Can't go out the while loop

Question

a = raw_input("please type 1 or 0")
while a != 0 or 1:
    print"please retype"
    a = raw_input("please type 1 or 0")
print "thanks"

I don't know why my output always repeats line 3 and 4 no matter what figure I typed in. Thank you

`while a != 0 or a !=1:` is needed. `1` is always true. Edit: actually a cant be both 1 and 0, so it's always either not 0 or not 1, so what you're looking for is probably `while a!=0 and a!=1` — 098799, Apr 19 '17 at 14:53
Possible duplicate of [Asking the user for input until they give a valid response](http://stackoverflow.com/questions/23294658/asking-the-user-for-input-until-they-give-a-valid-response) — Chris_Rands, Apr 19 '17 at 14:54
2nd part of dupe: http://stackoverflow.com/questions/15112125/how-do-i-test-one-variable-against-multiple-values — Chris_Rands, Apr 19 '17 at 14:55
Possible duplicate of [How do I test one variable against multiple values?](http://stackoverflow.com/questions/15112125/how-do-i-test-one-variable-against-multiple-values) — gyre, Apr 19 '17 at 14:59

gyre · Accepted Answer · 2017-04-19T15:02:18.693

a != 0 or 1 is actually interpreted as (a != 0) or (1), where the expressions enclosed in parentheses are evaluated as booleans. Since 1 is always True, the while loop continues to run forever, irrespective of the value of a.

See the near-duplicate How do I test one variable against multiple values? for more information on why the or operator does not serve the purpose you might expect in this situation.

You should use strings in your condition, since raw_input returns a string, and check whether a is one of several possibilities using the not in operator and a tuple:

a = raw_input("please type 1 or 0")
while a not in ('1', '0'):
    print("please retype")
    a = raw_input("please type 1 or 0")
print("thanks")

score 0 · Answer 2 · answered Apr 19 '17 at 14:58

0

Just another example:

a = raw_input("please type 1 or 0\n")
while True:
    if a in ("1", "0"):
        break
    print "please retype"
    a = raw_input("please type 1 or 0\n")
print "thanks"

answered Apr 19 '17 at 14:58

zipa

27,316
6
40
58

score 0 · Answer 3 · answered Apr 19 '17 at 15:47

You could use a helper variable to determine if the criteria are met.

b = True
while b:
    a = input("please type 1 or 0")
    if a == "0" or a == "1":
        b = False
    else:
        print("please retype")

print("thanks")

This also prevents non integer characters from throwing an error.

Can't go out the while loop

3 Answers3