从字符串列表到浮点列表

Question

I have a text file (data.txt) delimited by tab as follows:

name    height  weight
A   15.5    55.7
B   18.9    51.6
C   17.4    67.3
D   11.4    34.5
E   23.4    92.1

The program below gives the result as the list of strings.

with open('data.txt', 'r') as f:
    col1 = [line.split()[0] for line in f]
    data1 = col1 [1:]
    print (data1)
with open('data.txt', 'r') as f:
    col2 = [line.split()[1] for line in f]
    data2 = col2 [1:]
    print (data2)
with open('data.txt', 'r') as f: 
    col3 = [line.split()[2] for line in f]
    data3 = col3 [1:]
    print (data3)

The results are as follows:

['A', 'B', 'C', 'D', 'E']
['15.5', '18.9', '17.4', '11.4', '23.4']
['55.7', '51.6', '67.3', '34.5', '92.1']

But, I want to get data2 and data3 as the list of floats. How can I correct above program? Any help, please.

Answer 1

There is no need of reading the file 3 times here, you can do this by defining a simple function that returns float value of the the item if it is a valid number otherwise returns it as it is.

Now read all the lines one by one using a list comprehension and apply this function to the items of each line. So now you've a list of lists, and it's time to unzip that list of lists using zip(*) and assign the return value to data1 , data2 , data3

def ret_float(x):
    try:
        return float(x)
    except ValueError:
        return x

with open('data.txt') as f:
    next(f) #skip the header
    lis = [ map(ret_float,line.split()) for line in f]
    #[['A', 15.5, 55.7], ['B', 18.9, 51.6], ['C', 17.4, 67.3], ['D', 11.4, 34.5], ['E', 23.4, 92.1]]
    #unzip the list
    data1, data2, data3 = zip(*lis)

    #if you want data1,data2,data3 to be lists then use:
    #data1, data2, data3 = [list(x) for x in  zip(*lis)]
...     
>>> data1
('A', 'B', 'C', 'D', 'E')
>>> data2
(15.5, 18.9, 17.4, 11.4, 23.4)
>>> data3
(55.7, 51.6, 67.3, 34.5, 92.1)

Update : Fixing your solution

with open('data.txt', 'r') as f:
    col2 = [line.split()[1] for line in f]
    data2 = list(map(float, col2 [1:]))   # apply float to each item using `map`
                                          # as `map` returns a `map` object in py3.x
                                          # you have to pass it to list() 
with open('data.txt', 'r') as f: 
    col3 = [line.split()[2] for line in f]
    data3 = list(map(float, col3 [1:]))
    print (data3)

help on map :

>>> print(map.__doc__)
map(func, *iterables) --> map object

Make an iterator that computes the function using arguments from
each of the iterables.  Stops when the shortest iterable is exhausted.

Answer 2

Use the float() function. It takes 1 arg, which would be the string/var you want to turn into a float.

Answer 3

Use the float() command:

with open('data.txt', 'r') as f:
    col2 = [float(line.split()[1]) for line in f]
    data2 = col2[1:]
    print(data2)

Answer 4

Convert the strings to float as you read them:

data1 = col1 [1:]

becomes

data1 = float(col1 [1:])

---

But--and I know you didn't ask this--your approach is a little sideways. You might want to consider a program structure more like this:

data0 = []
data1 = []
data2 = []
with open('data.txt', 'r') as f:
    f.readline()
    for line in f:
        vals = line.split()
        data0.append(vals[0])
        data1.append(vals[1])
        data2.append(vals[2])
print(data0)
print(data1)
print(data2)

Hope it helps!

(Disclaimer: works with Python 2.7; not sure about 3.x.)

Answer 5

with open('data.txt', 'r') as f:
    col1 = [line.split()[0] for line in f]
    data1 = col1 [1:]
    print (data1)
with open('data.txt', 'r') as f:
    col2 = [line.split()[1] for line in f]
    data2 = float(col2 [1:])
    print (data2)
with open('data.txt', 'r') as f: 
    col3 = [line.split()[2] for line in f]
    data3 = float(col3 [1:])
    print (data3)