ValueError: could not convert string to float: when using input function

Question

I read similar threads with my error and could not find the solution. In most other threads it seemed that people were inputting data incorrectly, but as you can see I am inputting the values with the correct typology. I can't seem to find what else I could be missing. Any ideas?

def main():
    a, b, c, d, e, f  = float(input("Enter a, b, c, d, e, f: "))



    x = ((e * d) - (b * f))/ (
        (a * d) - (b * c) )

    y = ((a * f) - (e * c)) / (
        (a * d) - (b * c) )

    print("x is %f and y is %f" % (x,y))

if __name__ == "__main__":
    main()

Error message:

>>>Enter a, b, c, d, e, f: 9.0, 4.0, 3.0, -5.0, -6.0, -21.0
ValueError: could not convert string to float: '9.0, 4.0, 3.0, -5.0, -6.0, -21.0'

mhlester · Accepted Answer · 2014-02-04 00:40:31Z

2

the float() function tries to convert the entire string to a single float. Technically this would work:

a, b, c, d, e, f  = [float(number) for number in input("Enter a, b, c, d, e, f: ").split(', ')]

But keep in mind there's no error handling if it doesn't

answered Feb 4, 2014 at 0:40

mhlester

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John La Rooy · Accepted Answer · 2014-02-04 00:44:31Z

1

If you ensure the decimal point is always included

from ast import literal_eval
a, b, c, d, e, f = literal_eval(input("Enter a, b, c, d, e, f: "))

If the decimal point is missing, the corresponding variable will be an int instead of a float

answered Feb 4, 2014 at 0:44

John La Rooy

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2 Comments

chopper draw lion4 Over a year ago

I was taught it is bad coding practice to use eval() function for such a task. Is literal_eval different?

John La Rooy Over a year ago

@user3268003, yes, completely different. literal_eval only works for valid literals. eval can erase your harddrive and other evil things

steveha · Accepted Answer · 2014-02-04 00:41:52Z

The problem is that a series of float values, separated by commas, cannot be converted to a single float.

Either ask for each float individually, or split up the input string on commas and loop through them converting.

I recommend asking for each float individually. Write a function input_float() that takes input from the user, attempts to convert to float, and tells the user if there was a problem; it can have a loop that keeps looping until the user successfully enters a float value.

You might want to make your input_float() function accept a string to display as a prompt, another string to show on error, and a function to validate the input. So if the user must enter a float value between 0 and 100, you could make a validation function that performed this check.

Once you have written a flexible input_float() function, you can use it over and over again in your program.

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