How can I fix a ValueError when trying to terminate a while loop in Python?

[antoniacarol]

TL;DR Summary

Creating a python code--need help terminating a while loop

Please help!

I am trying to create a Python program for an assignment that will convert Celsius to Fahrenheit and vice versa depending on the input from the user. The numeric conversion portion of the code is working, however, if the user inputs "exit" the while loop is supposed to terminate and output "Goodbye". Additionally, if the user inputs anything else (like a word) the program is supposed to output "I do not understand. Please try again.".

I have tried moving the exit statement before temp value, I have tried saying "if temp =="exit" " and these have all resulted with the same error.
Whenever I input "exit" there is a ValueError stating it cannot convert string to float: "exi".

Please see the code I have so far in the photo below:

 

[jedishrfu]

Try placing a break statement or a sys.exit(0) statement following the print("Goodbye") statement. The break will do stuff after the loop where the sys.exit(0) will terminate the script then and there.

An example of break and continue statements:

https://www.tutorialspoint.com/python/python_loop_control.htm

An example, of sys.exit():

https://www.programcreek.com/python/example/2/sys.exit

Just typing exit while your program is running will do nothing unless the program recognizes it and calls sys.exit().

 

[pbuk]

If you post your code by copying and pasting text between [CODE=python][/CODE] tags instead of a screenshot it makes it much easier to help.

Read what the error message is telling you. Why do you think the interpreter is trying to convert a string "exi" to a numerical float value when you respond to the input prompt with the string "exit"? How do you think you can fix this?

Look at the line if input == "exit":. What do you think the value of input could be at this point?

Where do you think the code will go after the line print("... please try again")? Is that what you want to happen?

 

[PeterDonis]

Moderator's note: Moved to programming forum, and added Python tag.

 

[sysprog]

Immediately after you take the input you treat it as numeric (float) but if the input is the character string 'exit', it isn't numeric, so you get a data type exception.

 

[PeterDonis]

Whenever I input "exit" there is a ValueError stating it cannot convert string to float: "exi".

That's because at the line of code tempvalue=float(temp[:-1]), which is where the ValueError is being raised, the value of the temp variable is the string "exit", because that's what you typed. And Python can't convert that string to a float, which is what the line of code is telling it to do, so it raises ValueError.

What you need to do is check for the special values of the temp variable before you try converting it to a float. This is more or less what the code in your else block is trying to do, but those checks don't belong there, they belong in between the line where the temp variable is filled in with what the user types, and the line I quoted above where the float conversion is done. (Also, checking the value of input in an if statement makes no sense, because input is a built-in function. The string the user types in is in the variable temp.)

 

[PeterDonis]

Just typing exit while your program is running will do nothing unless the program recognizes it and calls sys.exit().

The program doesn't need to call sys.exit(), it just needs to exit whatever loop it is in and reach the end of the code. The interpreter will exit automatically when that happens.

 

[PeterDonis]

Try placing a break statement or a sys.exit(0) statement following the print("Goodbye") statement.

This would be a no-op since after that print statement the program will reach the end of its code and exit anyway. That's not the problem the OP is having.

 

[jedishrfu]

I guess i got confused by the while else construct and thiught something was amiss with the code.

I didnt know python had this feature and will have to try it out.

https://www.tutorialsteacher.com/python/python-else-loop

 

[pbuk]

I am trying to create a Python program for an assignment...

Not sure why the thread has been moved here from a homework forum?

As I hinted in my post #3 there are some pretty fundamental problems here which need more than a couple of tweaks - but the OP hasn't been back anyway.

 

[PeterDonis]

Not sure why the thread has been moved here from a homework forum?

It wasn't, it was moved from Computing and Technology because it's specifically about programming.

 

[pbuk]

It wasn't, it was moved from Computing and Technology because it's specifically about programming.

Sorry, my bad.