Why this logic function prints 7 in option A and not in B?

Question

Why does option a) prints x, while on b) it doesn't? By the way, both operations should bring the result of b).

a)

x = 7

if x >3 & x<=6:
    print(x)

b)

x = 7

if x >3:
    if x<=6:
        print(x)

and is spelled `and` in Python (not `&` - which is the binary and operator). — thebjorn, Jul 07 '20 at 20:02
`3 & x` is 3. Your first code is checking `x > 3` and `3 <= 6`. — khelwood, Jul 07 '20 at 20:04
@thebjorn probably OP's intention were actually to perform a logical `and`. But the question is why the condition is true with & operator. — Roberto Caboni, Jul 07 '20 at 20:07

esqew · Answer 1 · 2020-07-07T20:16:02.497

4

The ampersand (&) character is a bitwise operator. From the Python Wiki page on Bitwise operators:

x & y

Does a "bitwise and". Each bit of the output is 1 if the corresponding bit of x AND of y is 1, otherwise it's 0.

If you evaluate your expression x >3 & x<=6, the result is:

x >3 & x<=6
7 > 3 <= 6
True <= 6
True

... which is why your code reaches the final condition and outputs 7.

What you're looking for is the logical and token:

x = 7

if x >3 and x<=6:
    print(x)

edited Jul 07 '20 at 20:16

answered Jul 07 '20 at 20:02

esqew

@RobertoCaboni Updated to reflect this aspect of the question, thanks – esqew Jul 07 '20 at 20:09
2

`x >3 & x<=6` is not parsed as `1 & 1`, as `&` has a higher priority than `<`. See @khelwood's answer for the correct explanation. – Thierry Lathuille Jul 07 '20 at 20:12
@ThierryLathuille Ah yes... seems I totally missed this point in my haste to answer! I've updated to reflect the correct logic - thanks! – esqew Jul 07 '20 at 20:16

score 4 · Answer 2 · answered Jul 07 '20 at 20:06

4

This condition:

if x >3 & x<=6

is checking if x > (3&x) and (3&x) <= 6.

x is 7, so 3&x is equal to 3. So both conditions are true.

In general, if you want to check if two conditions are both true, use and.

if x > 3 and x <= 6:

For what you want in this case, you can do it more concisely:

if 3 < x <= 6:

answered Jul 07 '20 at 20:06

khelwood

2

This is the only correct answer: `&` has higher priority than `<`, so `3&x` gets evaluated first, then the comparisons get chained. – Thierry Lathuille Jul 07 '20 at 20:14

score 1 · Answer 3 · answered Jul 07 '20 at 20:02

1

With the first, you are using the bitwise operator for and, instead of and which correctly would not print x in the case of 7.

answered Jul 07 '20 at 20:02

duckboycool

3 Answers3