Nested while loop - Python

Question

I'm developing a simple conversion program, but I got stuck on preventing the user from typing in a string where integers are expected:

    choicecheck = 1 
    inputcheck = 1 
    while choicecheck == 1: 
        choice = input ("""please choose type of convert:
        1. Centimeters to inches
        2. Inches to centimeters
        3. Exit""") 


        while inputcheck == 1:
            if choice == "1":
                cm = int(input ("Please type in value in centimeters.")) 
                if type(cm) == int:
                    cmsum = cm * 0.39 # multiplies user input by 0.39
                    print (cm, " centimeters is ", cmsum, " inches") 
                    choicecheck = 0
                    inputcheck = 0
                else:
                     print("Sorry, invalid input")

When it comes to inputcheck, the else part of the if statement does not work, and I have no idea why is that. Please help.

type(cm) can only ever be int because cm was constructed using a call to int() on the user's input. But that call fails if the user happens to enter something that can't be interpreted as an integer. For this, you use a try/except statement like the one @mhlester proposes. Type checking is rarely ever done in Python. — Tim Pietzcker
– Tim Pietzcker, Commented Feb 6, 2014 at 20:38

mhlester · Accepted Answer · 2014-02-06 20:36:10Z

3

while True:
    try:
        choice = int(input('please choose type of convert'))
    except ValueError:
        # user entered an input that couldn't be converted to int
        continue
    else:
        if not 1 <= choice <= 3:
            # valid int, invalid choice
            continue
        # success. stop looping
        break

Now you have an int that represents their input. It will continue asking until they've successfully entered an int

answered Feb 6, 2014 at 20:36

mhlester

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Comments

kylieCatt · Accepted Answer · 2014-02-06 21:03:08Z

This will keep the program looping until the user selects to exit. I used a float for the second user input since the user may need to find a value for 2.5 inches or something similar.

while True:
    try:
        choice = int(input("""please choose type of convert:
        1. Centimeters to inches
        2. Inches to centimeters
        3. Exit"""))

        if choice == 1:
            while True:
                 try:
                    cm = float(input ("Please type in value in centimeters.")) 
                    cmsum = cm * 0.39 # multiplies user input by 0.39
                    print (cm, " centimeters is ", cmsum, " inches")
                    break
                 except ValueError:
                    print('error')

        elif choice ==2:
            while True:
                try:
                    stuff()
                    break
                except ValueError:
                    print('error')

        elif choice == 3:
            print('Thanks!')
            break

    except ValueError:
        print('error')

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