How to write a function that takes a positive integer N and returns a list of the first N natural numbers

Question

I m taking this online Python course and trying to solve the following problem called Coding Exercise  It s Natural      Write a function naturalNumbers which takes a positive integer n as input  and returns a list  1  2       consisting of the first n natural numbers    Do I even need a for loop to create a list  Here s my code  which doesn t work obviously   Keep in mind  they have not taught list comprehension  I found this concept on stackoverflow   def naturalNumbers n      list    n 1 for i in n     return list   Should I take another approach where I create multiple lists of 1 2 3   n and concatenate them all together like  1     2     3

User · Answer

Here are a few ways to create a list with N of continuous natural numbers starting from 1.

1 range:

def numbers(n): 
    return range(1, n+1);

2 List Comprehensions:

def numbers(n):
    return [i for i in range(1, n+1)]

You may want to look into the method xrange and the concepts of generators, those are fun in python. Good luck with your Learning!

User · Answer

Do I even need a for loop to create a list    No  you can  and in general circumstances should  use the built-in function range      gt  gt  gt  range 1 5   1  2  3  4    i e   def naturalNumbers n       return range 1  n   1    Python 3 s range   is slightly different in that it returns a range object and not a list  so if you re using 3 x wrap it all in list    list range 1  n   1

User · Answer

There are two problems with your attempt  First  you ve used n 1 instead of i 1  so you re going to return something like  5  5  5  5  instead of  1  2  3  4   Second  you can t for-loop over a number like n  you need to loop over some kind of sequence  like range n   So  def naturalNumbers n       return  i 1 for i in range n    But if you already have the range function  you don t need this at all  you can just return range 1  n 1   as arshaji showed  So  how would you build this yourself  You don t have a sequence to loop over  so instead of for  you have to build it yourself with while  def naturalNumbers n       results          i   1     while i  lt   n          results append i          i    1     return results  Of course in real-life code  you should always use for with a range  instead of doing things manually  In fact  even for this exercise  it might be better to write your own range function first  just to use it for naturalNumbers   It s already pretty close    There is one more option  if you want to get clever  If you have a list  you can slice it  For example  the first 5 elements of my list are my list  5   So  if you had an infinitely-long list starting with 1  that would be easy  Unfortunately  you can t have an infinitely-long list    but you can have an iterator that simulates one very easily  either by using count or by writing your own 2-liner equivalent  And  while you can t slice an iterator  you can do the equivalent with islice  So  from itertools import count  islice def naturalNumbers n       return list islice count 1   n

[python] How to write a function that takes a positive integer N and returns a list of the first N natural numbers

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