Confused about while statement

[EAS]

In theory, the following code should ask for the user to enter a value for h
until he/she enters hello or goodbye.

h = "hi"
while h != "hello" or "goodbye":
h = raw_input("Value for h:")

But the program keeps asking for a value no matter what I enter. Why doesn't
it work?

 

[Erik Max Francis]

In theory, the following code should ask for the user to enter a value
for h
until he/she enters hello or goodbye.

h = "hi"
while h != "hello" or "goodbye":
h = raw_input("Value for h:")

But the program keeps asking for a value no matter what I enter. Why
doesn't
it work?

You meant

while h != "hello" and h != "goodbye": ...

 

[Roy Smith]

You meant

while h != "hello" and h != "goodbye": ...

Or, perhaps even better,

while h not in ("hello", "goodbye"):

The meaning is the same, but I think idiomatically, it's a closer match
to the way you would say it in natural language.

 

[John Roth]

In theory, the following code should ask for the user to enter a value for h
until he/she enters hello or goodbye.

h = "hi"
while h != "hello" or "goodbye":
h = raw_input("Value for h:")

But the program keeps asking for a value no matter what I enter. Why doesn't
it work?

Operator precedence and a misunderstanding about how "or" works.

Operator precedence means that the expression is equivalent to:

(h != "hello") or "goodbye"

The result of h != "hello" is either True or False.
The way "or" works, if the result was false, the
second operand would be substituted, so you
would get "goodbye" which is true. In other words,
the result of the entire expression is either True
or "goodbye", which is also true. So the loop never terminates.

The "proper" way to write this test is:

while h not in ("hello", "goodbye"):

There are other ways, but this is probably the most readable.

John Roth

 

[Georgy]

h != "hello" or "goodbye"
is
(h != "hello") or "goodbye"

It's always true because even if (h != "hello") evaluates to False,
then "goodbye" is 'tested' and considered to be true.

You probably meant:
h != "hello" and h != "goodbye"
that can be written even better as:
h not in ("hello", "goodbye")

Georgy

| In theory, the following code should ask for the user to enter a value for h
| until he/she enters hello or goodbye.
|
| h = "hi"
| while h != "hello" or "goodbye":
| h = raw_input("Value for h:")
|
| But the program keeps asking for a value no matter what I enter. Why doesn't
| it work?