尝试在使用方法 insert() 时在列表中添加新的最后一个元素

Question

thanks for reading.

I'm currently learning python, yet a silly question popped in my head.

I was writting some lines of code usinng the things I've learnt and I noticed something that doesn't make "sense" (whatever that means) to me.

Here is my code:

//printing the last element of my list

numbers = [5, 4 ,3 ,2 ,1]

print(numbers[-1]) 

//changing values in my list

numbers[0] = 1

numbers[-1] = 1



//inserting values on my list

print(numbers)

numbers.insert(0,"Hello")

**numbers.insert(-1, "Hello")**

print(numbers)

the question pops with the line I marked as ****BOLD****: Why I can't modify the last element of my list when I put -1?, I touhght that when you call the element [-1] in a list it shows the last element, why this doesn't work when I'm using the insert () method and calling -1?

this is what happens when you run all the code.

1

[1, 4, 3, 2, 1]

['Hello', 1, 4, 3, 2, 'Hello', 1]

Answer 1

When you use insert , you are inserting at the specified index position. Remember that lst[-x] is the same as lst[len(lst)-1-x] . For example, lst[-1] and lst[len(lst)-1-1] both refer to the last index position in the list. Once you know this, the behavior you are seeing with the negative index positions starts to make more sense:

nums = [1, 2, 3, 4, 5]

nums.insert(1, 'Hello')  # Insert at index position 1.
# Result: [1, 'Hello', 2, 3, 4, 5]

nums.insert(-1, 'Bye')  # Insert at index position len(nums)-1-1 = 4.
# Same as: nums.insert(4, 'Bye')
# Result: [1, 'Hello', 2, 3, 4, 'Bye', 5]

nums.insert(-2, 'Good')  # Insert at index position len(nums)-1-2 = 5.
# Same as: nums.insert(5, 'Good')
# Result: [1, 'Hello', 2, 3, 4, 'Good', 'Bye', 5]

To insert at the end of the list using insert :

nums = [1, 2, 3]
nums.insert(len(nums), 'Hello')
# Result: [1, 2, 3, 'Hello']

As you can see, 'Hello' was inserted in index position 3 (the original len(nums) ) as specified.

If all you want to do is add an element at the end of a list, you could use append instead:

nums = [1, 2, 3]
nums.append('Hello')
# Result: [1, 2, 3, 'Hello']

Answer 2

To insert at the end of a list, you have to use

numbers.insert(len(numbers), 'Hello')

This will output

[5, 4, 3, 2, 1, 'Hello']

The problem you faced occurs because numbers[-i] is equivalent to numbers[len(numbers) - 1 - i] . So, numbers[-1] is actually numbers[5 - 1 - 1] , ie, numbers[3] , which is exactly what was happening in your program. Insertion was happening at the 3rd index.

Answer 3

Why I can't modify the last element of my list when I put -1?, I touhght that when you call the element [-1] in a list it shows the last element, why this doesn't work when I'm using the insert () method and calling -1?

Because -1 is len(x)-1 , it's the position of the last item but it "points to" right before the last item ⁰ , in the same way 0 points right before the first item. Basically:

 [a][b][c][d][e]
 0  1  2  3  4  5
-5 -4 -3 -2 -1

Thus in the same way insert(0, ...) inserts before the first item, insert(-1, ...) inserts right before the last item.

To add an element at the end, you need to insert right after the last item.

[0] that's the "pointer" view of indexing, shared by most 0-indexed languages: the index is seen as an offset from the array pointer.