将列表转换为字符串然后整数

Question

I have a list with 1550500 numbers, and all of them with quotes. Example: list('100', '150', '200', '250') etc... I need to sum all the numbers, but before that I need to convert it to INT.

List Name: trip_list

My code:

mean_tripstr = str(trip_list)
mean_trip = [int(x) for x in mean_tripstr]
print(type(mean_trip))

Error message:

Traceback (most recent call last):
  File "projeto1.py", line 235, in <module>
    mean_trip = [int(x) for x in mean_tripstr]
  File "projeto1.py", line 235, in <listcomp>
    mean_trip = [int(x) for x in mean_tripstr]
ValueError: invalid literal for int() with base 10: '['

What am I doing wrong? I am new to coding...

Answer 1

You have to convert each element to int :

mean_tripstr = map(str,trip_list)
mean_trip = list(map(int,mean_tripstr))

The code above uses a generator , what is more efficient in cases when you just have to iterate in a list. The last line convert to a list again properly.

But, as you said, if you already have a list of strings, you can just do:

mean_trip = list(map(int,trip_list))

If you know numpy , you can do too:

import numpy as np

trip_list = np.array(trip_list)

mean_trip = trip_list.astype(np.int)

Answer 2

Python has a map function, this takes a function and an iterable. There is also the sum function, which returns the sum of an iterable.

You can use this:

sum(map(int(trip_list))

Note that the map function does not return a list, it returns a generator. To convert it to a list, use

list(sum(map(int, trip_list)))

(this may take a while as it requires iterating over the entire list, and yours is quite long).

The error with your code is converting your list to a string, that is,

>>> my_list = ["5", "6"]
>>> my_list_str = str(my_list)
>>> my_list_str
"['5', '6']"
>>> type(my_list_str)
<class 'str'>
>>> type(my_list)
<class 'list'>

So when you try to iterate over the string, the first x is [ which is not a number (thus the exception).

As a sidenote, using list(map(int, a_list)) is faster than [int(i) for i in a_list]

>>> c1 = "list(map(int, a_list))"
>>> c2 = "[int(i) for i in a_list]"
>>> s = "a_list = [str(i) for i in range(1000)]"
>>> import timeit
>>> timeit.timeit(c1, setup=s, number=10000)
1.9165708439999918
>>> >>> timeit.timeit(c2, setup=s, number=10000)
2.470973639999997