列表是不可变的？ 无法转换字符串列表 ---> 浮点数列表

Question

I have a list of values, titled 'list', with values 23.4158, 25.3817, 26.4629, 26.8004, 26.6582, 27.7, 27.8476, 28.025. Each value is a string, not a float. Thus, I would like to convert this to a list of floats.

When I create a for loop to reassign the strings as floats, using the float() function, within the loops it shows me that the str has been successfully converted to a float. But when I check the type outside the loop, it shows me they are still strings.

for i in list:
    i = float(i)
    print(i,"=", type(i))
print(type(list[0]))

HOWEVER. When I create an empty list (new_list), and append the converted floats into said list, it shows exactly what I want. In other words, the str--->float conversion is successful. Code as such:

new_list = list()
for i in list:
    i = float(i)
    print(i,"=", type(i))
    new_list.append(i)
print(type(new_list[0]))

Why is it that the reassignment does not 'stick' unless the values are appended to new_list? Lists are mutable, so the old list should be able to be modified. Am i missing something?

Answer 1

The reassignment does not "stick" because you are not converting the item inside the list, you are converting i , which is another value inside the loop, completely detached from the list object. You are just converting a new variable i to float , you are not converting the item from within the list. The variable i is a new variable and python just copied the value from the item inside the new variable i . You are not converting the item from the list, you are converting a new variable i that has the same value as the item that the loop is currently at.

If you want to convert the item from withing the list using a for loop, you must target the item itself using its index:

values = [
    "23.4158", "25.3817", "26.4629", "26.8004", "26.6582", "27.7", "27.8476", "28.025"
]

for i in range(len(values)):
    values[i] = float(values[i])

print(values)
print(type(values[0]))

The reason why it works when appending to a list is because you are appending the newly converted i , which indeed, is a float.

I suggest reading the following:

• What does Python treat as reference types?
• https://www.geeksforgeeks.org/pass-by-reference-vs-value-in-python/
• https://www.tutorialsteacher.com/csharp/csharp-value-type-and-reference-type