PYTHON: Changing list item from string to float

Question

I am creating a list in which I take values and convert them to floats. If the user enters a character A-Z/a-z, however, I must change that value to 0.0 and specify the location where the value has been changed. This is where I'm having trouble. I'm not exactly sure how I would find the values and change them to 0.0 if they're not numeric. Here is my code so far:

def main():
    # Creating the list
    num_list = []
    val = input("Enter a number or 0 to stop: ") 

    while val != '0': 
        num_list += [val] 
        val = input("Enter a number or 0 to stop: ") 
    #The list before values are changed to floats    
    print("Before: ", num_list) 

    try: 
        if val.isdigit():
            newnumlist = [] 
            for val in list:
                newnumlist.append(float(val)) 
        print(newnumlist)
    except ValueError: 

main()

After my try statement, I keep getting a TypeError. Do I need to use a variable, such as i, to get the values to change to floats? And in my except body, do I need a variable as well? How would I find the alphabetical characters in my list in order to change them?

Thank you in advance.

where would you like to store the location? would you be making the item something like (0.0,index) if val is not a number? — R Nar
– R Nar, Commented Nov 18, 2015 at 22:33

Riccati · Accepted Answer · 2015-11-18 23:04:12Z

2

(1) change

for val in list:

to

for val in num_list:

(2) change

except ValueError:

to

except ValueError:
    pass

(or whatever you want the program to do in the event of a ValueError).

This will work:

try:
    newnumlist = []
    for val in num_list:
        if val.isdigit():
            newnumlist.append(float(val))
        else:
            newnumlist.append('0.0')
    print(newnumlist)
except ValueError:
    pass

However, I have a feeling that you are trying to learn about exceptions so try (pun intended) this:

newnumlist = []
for val in num_list:
    try:
        newnumlist.append(float(val))
    except ValueError:
        newnumlist.append('0.0')

print(newnumlist)

Thanks ekhumoro!

edited Nov 18, 2015 at 23:04

answered Nov 18, 2015 at 22:34

Riccati

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2 Comments

Joshua Over a year ago

For the ValueError, though, how would I change the alphabetical characters to a float? I'd use the replace method, I assume, but how would I be able to determine which characters are not numeric?

ekhumoro Over a year ago

In the first example, the try/except is redundant, because isdigit protects against value errors. In the second example, the try/except should go inside the loop (i.e. it just replaces the isdigit check).

sami ghazouane · Accepted Answer · 2015-11-19 09:57:04Z

-1

You cant use isdigit to test if a string is a float. You need to make it your self and then map your list with this function:

def parse(string):
    try:
        return float(string)
    except Exception:
        raise TypeError

old_list = ["3.2","2.1"]
new_list = [parse(i) for i in old_list]

one line (without try/except) :

new_list = list(map(float,old_list))
# or other style
new_list = [float(i) for i in old_list] # certainly faster

Its exactly the same (certainly slower):

new_list = []
for i in old_list:
    new_list += [float(i)] # or [parse(i)]

edited Nov 19, 2015 at 9:57

answered Nov 18, 2015 at 22:49

sami ghazouane

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2 Comments

TigerhawkT3 Over a year ago

Your parse() function is way off. Even if throwing an exception was the right thing to do (it isn't), throw isn't a thing.

sami ghazouane Over a year ago

Omg... i just did a fabulous mix language ^^. Thx you. Revised.

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