Convert string values to floating point

Question

My small script returns a comma separated list of 10 floating point values, loops through them, and holds them in variable vals.

I sum these up into new variable 'total' like this:

total = sum(map(float, vals))

Easy enough, BUT oftentimes None occurs as one or more values in the list,

4.234,None,0.2398,None,None,0.0166666666667,None,None,None,None

then I get the ValueError:

ValueError: could not convert string to float: N

How can I convert these None values to 0 so that the values sum up without error?

I would loop over the list before calling sum and replace None with 0. I wonder if there a better way — Robert Ekendahl
– Robert Ekendahl, Commented Jun 25, 2014 at 19:05
Your error message makes it seem like vals is a string, not a list. Is that correct? — DSM
– DSM, Commented Jun 25, 2014 at 19:06
Python newbie and my understanding still evolving. I believe it is a string. — a ekandem
– a ekandem, Commented Jun 25, 2014 at 19:25

C.B. · Accepted Answer · 2014-06-25 19:20:04Z

3

total = sum(map(float, filter(None,vals)))

From the docs:

Note that filter(function, iterable) is equivalent to [item for item in iterable if function(item)] if function is not None and [item for item in iterable if item] if function is None.

If vals is a string literal (as DSM hints it looks like it is) use can use the ast library

>>> vals = '[4.234,None,0.2398,None,None,0.0166666666667,None,None,None,None]'
>>> import ast
>>> vals_list = ast.literal_eval(vals)
>>> vals_list
[4.234, None, 0.2398, None, None, 0.0166666666667, None, None, None, None]

Then you can simply apply sum and filter

>>> total = sum(filter(None,vals_list))
>>> total
4.4904666666667

Note -- I did append square brackets to the string, so if they are missing that would be a step you would need

edited Jun 25, 2014 at 19:20

answered Jun 25, 2014 at 19:06

C.B.

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4 Comments

Nathaniel Over a year ago

It's important to note that if item behaves differently from item is not None. Also, the function call will be slightly slower than a generator expression.

C.B. Over a year ago

Sure - if the case is more complex than stated above, then there would probably need to be more pruning

a ekandem Over a year ago

Thanks C.B. I would then have to add code to add brackets to vals, and that seems unnecessary. I liked your line, total = sum(map(float, filter(None,vals))), except I get same error as originally.

C.B. Over a year ago

@aekandem If you don't go that route, then you will need to use split() and a generator expression that is filtering on the string None, which to me isn't as explicit. You would then have to apply the map(float,iterable) as well. You will have to convert the string into a list one way or another.

Nathaniel · Accepted Answer · 2014-06-25 19:13:42Z

1

total = sum(float(val) for val in vals if val is not None)

edited Jun 25, 2014 at 19:13

answered Jun 25, 2014 at 19:04

Nathaniel

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1 Comment

Wooble Over a year ago

The [] are unnecessary; using a generator expression instead of a list comprehension will be more efficient.

Daiver · Accepted Answer · 2014-06-25 19:07:10Z

0

sum(map(lambda x: float(x) if x else 0.0, vals))

edited Jun 25, 2014 at 19:07

answered Jun 25, 2014 at 19:05

Daiver

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1 Comment

ZeMoon Over a year ago

Please explain this code, it has been flagged as low quality.

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