Python：生成数字1-10的列表，但是1最终出现在列表的末尾吗？

Question

I have just started learning how to program using Python, and have been going through some exercises to help me improve. For one of the exercises, I have to program a Sieve of Eratosthenes, and as part of doing this, I wanted to generate a list of numbers 1 through n .

My code is as follows:

def primelist(n): #returns a list of all primes lower than or equal to n
    grid=[]
    k=1
    while k in range (1, n+1):
        grid.insert(-1, k)
        k+=1
    return grid

This is as far as I got, because when I tested what I had written so far with:

print(primelist(10))

my code output the list [2, 3, 4, 5, 6, 7, 8, 9, 10, 1] and I can't figure out why it is doing this. Any explanation would be much appreciated!

Answer 1

In Python, list.insert(index, item) actually inserts item BEFORE index .

So list.insert(-1, k) inserts k before -1. Hence your results.

To do what you want to do properly, the following two lines are equivalent

list.insert(len(list), k)
list.append(k)

Both methods modify the actual list (instead of returning a new list), as expected.

See more here: https://mail.python.org/pipermail/tutor/2010-January/073754.html

Answer 2

insert method for list inserts the element at the index before the first argument. -1 is the index of last element. So, when you are inserting at -1 , you are always inserting just before the last element. For example you have a list a as:

a = [1,2,3,4]
print a[-1]  # It will print 4

So, when you are trying to insert first element (1) in your list using grid.insert(-1, k) , it goes to last (or first as it will be only element), as there is no other element. All insert operations after first element add the elements to the second last position in the list.

This leads you see only 1 at last position while all other elements are in order as you expect.

As already mentioned by @coln, you can either use list.append(k) or list.insert(len(list), k) to insert/append/add elements to the end of the list.

Answer 3

I don't know why it's backwards, but your code is not the way to add elements to a list. Also you don't use the variable n in your code (probably a typo), so I will just add the function to generate a list through 1 to n .

def primelist(n):
    grid=[]
    for k in range(1,n+1):
        grid.append(k)
    return grid

This code will return [1,2,3,4,5,6,7,8,9,10] as you requested.

Answer 4

xrange can do this for you, remember to give xrange a end value that is one more than the one you want.

So to generate numbers from 1 to n : xrange(1,n+1)

The example below will give you a list from 1 to 10.

>>> primelist=list(xrange(1,11))
>>> primelist
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]