The Empty List Blocks Recursion

[leoahum]

List = [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]
step = 4



def ResetCR (l, stp):
    K = []
    if stp != 0:
        for i in range(1, int(len(l)/stp)+1):
            K.append(l[i*stp - 1])
        l = [x for x in l if x not in K]
        stp = stp - 1
        return ResetCR(l,stp)
    if stp == 0:
        return K


F = ResetCR(List,step)

print (F)

I'm trying to change the order of a list. If the "K = []" is out side the function, it works. However, I'm wondering is there a way to keep the variable 'K' inside the function?

[ichabod801]

Is there some reason you need to use recursion?

def reset2 (l, stp):
    k = []
    for s in range(stp, 0, -1):
        for i in range(1, int(len(l) / s) + 1):
            k.append(l[i * s - 1])
        l = [x for x in l if x not in k]
    return k

[leoahum]

Is there some reason you need to use recursion?

def reset2 (l, stp):
    k = []
    for s in range(stp, 0, -1):
        for i in range(1, int(len(l) / s) + 1):
            k.append(l[i * s - 1])
        l = [x for x in l if x not in k]
    return k

Just to practice recursion... Thank you for the solution!

[ichabod801]

In terms of recursion, my first thought was to have K be a parameter, defaulting to an empty list, and passing the modified K when you recurse. However, there are problems with using empty lists as parameter defaults. You would have to use None as the default, and then replace it with a list in the body of the function (if it is None).

[leoahum]

In terms of recursion, my first thought was to have K be a parameter, defaulting to an empty list, and passing the modified K when you recurse. However, there are problems with using empty lists as parameter defaults. You would have to use None as the default, and then replace it with a list in the body of the function (if it is None).

My question is if I replace K in the function with a list and the list is not empty, how does the ".append" effect the list or should I still use ".append" to change the list? Also, in that way, I need a K outside the function.

[ichabod801]

I'm not sure I understand the question. If you have a K in the function as a non-empty list, append will add to what is already in the list. But if you are defining K in the function, you will have the same problem of it resetting every time you recurse. But if you have a default, and pass the modified list each time you recurse, it doesn't reset.

[leoahum]

I'm not sure I understand the question. If you have a K in the function as a non-empty list, append will add to what is already in the list. But if you are defining K in the function, you will have the same problem of it resetting every time you recurse. But if you have a default, and pass the modified list each time you recurse, it doesn't reset.

Thank you! It is very helpful. I think I figured it out.

List = [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]
step = 4



def ResetCR (l, stp, K = None):
    if K is None:
        K = []
    if stp != 0:
        for i in range(1, int(len(l)/stp)+1):
            K.append(l[i*stp - 1])
        l = [x for x in l if x not in K]
        stp = stp - 1
        return ResetCR(l,stp,K)
    if stp == 0:
        return K


F = ResetCR(List,step)

print (F)