CODEWITHSUNDEEP
pythonpython-3.x
muffinman
muffinman

Reputation: 25

python - converting objects to float in list of lists

I'm trying to make a program in Python 3 where user gives multiple values in one input separated by space and these values are added to a list in form of a list, thus creating a list of lists. But I just cant convert these values to float.

This is the "base" of the code:

lst = []
ln = input()
values = ln.split(" ")
lst.extend([values])
while ln != "":
    ln = input()
    values = ln.split(" ")
    lst.extend([values])

Which would give me list like this

[['1', '2'], ['3', '4']]

So the problem is that I cannot convert these strings to floats. One of the ways I've tried is 

for i in lst:
    for j in i:
        j = float(j)

Which just gives me "ValueError: could not convert string to float:". I've also tried mapping but that doesn't work either.

Upvotes: 1

Views: 7904

Answers (2)

timgeb
timgeb

Reputation: 78680

The most straight forward approach is to convert the strings in your sub-lists to floats before appending the sub-lists to your final list. That means use

values = list(map(float, ln.split(" ")))

Alternatively, you can post-process your list of lists like this:

>>> lst = [['1', '2', '3'], ['4', '5', '6']]
>>> lst = [list(map(float, sublist)) for sublist in lst]
>>> lst
[[1.0, 2.0, 3.0], [4.0, 5.0, 6.0]]

... which also has a nested list-comprehension version:

>>> [[float(x) for x  in sublist] for sublist in lst]
[[1.0, 2.0, 3.0], [4.0, 5.0, 6.0]]

... which if you are new to Python might want to write as

>>> lst = [['1', '2', '3'], ['4', '5', '6']]
>>> result = []
>>> 
>>> for sublist in lst:
...     float_sublist = []
...     for x in sublist:
...         float_sublist.append(float(x))
...     result.append(float_sublist)
... 
>>> result
[[1.0, 2.0, 3.0], [4.0, 5.0, 6.0]]

In addition, but unrelated, use lst.append(values) instead of lst.extend([values]). The effect is the same, but the latter one looks weird and goes against the intended use-case for extend - appending objects from an iterable one by one, which you suppress by wrapping your iterable values in another iterable by passing [values].

Upvotes: 5

Ben
Ben

Reputation: 6348

Python has less than intuitive ways to modify primitive objects within a list- many times the easiest way to get around that is to create a new list:

lst = [['1', '2'], ['3', '4']]

floatlist = []
for l in lst:
    floatlist.append([float(i) for i in l])

print(floatlist)

Upvotes: 1

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