Using the while loop to give the user 3 attempts for a valid input value

Question

counter = 0
miles = float(input('How many miles do you want converted into kilometers? '))
while miles < 0:
    print('You cannot enter a negative value!')
    miles = float(input('Enter the correct number of miles: '))
    counter = counter + 1
    if counter > 2:
        break
if counter <= 2:
    milesToKm = (miles*1.6)
    print(miles, 'miles is', round(milesToKm,2), 'kilometers')
else:
    print('Error: cannot exceed three attempts')
    exit()

Hi everybody. This is my fifth week of learning to code. I'm supposed to create a program that will give the user three attempts to enter a valid value. If the value on the third attempt is invalid (a negative number), then it will prompt an error message and terminate.

The problem is that I can enter two invalid values followed by a valid value, and it still gives the error message and terminates. The program should calculate the third valid value and do the mathematics then print the conversion.

For example:

How many miles do you want converted into kilometers? -1

You cannot enter a negative value!

Enter the correct number of miles: -1

You cannot enter a negative value!

Enter the correct number of miles: 5

Error: cannot exceed three attempts

Can you guys help?

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Prune Prune · Accepted Answer · 2016-02-24T01:03:59

It's just a small confusion in managing both exit conditions. Check both of them in the while statement, and you'll be fine. I tested this with sequences (-1, -1, -1), (-1, -1, 5), and (-1, 5). It worked fine each time.

counter = 0
miles = float(input('How many miles do you want converted into kilometers? '))
while miles < 0 and counter <= 2:
    print('You cannot enter a negative value!')
    miles = float(input('Enter the correct number of miles: '))
    counter = counter + 1

if counter <= 2:
    milesToKm = (miles*1.6)
    print(miles, 'miles is', round(milesToKm,2), 'kilometers')
else:
    print('Error: cannot exceed three attempts')
    exit()

Another way to handle this is with the else clause of a loop. If you exit the loop normally, its else clause gets executed; if you break out of the loop, the else gets skipped. This lets you handle your logic with a for loop:

for counter in range(3):
    miles = float(input('How many miles do you want converted into kilometers? '))
    if miles >= 0:
        break
    print('You cannot enter a negative value!')

else:
    print('Error: cannot exceed three attempts')
    exit()

milesToKm = (miles*1.6)
print(miles, 'miles is', round(milesToKm,2), 'kilometers')

Using the while loop to give the user 3 attempts for a valid input value

2 Answers