Cannot convert a string using float - Python 3

Question

I am pretty sure the solution is easy but somehow the usual float doesn't work.

output = open('output.txt', 'wt')
with open('input.txt', 'r') as file:
    file = file.readlines()
    for x in range(0,len(file)):
        if file[x][0:6] == 'NUMBER':
            print(file[x][11:18]) 
            float(file[x][11:18])

print will give me '11111' , but float gives me error ValueError: could not convert string to float: What is happening here. I need it as float to do some math operation.

Update. I guess it's because i am trying to convert an empty space. The first line cause this error. float('') will give error.

NUMBER
*
NUMBER 111111111111111111111

The error message on my Python includes the repr of the string that it couldn't convert. If that's not printing, try print(repr(file[x][11:18])) to see the repr. Might be some non-printing character (e.g. \0) embedded in the string; float ignores leading and trailing whitespace, but not arbitrary non-numeric text or embedded whitespace. — ShadowRanger
– ShadowRanger, Commented Mar 3, 2017 at 2:58
You can put a try and except condition to ignore if there is no number in the line — Bijoy
– Bijoy, Commented Mar 3, 2017 at 3:11
So what should the value be when the string is empty? Or is the line ignored? — ShadowRanger
– ShadowRanger, Commented Mar 3, 2017 at 3:32

John Coleman · Accepted Answer · 2017-03-03 03:26:05Z

2

A version of the idea that @Bijoy gave in the comments: If the problem is that the string which follows 'NUMBER' is sometimes empty and such cases should always map to the float 0.0, you can use error trapping to write a special-purpose version of float():

def getfloat(s):
    try:
        return float(s)
    except ValueError:
        return 0.0

This will work for your situation, though has the counter-intuitive behavior where e.g. getfloat("hello world") == 0.0

answered Mar 3, 2017 at 3:26

John Coleman

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1 Comment

kacangputih Over a year ago

Thank you. This is what I am looking for.

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