Reading a text file and converting string to float

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I have a text file called "foo.txt", with a list of numbers, one on each line, for example:


I now want to read these numbers into a Python list. My code to do this is as follows:

x = []
file_in = open('foo.dat', 'r')
for y in'\n'):

But this gives me the error:

ValueError: could not convert string to float

What am I doing wrong?


commented by martineau: you can also use if y: to eliminate None or empty string.

Original Answer:

It fails due to you are using newline character as a separator, therefore the last element is empty string

you can add y.isdigit() to check whether y is numeric.

x = []
file_in = open('sample.csv', 'r')
for y in'\n'):
    if y.isdigit():


you can change read().split("\n") to readlines()


remove the leading/trailing characters from y. it handles the lines with extra whitespaces

for y in file_in:
    trimed_line = y.strip()  # leading or trailing characters are removed

How to convert a string into a float when reading from a text file , At the end of the second line, you have a comma, so you have an empty string in the list. float('') raise an exception, hence your error: for line in  Python CSV formatting with string and float and write How do you format a float to 2 decimal points with mixed data type? I'm fetching a table and writing rows to csv file. My data is (string, string, float, float, float) sql = 'select * from testTable' c.execute(sql) columnNames = list(map(lambda x:

How about this approach:

x = []
with open('foo.dat', 'r') as f:
    for line in f:
        if line: #avoid blank lines


with open('foo.dat', 'r') as f:
    lines = (line.strip() for line in f if line)
    x = [float(line) for line in lines]

Finally more compact:

with open('foo.dat', 'r') as f:
    x = [float(line.strip()) for line in lines if line]

This way you don't have to worry about blank lines and you make proper conversion from string to float

[PDF] Reading data from a file Example: reading a file with numbers (part , We can read text in the file into strings in a program load all lines into a list of strings (lines): For each word after the first: if word is not "no", convert to float. csv.QUOTE_NONNUMERIC: Instructs the reader to convert all non-quoted fields to type float. That works if you're ok with all unquoted fields (integer, float, text, Boolean etc.) being converted to float, which is generally a bad idea for many reasons (missing or NA values in Booleans or factors will get silently squelched).

Usually files has empty line at the end so probably you're trying to cast empty string to float.

Instead of'\n') you could use:

for line in file_in.readlines():

Method readlines returns list of all lines from given file and skips last empty line if present.

Reading a file line by line, Before trying to let a program read a file, we must know the file format, i.e., what the The reason is that lines holds each line ( number ) as a string, not a float or int that we can add to other numbers. A fix is to convert each line to a float :. Usually files has empty line at the end so probably you're trying to cast empty string to float. Instead of' ') you could use: for line in file_in.readlines(): x.append(float(line)) Method readlines returns list of all lines from given file and skips last empty line if present.

You can try using the default float function

>>> float("1.1")

You could also try using the python try else statement which will run the code until it catches a error and runs a else statement.

   try_this(whatever that might bring up a error)
except SomeException as exception:
   #Handle exception
   return something

There might be a possibility that there is a blank line end of the file which might create errors. Try using the try else statement because of it.

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I think You string like this : '0.1111, 0.1111' or other

file_in = open('foo.dat', 'r')
for y in file_in.readlines()[0]:

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