CODEWITHSUNDEEP
pythonlistrecursioncopy
spider
spider

Reputation: 13

recursively copy a list to another in python

I want to copy a list to another recursively in python. For that purpose, i have taken string as input, defined an empty list and send them to recursive function as lists, so that i can copy the list to another list. However, the error displays "NoneType" object has no attribute "append". What am i missing ? i have defined "S" as list in main(). If there are other recursive methods, they are welcomed.

Error shown:

line 35, in string_copy
return string_copy(k,s.append(k[i]),i+1)
AttributeError: 'NoneType' object has no attribute 'append'

The code is :

def string_copy(k,s,i):
  if (i == len(k)):
    return s;
  else:
    return string_copy(k,s.append(k[i]),i+1)


def main():
  print("enter the string you want to copy:");
  k = input();
  s = [None];
  i = 0;
  print("the type of k and s is:", type(k),type(s));
  res = string_copy(list(k),list(s),i);
  print("the desired results are:","\n", res);
if __name__ == "__main__": main()    `

Upvotes: 0

Views: 2208

Answers (2)

spider
spider

Reputation: 13

The final solution is:

def string_copy(k,s,i):
  if (i == len(k)):
    return s
  else:
    s.append(k[i])
    return string_copy(k,s,i+1)

def main():
   print("enter the string you want to copy:")
   k = input()
   s = ""
   i = 0;
   print("the type of k is:", type(k))
   res = string_copy(list(k),list(s),i)
   print("the desired results are:","\n", "".join(res))
if __name__ == "__main__": main()

Better way of approaching this problem:

def string_copy(k):
  if (len(k) == 0):
    return k
  else:
    return k[0] + string_copy(k[1:])

def main():
  print("enter the string you want to copy:")
  k = input()
  print("the type of k is:", type(k))
  res = string_copy(k)
  print("the copied string is:","\n",res)
if __name__ == "__main__": main()

Upvotes: 0

John Gordon
John Gordon

Reputation: 33335

return string_copy(k,s.append(k[i]),i+1)

The list append() method does not return the updated list; it adds the new item in-place and returns None.

Therefore the next call to string_copy() will have None as the value for s.

Upvotes: 1

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