Reading floats from file with python

问题

My input file has the form:

5.0, 1000.0, 100000000000000.0, 115.2712, 230.538, 345.796, 461.0408, 
1.053E-09, 1.839E-09, 1.632E-10, 1.959E-12, 4.109, 3.683, 3.586, 3.650

where every number is essentially in a line.

What I want to do is read all the floats, and then append only columns 7 through 10 to an array.

Here's what I've written:

T=[]
with open("test.txt", "r") as file1:
    for line in file1.readlines():
        f_list = [float(i) for i in line.split(",")]
        T.append(float(f_list[7]))
        T.append(float(f_list[8]))
        T.append(float(f_list[9]))
        T.append(float(f_list[10]))

When I run the above I get:

ValueError: could not convert string to float:

I think there's something wrong with the float(i) part, but I can't find a way around it.

I've seen people having similar problems here, but none of the fixes I've tried so far have helped. Any help is greatly appreciated.

回答1:

No the problem is that your first line ends with a comma:

5.0, 1000.0, 100000000000000.0, 115.2712, 230.538, 345.796, 461.0408,
1.053E-09, 1.839E-09, 1.632E-10, 1.959E-12, 4.109, 3.683, 3.586, 3.650

As a result, you want to process a string that only contains spaces (like ' '). And float(' ') fails since it is not a number (it actually reports this):

>>> float(' ')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: could not convert string to float: 
>>> float('a')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: could not convert string to float: 'a'

But a space simply is invisible when printed.

You can solve it by adding a filter statement to the list comprehension:

T = []
with open("test.txt", "r") as file1:
    for line in file1.readlines():
        f_list = [float(i) for i in line.split(",") if i.strip()]
        T += f_list[7:11]

Furthermore this will not work since none of the lines has 7-11 floats. So you will never add these floats anyway.

You can however use the following code:

with open("test.txt", "r") as file1:
    f_list = [float(i) for line in file1 for i in line.split(',') if i.strip()]
    T = f_list[7:11]

This will result in T being equal to:

>>> T
[1.053e-09, 1.839e-09, 1.632e-10, 1.959e-12]

回答2:

Your problem is that when you split line, the resulting list most likely contains whitespace. This would cause float() to fail. You need to sanitize your split list first by testing if an element is actually a valid float number. eg:

>>> def is_float(n):
    try:
        float(n)
        return True
    except:
        return False


>>> 
>>> line = '5.0, 1000.0, 100000000000000.0, 115.2712, 230.538, 345.796, 461.0408,'
>>> lst = [float(n) for n in line.split(',') if is_float(n)]
>>> lst
[5.0, 1000.0, 100000000000000.0, 115.2712, 230.538, 345.796, 461.0408]
>>>

来源：https://stackoverflow.com/questions/44975599/reading-floats-from-file-with-python

标签

python

floating-point