Python3 input() floats list

Question

I apologise if I am missing something simple here, but I don't understand...

list_rho = []
# start some sort of loop
val = input() 
list_rho.extend(val)

With this code I am attempting to create a list of rho values: the user inputs a floating point number. This is then added to the list. e.g. the user inputs the values 0.10, 0.15, 0.20, 0.30, 0.35

However, when I print the list it appears like so:

0,  .,  1,  0,  0,  .,  1,  5,  0,  .,  2,  0,  0,  .,  2,  5,  0,  .,  3,  0,  0,  .,  3,  5

Clearly, not what I want. The same happens if I try enclosing the input value in quotes or converting the user input to a string first.

Can anyone offer an explanation of what/why this is happening?

N.B. I could use a loop for the values of rho, but, the number of decimal places is important in my end use and I know there is no way of precisely representing 0.1 in base 2...

jonrsharpe · Accepted Answer · 2014-05-23 16:10:27Z

3

If you want to keep the strings, you can do:

list_rho.extend(val.split(", "))

or, to convert to floating point numbers:

list_rho.extend(map(float, val.split(", ")))

input always gives you a string, and list.extend iterates over its argument adding each item separately, so list.extend("foo") is equivalent to list.extend(['f', 'o', 'o']):

>>> l = []
>>> l.extend("hello, world")
>>> l
['h', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd']

>>> l = []
>>> l.extend("hello, world".split(", ")) # right
>>> l
['hello', 'world']

edited May 23, 2014 at 16:10

answered May 23, 2014 at 16:00

jonrsharpe

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jonrsharpe · Accepted Answer · 2014-05-23 16:13:46Z

0

input() returns a string.

list_rho.extend is expecting a list.

"0, ., 1, 0, 0, ., 1, 5, 0, ., 2, 0, 0, ., 2, 5, 0, ., 3, 0, 0, ., 3, 5" is the input string coerced into a list.

>>> m = "0.10, 0.15, 0.20, 0.30, 0.35"
>>> m
'0.10, 0.15, 0.20, 0.30, 0.35'
>>> n = list(m)
>>> n
['0', '.', '1', '0', ',', ' ', '0', '.', '1', '5', ',', ' ', '0', '.', '2', '0', ',', ' ', '0', '.', '3', '0', ',', ' ', '0', '.', '3', '5']
>>> g = []
>>> g.extend(m)
>>> g
['0', '.', '1', '0', ',', ' ', '0', '.', '1', '5', ',', ' ', '0', '.', '2', '0', ',', ' ', '0', '.', '3', '0', ',', ' ', '0', '.', '3', '5']

edited May 23, 2014 at 16:13

jonrsharpe

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answered May 23, 2014 at 16:01

desimusxvii

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1 Comment

jonrsharpe Over a year ago

list.extend expects an iterable object, which it gets. It doesn't coerce the str to a list (Python is strongly-typed, and doesn't implicitly coerce); it iterates over it. The problem here is that iterating over a string is done character-by-character, which isn't how the OP wants to interpret the input.

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