Python while loops

Question

I have this code in python 3,to check for errors on input but i need to determine that the input is an integer and if not to print the error message.

can anyone help me to figure out the code in my while loop. Thanks

price = 110;

ttt = 1;

while price < 0 or price > 100:

    price = input('Please enter your marks for Maths:');
    ttt =ttt +1;
    if ttt >= 2:
        print( 'This is an invalid entry, Please enter a number between 0 and 100')

Maybe this is what you're after: http://stackoverflow.com/questions/354038/how-do-i-check-if-a-string-is-a-number-in-python — demux, Dec 27 '12 at 22:49
Also `if ttt >= 2` will always evaluate to true the first time in the loop, is this really the behaviour you want? — Daniel Figueroa, Dec 27 '12 at 22:49
If the OP were using Python 3, then the `print` statement would be a SyntaxError. — DSM, Dec 27 '12 at 23:24

score 10 · Answer 1 · answered Dec 27 '12 at 22:51

10

Use the int() function to convert to an integer. This will raise a ValueError when it cannot do the conversion:

try:
    price = int(price)
except ValueError as e:
    print 'invalid entry:', e

answered Dec 27 '12 at 22:51

Roland Smith

42,427
3
64
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Stuart · Answer 2 · 2012-12-27T23:37:07.020

You probably want something like this, which will catch both whether the price is a whole number and whether it is between 0 and 100, and break the loop if these conditions are fulfilled.

while True:
    price = raw_input('Please enter your marks for Maths:')
    try:
        price = int(price)
        if price < 0 or price > 100:
            raise ValueError
        break
    except ValueError:
        print "Please enter a whole number from 0 to 100"

print "The mark entered was", price

Or since you have a manageably small number of possible values you could also do something like:

valid_marks = [str(n) for n in range(101)]
price = None
while price is None:
    price = raw_input('Please enter your marks for Maths:')
    if not price in valid_marks:
       price = None
       print "Please enter a whole number from 0 to 100"

can I kindly ask what this line means? `valid_marks = [str(n) for n in range(101)]` can this be written in updated format today? — KiritoLyn, Sep 12 '21 at 13:35
@kiritolyn it just converts the numbers in the range to strings resulting in `['0', '1', '2', ... '100']`. This is still valid and commonplace in modern python. — Stuart, Sep 12 '21 at 13:47

score 2 · Answer 3 · answered Dec 27 '12 at 22:50

First, use raw_input instead of input.

Also, place the ttt check before your input so errors display correctly:

price = 110;
ttt = 1;
while price < 0 or price > 100:
    if ttt >= 2:
        print 'This is an invalid entry, Please enter a number between 0 and 100';
    price = raw_input('Please enter your marks for Maths:');
    ttt = ttt +1;

score 0 · Answer 4 · answered Oct 20 '19 at 11:52

0

I would say that the simplest thing would be to instead of doing

price=int(price)

do

price=int(110)

answered Oct 20 '19 at 11:52

Vito Kekin

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Python while loops

4 Answers4