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In Python, how do I convert all of the items in a list to floats?

Posted by: admin November 6, 2017 Leave a comment

Questions:

I have a script which reads a text file, pulls decimal numbers out of it as strings and places them into a list.

So I have this list: ['0.49', '0.54', '0.54', '0.54', '0.54', '0.54', '0.55', '0.54', '0.54', '0.54', '0.55', '0.55', '0.55', '0.54', '0.55', '0.55', '0.54', '0.55', '0.55', '0.54']

How do I convert each of the values in the list from a string to a float?

I have tried:

for item in list:
    float(item)

But this doesn’t seem to work for me.

Answers:
[float(i) for i in lst]

to be precise, it creates a new list with float values. Unlike the map approach it will work in py3k.

Questions:
Answers:

map(float, mylist) should do it.

(In Python 3, map ceases to return a list object, so if you want a new list and not just something to iterate over, you either need list(map(float, mylist) – or use SilentGhost’s answer which arguably is more pythonic.)

Questions:
Answers:

float(item) do the right thing: it converts its argument to float and and return it, but it doesn’t change argument in-place. A simple fix for your code is:

new_list = []
for item in list:
    new_list.append(float(item))

The same code can written shorter using list comprehension: new_list = [float(i) for i in list]

To change list in-place:

for index, item in enumerate(list):
    list[index] = float(item)

BTW, avoid using list for your variables, since it masquerades built-in function with the same name.

Questions:
Answers:

You can use numpy to convert a list directly to a floating array or matrix.

    import numpy as np
    list_ex = [1, 0] # This a list
    list_int = np.array(list_ex) # This is a numpy integer array

If you want to convert the integer array to a floating array then add 0. to it

    list_float = np.array(list_ex) + 0. # This is a numpy floating array