将文本列表转换为数字？

Question

I am trying to use float() to convert the following list to numbers. But it always says ValueError: could not convert string to float .

I understand that this error occurs when there is something in text that cannot be treated as a number. But my list seems okay.

a = ['4', '4', '1', '1', '1', '1', '2', '4', '8', '16', '3', '9', '27', '81', '4', '16', '64', '256', '4', '3']

b = [float(x) for x in a]

Answer 1

a = ['4', '4', '1', '1', '1', '1', '2', '4', '8', '16', '3', '9', '27', '81', '4', '16', '64', '256', '4', '3']

b = [float(x) for x in a]

Works perfectly.

['4', '4', '1', '1', '1', '1', '2', '4', '8', '16', '3', '9', '27', '81', '4', '16', '64', '256', '4', '3']

b = [float(x) for x in a]

Not so much.

Your error message shows that one of you entries is a single quotation mark or an empty element in your list. For example,

a = ['the', '4', '1', '1', '1', '1', '2', '4', '8', '16', '3', '9', '27', '81', '4', '16', '64', '256', '4', '3']

b = [float(x) for x in a]

throws the error: ValueError: could not convert string to float: the

Answer 2

The list you have posted is perfectly ok with your method.

• Either you have a already define to a string and don't have a pointed to that list.

• The list contains a number.

Just to verify try

map(int,a) #if no errors you have all numbers

Then try

set(map(type,a)) #if outputs {str} you should be good. 

Either way your error is not reproducible and post more details in your question.

Answer 3

The problem is exactly what the Traceback log says: Could not convert string to float

1. If you have a string with only numbers, python's smart enough to do what you're trying and converts the string to a float.
2. If you have a string with non-numerical characters, the conversion will fail and give you the error that you were having.

You can strip whitespaces and then check for digits in the string.

 f = open('mytext.txt','r') 
 a = f.read().split()
 a = [each.strip() for each in a]
 a = [each for each in a if each.isdigit() ]
 b = [float(each) for each in a]  # or b = map(float, a)
 print b
 # just to make it clear written as separate steps, you can combine the steps

Answer 4

you can convert list using

if you have any string in your list then you get error message ValueError: could not convert string to float like that

>>> a = ['4', '4', '1', '1', '1', '1', '2', '4', '8', '16', '3', '9', '27', '81', '4', '16', '64', '256', '4', '3']
>>> b = list(map(float, a))
>>> b

output

[4.0, 4.0, 1.0, 1.0, 1.0, 1.0, 2.0, 4.0, 8.0, 16.0, 3.0, 9.0, 27.0, 81.0, 4.0, 16.0, 64.0, 256.0, 4.0, 3.0]