如何将数字添加到列表中的某个索引？

Question

list = [4, 7, 5, 3]

In this list i understand that the number 4 has an index of 0, 7 has 1, 5 has 2 and 3 has 3, but how would i add a value to an individual index?

(list[1] + 1)

I thought that the above would would make it so that when i print the list it would give:

print(list)

[4, 8, 5, 3]

Answer 1

You need to do (you weren't actually giving a value with = , so once you did the addition the result was just thrown away):

list[1] += 1  # Short for list[1] = list[1] + 1

Integers are immutable , so you can't modify them in place. You need to re-assign to the name to change its value - so here we assign a new number which is 1 greater than the last.

Now it works with a demo:

>>> list1 = [4, 7, 5, 3]
>>> list1[1] += 1
>>> print(list1)
[4, 8, 5, 3]

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Only don't name a variable list , it masks the built-in.

Answer 2

>>> lst = [4, 7, 5, 3]
>>> lst[1]
7

To modify value of an item in a list, you assign new value to it by lst[i] = new_value

To increment existing value, you may calculate new value to assign:

>>> lst[1] = lst[1] + 1
>>> lst[1]
8

There is also short notation for adding value to an existing value using += :

>>> lst[1] += 1
>>> lst[1]
9