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My code:

distance = ['0.12', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.02', '0.12', '0.01', '0.13', '0.02', '0.12', '0.02', '0.12', '0.02', '0.13', '0.02', '0.13', '0.02', '0.12', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.01', '0.13', '0.02', '0.13', '0.01']

the values are from a CSV file and appending it to distance list. how do I sum them so I can get a total distance? I've tried some of the answers given on the website but they haven't worked :(

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3 Answers 3

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5

Convert the strings to floats using map, and then sum the resulting numbers with sum.

>>> sum(map(float, distance))
2.9199999999999986

Those 9s are due to the inherent imprecision of floats. If you want something exact, you can use Decimal.

>>> from decimal import Decimal
>>> sum(map(Decimal, distance))
Decimal('2.92')
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6 Comments

A generator expression might be considered more Pythonic: sum( float(x) for x in distance).
@chepner : why? would you replace all usage of map by a generator? (itertools.imap would return a generator as well, hence eliminating the creation of the list map returns, too)
You can use math.fsum instead of sum to get more accurate results.
@njzk2 stackoverflow.com/q/1247486/1126841
Using map (or imap) is a lot more efficient, as you are not interested in the resulting list. See also my answer on this question.
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4

Convert each element to a float and use the sum builtin:

distance = ['0.12', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.02', '0.12', '0.01', '0.13', '0.02', '0.12', '0.02', '0.12', '0.02', '0.13', '0.02', '0.13', '0.02', '0.12', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.01', '0.13', '0.02', '0.13', '0.01']

distance = [float(s) for s in distance]
total = sum(distance)

total
>>> 2.9199999999999986

To take care of the extra 9s, use Decimal.

from decimal import Decimal

distance = [Decimal(s) for s in distance]
total = sum(distance)
total
>>>Decimal('2.92')
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Comments

3

Before you use the built-in sum you need to convert the string values to floats.

This does not work:

>>> sum(['1', '2'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'str'

But this does:

>>> sum([1,2])
3

Therefor you need to step through every value of your list distance and convert to a float before handing the list to sum.

You can use a list comprehension, map, a loop or a generator expression:

>>> distance = ['0.12', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.02', '0.12', '0.01', '0.13', '0.02', '0.12', '0.02', '0.12', '0.02', '0.13', '0.02', '0.13', '0.02', '0.12', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.02', '0.13', '0.01', '0.13', '0.02', '0.13', '0.01']
>>> sum(float(e) for e in distance)
2.9199999999999986

For a more accurate result, use fsum:

>>> math.fsum(float(e) for e in distance)
2.92
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1 Comment

Technically, this is a generator expression, as no list is created as would be with sum([float(e) for e in distance]).

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