如何将csv中的每一列保存到列表中？

Question

I'm trying to make each column into a list the following way:

f = open('file.csv', 'r') 
a = [float(item.split('.')[0]) for item in f]
b = [float(item.split('.')[1]) for item in f]

When I print a I get a list with all the values of the first column, but when I print b it shows an empty list. I tried changing the columns of a with that of b and a gives me the column I'm asking for, just after the first defined list it keeps showing empty lists.

Answer 1

You can also just use a csv.reader :

import csv

list_a = []
list_b = []

with open('file.csv', newline='') as csv_file:
    reader = csv.reader(csv_file, delimiter='.')
    next(reader, None)  # Skip the header.
    for a, b in reader:
        list_a.append(float(a))
        list_b.append(float(b))

---

Or with the zip trick:

with open('file.csv', newline='') as csv_file:
    reader = csv.reader(csv_file, delimiter='.')
    next(reader, None)  # Skip the header.
    list_a, list_b = zip(*(map(float, row) for row in reader))

Answer 2

a = [float(item.split('.')[0]) for item in f] reads everything from the file. There is nothing left for the second list comprehension. You must close the file and open it again, eg:

with open('file.csv', 'r') as f:
    a = [float(item.split('.')[0]) for item in f] 
with open('file.csv', 'r') as f:
    b = [float(item.split('.')[1]) for item in f]

But it is even better to read the file only once and process both a's and b's in one loop:

with open('file.csv', 'r') as f:
    a, b = zip(*[map(float, item.split('.')) for item in f])