ValueError: could not convert string to float: '.'

Question

I have a list of strings (CD_cent) like this:

2.374 2.559 1.204

and I want to multiply these numbers with a float number. For this I try to convert the list of strings to a list of floats for example with:

CD_cent2=[float(x) for x in CD_cent]

But I always get the error: ValueError: could not convert string to float: '.'. I guess this means, that it can't convert the dot to a float (?!) But how could I fix this? Why doesn't it recognize the dot?

Is CD_cent == "2.374 2.559 1.204"? If so, for x in CD_cent will iterate over characters, ie, x = 2, x = '.', x = 3, etc. That's why it's complaining, because '.' can't be converted to float. — darksky
– darksky, Commented Jul 12, 2017 at 0:09
yes the command print(CD_cent) gives me '2.374 2.559 1.204' — Leo
– Leo, Commented Jul 12, 2017 at 0:12

pythad · Accepted Answer · 2017-07-12 00:42:23Z

12

You need to split each string as the string has multiple values:

your_str = "2.374 2.559 1.204"

floats = [float(x) for x in your_str.split(' ')]

Having a list you can do something like this:

li = [...]
floats = []

for s in li:
    floats.extend([float(x) for x in s.split(' ')])

In your exact situation you have a single string CD_cent = 2.374 2.559 1.204, so you can just:

floats = [float(x) for x in CD_cent.split(' ')]

edited Jul 12, 2017 at 0:42

answered Jul 12, 2017 at 0:10

pythad

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6 Comments

darksky Over a year ago

This is the right way to do it. But I would explicitly add str.split(" ") to make it clear how the string should be split.

pythad Over a year ago

@darksky, yes, sure, forgot to add ' '. Updated the answer. Thanks

Andrea Corbellini Over a year ago

Or even just split() without any arguments, so that this works with any kind of whitespace delimiter

Leo Over a year ago

Thanks guys! That was what i needed :)

Kevin Over a year ago

Bleh, don't name strings str. str is the string class/constructor, and now you shadowed it.

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Ofer Sadan · Accepted Answer · 2017-07-12 00:11:18Z

When I ran your line with the provided data everything worked fine and all the strings converted to floats without error. The error indicates that somewhere in your CD_cent there is a single DOT . that really can't be converted to float.

To try to solve this do:

CD_cent2=[float(x) for x in CD_cent if x != '.']

And if that doesn't work because of other strings you will have to try...except like this:

CD_cent2 = []
for x in CD_cent:
    try:
        CD_cent2.append(float(x))
    except ValueError:
        pass

All of that is because I assume CD_cent is not just a long string like '2.374 2.559 1.204' but it is a list like [2.374,2.559,1.204]. If that is not the case than you should split the line like this

CD_cent2=[float(x) for x in CD_cent.split()]

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