Simple python if statement

Question

Sorry for asking a very simple question. I'm trying to teach myself python which seems pretty easy but I get stuck on some things.

x = input("Enter a number: ")

if x > 10 & x < 20:
   print "YES!!!"

else:
   print "Not between 10 and 20"

In C (pseudocode) I would do

if(x > 10 && x < 20)
     "yes"
else
     "no"

and it would work.. but in Python it just keeps printing out YES!!! no matter what I input. What am I doing wrong?

When I run it:

Enter a number: 3
YES!!!
>>> ================================ RESTART ================================
>>> 
Enter a number: 13
YES!!!
>>> ================================ RESTART ================================
>>> 
Enter a number: 6
YES!!!
>>> ================================ RESTART ================================
>>> 
Enter a number: 13
YES!!!
>>> ================================ RESTART ================================
>>> 
Enter a number: 4
YES!!!
>>> ================================ RESTART ================================
>>> 
Enter a number: 8
Not between 10 and 20
>>> ================================ RESTART ================================
>>> 
Enter a number: 13
YES!!!
>>> ================================ RESTART ================================
>>> 
Enter a number: 3
YES!!!
>>> ================================ RESTART ================================
>>> 
Enter a number: 8
Not between 10 and 20

@arshajii: Wouldn't Python 3.x require parentheses around the argument to print? — Keith Thompson
– Keith Thompson, Commented Sep 5, 2013 at 22:42

Henry Keiter · Accepted Answer · 2013-09-05 22:57:18Z

8

& is the bitwise "and" operator in python. What you want is the logical "and", which is just the keyword: and.

if x > 10 and x < 20:

---Pedantic Explanation---

Now, if that were the only issue at play, you'd actually be fine, since Python True and False are just 1 and 0. However, there's something else going on that actually causes the problem. Specifically, the bitwise operators have higher precedence than the comparison operators. As a result, Python sees (for example, if x = 5) 5 > 10 & 5 < 20 and parses it as 5 > (10 & 5) < 20, which is a chained comparison. This is further expanded to:

(5 > (10 & 5)) and ((10 & 5) < 20)

Which evaluates to True, since 10 & 5 is zero:

(5 > 0) and (0 < 20)
True

Notice that 10 & 8 evaluates to 8, which is why when you input 8 you get the "correct" result of not satisfying the condition (since 8 is not less than 8 for the first condition):

(8 > (10 & 8)) and ((10 & 8) < 20)
(8 > 8) and (8 < 20)
False

edited Sep 5, 2013 at 22:57

answered Sep 5, 2013 at 22:42

Henry Keiter

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3 Comments

user2357112 Over a year ago

This is primarily important here because of precedence; x > 10 & x < 20 is interpreted as the chained comparison x > (10 & x) < 20.

Keith Thompson Over a year ago

And unlike in most other languages, 10 < x < 20 is equivalent to x > 10 and x < 20. (Don't try that in C; it's legal but has a completely different meaning.)

Henry Keiter Over a year ago

@user2357112 Yeah; I'm editing in a clarification to that effect as we speak :-)

JeFf · Accepted Answer · 2013-09-05 22:46:35Z

1

Operator & is binary operator. You probably want to use 'and' logical operator.

example: (a and b) is true.

Replace '&' for 'and'.

answered Sep 5, 2013 at 22:46

JeFf

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Comments

Andrew Clark · Accepted Answer · 2013-09-05 22:52:33Z

1

In Python you need to use the Boolean operators and and or to combine Boolean expressions, not &, |, or any other variant involving those characters:

if x > 10 and x < 20:
   print "YES!!!"
else:
   print "Not between 10 and 20"

In this case you can also shorten it a bit:

if 10 < x < 20:

As for what is happening with your current code, & is a bitwise "and" operator, and it has higher operator precedence than comparisons so you are ending up with x > (10 & x) < 20 which is equivalent to (x > (10 & x)) and ((10 & x) < 20). Obviously this is pretty far from what you are trying to do, but as it turns out this won't always evaluate to True, it evaluates to False if x is a negative number, 0, 2, 8, or 10.

answered Sep 5, 2013 at 22:52

Andrew Clark

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Comments

diegoperini · Accepted Answer · 2013-09-05 22:43:38Z

0

Logical AND in python is and keyword, not &. That's bitwise and operation.

answered Sep 5, 2013 at 22:43

diegoperini

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