Python ValueError: could not convert string to float:

Question

So, I've been having a problem while trying to convert the variable t into a float, and it's giving me an error, saying "could not convert string to float: " with a space as false.

def FtoC(a):
    a=(t-32)*1.8
    return a
def FtoK(a):
    a=(a-32)*1.8+273
    return ta
def CtoF(a):
    a=a*1.8+32
    return a
def CtoK(a):
    a=a+273
    return t
def KtoC(a):
    a=a-273
    return a
def KtoF(a):
    a=(a+459.67)*5/9
    return a
########################################################
import re
t=input()
while True:
    if t=='end':
          break
    tl=list(t)
    t=re.findall('\d+', t)
    t=''.join(t)
    t=float(t)
    if tl[-1]!='C' or tl[-1]!='K' or tl[-1]!='F' or t>200 or t<-100:
        print("Wrong")
    else:
        if tl[-1]=='C':
            print("%.2f" % t,'C',CtoF(t),'F',CtoK(t),'K')
        if tl[-1]=='K':
            print("%.2f" % t,'K',KtoC(t),'C',KtoF(t),'F')
        if tl[-1]=='F':
            print("%.2f" % t,'F',FtoC(t),'C',FtoK(t),'K')
    t=input()

I've been testing out with print commands inbetween and it prints the float value just alright, but in the if command it shows up an error, so it just prints "Wrong" every single time. When I remove:

if tl[-1]!='C' or tl[-1]!='K' or tl[-1]!='F' or t>200 or t<-100:
            print("Wrong")

The code works jus fine. Where is the problem?

I've been running the program on a private university platform, and this is what it returns:

Non-zero exitcode 1
*** Program stderr output following ***
Traceback (most recent call last):
  File "my_code.py3", line 28, in 
    t=float(t)
ValueError: could not convert string to float:

When using Python it just prints "Wrong", without any error.

As an aside, why are you detecting `t>200` as an error condition without considering units? 15°C ≈ 59°F ≈ 288 Kelvin. — Chris, Nov 09 '16 at 15:09
The only way to get past that check would be for the final character to simultaneously be equal to 'C', 'K', and 'F'. — jasonharper, Nov 09 '16 at 15:10
@jasonharper What? No. `if c1 or (...) or cN:` will evaluate to `True` if any of the conditions (c1 (...) cN) is `True` — Gormador, Nov 09 '16 at 15:13
the problem is using negation with the or's two out of three will always be True therefore you print wrong — Tom, Nov 09 '16 at 15:16
btw your code has 19 lines instead of 28 lines, at 28 is where the error was thrown, try to put the entire example — Zinov, Nov 09 '16 at 15:19
I guess you have Python 2 and the university has Python3. `input()` does not have the same behavior between those version, see https://stackoverflow.com/questions/3800846/differences-between-input-and-raw-input — Guillaume, Nov 09 '16 at 15:21
For which input does your program fails/behave differently exactly ? — Guillaume, Nov 09 '16 at 15:23
@Chris It doesn't matter. This code is for an exercise at the uni, and it just says that the number should be between 200 and -100 without the units — Nežumi, Nov 09 '16 at 15:26
@Zinov I updated the post, but it the lines that were missing were functions used when printing. — Nežumi, Nov 09 '16 at 15:27

Tom · Accepted Answer · 2016-11-09T15:37:21.870

Your problem isn't with converting to a float but with your condition.

write out the truth table and you'll find your mistake.

As a side note it's quite confusing using negation in a condition unless it's absolutely necessary.

try using the condition:

allowed_scales = ["C", "K", "F"]
if tl[-1] in allowed_scales and -100<t<200:
    # do your work here
else:
    print("wrong")

You also have a problem with your regex cutting out any minus signs so you will never get a negative temperature.

You should probably also check that the input actually contains some digits to convert to a float as trying to convert an empty string "" will return the error you are seeing.

while True:
    # in python 2:
    t = raw_input()
    # in python 3:
    t = input()
    if t=='end':
          break
    try:
        t=float(t[:-1]) # -1 to cut off the scale type
    except ValueError:
        print("Not valid input. try again.")

    allowed_scales = ["C", "K", "F"]
    if tl[-1] in allowed_scales and -100<t<200:
        if tl[-1]=='C':
            print("%.2f" % t,'C',CtoF(t),'F',CtoK(t),'K')
        if tl[-1]=='K':
            print("%.2f" % t,'K',KtoC(t),'C',KtoF(t),'F')
        if tl[-1]=='F':
            print("%.2f" % t,'F',FtoC(t),'C',FtoK(t),'K')
    else:
        print("wrong")

working with user input is always tricky since they can always type whatever they want in the box. using a try-except will handle most user error in this case.

Thank you very much. I hasn't been long since I started learning Python, and programming in general, so thank you vor pointing my mistake. I also changed the code a little bit to include negatives, I just used strings instead of lists. I still have to work a little bit on the code but that was my biggest worry. — Nežumi, Nov 09 '16 at 15:39

Python ValueError: could not convert string to float:

1 Answers1