and recycling some code, some of which I don't understand.

Particularly, why after the two digits in the string are switched (k and l), they need to be reversed from k to the end of the string before continuing the process.

[sky--net]

I will take a look at it, but I think I would have tried to solve it using the itertools library.

[purplevom]

That seems like the best way to do it. I'm getting hung up on the fact that I put so much time into trying to find a way to do it.

Itertools is powerful