How can I read a number from a file and store it as a float in a list? / Python

Question

(this is my first ever question, so I apologise if it's not very clear)

I have a text file which looks something like this:

106.33333333333333
200.79263333333
68.214276154075

and I would like to put each number into a list (as a float) which can then be used to work out the average, maximum and sum. ( mean() , max() , sum() )

---

I've tried a number of variations based on similar question responses of: (simplified code)

    whod = []

    filename = (whouser+' '+d)

    for line in filename:
        numbers = line.rstrip('\n')
        whod = whod + [float(numbers)]

    print(whod)

then to check,

print(list)

Output:

ValueError: could not convert string to float: 's'

---

To be honest I didn't expect it work but if you have any suggestions that would be great.

Thank you.

Answer 1

Use a list comprehension and a with statement to open the file:

with open('text.txt') as text:
    data = [float(i) for i in text]

Then you can simply call your built-in methods on the data list:

max_value = max(data)
sum_value = sum(data)

Answer 2

First a quick note: As MattDMo pointed out, you want to avoid using builtins as variable names. In your example, thats list and file

https://docs.python.org/2/library/functions.html#list https://docs.python.org/2/library/functions.html#file

---

with open('filename.txt','r') as numbers_file:
    numbers = [float(n) for n in numbers_file]

print 'numbers: %s' % numbers
numbers_max = max(numbers)
print 'max: %s' % numbers_max
numbers_sum = sum(numbers)
print 'sum: %s' % numbers_sum
numbers_mean = numbers_sum / float(len(numbers))
print 'mean: %s' % numbers_mean

If you are still getting this error:

ValueError: could not convert string to float: 's'

You need to make sure that every item in the file can be cast as a float exactly as is. If you want to read the file in a more resilient way:

numbers = []
with open('filename.txt','r') as numbers_file:
    for line in numbers_file:
        try:
            num = float(line.strip())
            numbers.append(num)
        except ValueError:
            continue

This will ensure that you only read the lines from the file that are numbers.

---

The code inside the brackets, [float(n) for n in numbers_file] will iterate over the lines in the numbers_file (filename.txt) and cast each item as float while building the list. The resulting list is then passed to extend which updates the original numbers list. Iterating over a list like this in one line of code is called a list comprehension .

Here's the python docs on list methods:

https://docs.python.org/2/tutorial/datastructures.html#more-on-lists

Answer 3

my_list = my_list + [float(numbers)]