if coin == 25 | 10 | 5:

If I replace the ‘|’ with ‘or’ the code runs just fine. I’m not sure why I can’t use ‘|’ in the same statement.

Doing the following doesn’t work either:

if coin == 25 | coin == 10 | coin == 5:

I know bitwise operators can only be used with integers, but other then that is there another difference from logical operators?

  • ExtremeDullard@lemmy.sdf.org
    2·
    2 months ago

    Much to unpack here…

    coin == 25 | 10 | 5

    …will evaluate as True if coin is equal to the bitwise OR of 25, 10 and 5 - i.e. 31. In other word, it’s equivalent to coin == 31. That’s because the bitwise OR has precedence over the == operator. See operator precedence in Python.

    If I replace the ‘|’ with ‘or’ the code runs just fine.

    It probably doesn’t. If you replace | with or, you have the statement coin == 25 or 10 or 5 which is always True in the if statement because it’s evaluated as (coin == 25) or (not 0) or (not 0) in an if statement.

    coin == 25 | coin == 10 | coin == 5

    …will evaluate as coin == (25 | coin) == (10 | coin) == 5. Again, operator precedence.

    What you want to do is this:

    if coin in [25, 10, 5]:

    or

    if coin in (25, 10, 5):

    or simply

    if coin == 25 or coin == 10 or coin == 5:

    Don’t create problems and confusion for the next guy who reads your code for nothing. Simple and readable are your friends 🙂

    • milon@lemm.eeOP
      1·
      2 months ago

      Thanks. I think I understand why I wouldn’t want to use it in this case. But what is an example of where I can use it? This makes me think I should avoid using bitwise operators with integers and keep it to strings only, but I know that’s not true from what I’ve learned.

      • wewbull@feddit.ukEnglish
        1·
        2 months ago

        When you’re working with the binary representation of numbers.

        In your code you had three numbers 25, 10 and 5. If we write those number in binary we get:

        • 25: 0b00011001
        • 10: 0b00001010
        • 5: 0b00000101

        (The 0b at the start is just a way of saying “this is binary”)

        When you do a bitwise-or, it’s a bit like adding up but you don’t bother with carrying anything. So let’s do 25 | 10, starting at the right-hand end going bit by bit (bitwise):

        • 0 | 1 = 1
        • 1 | 0 = 1
        • 0 | 0 = 0
        • 1 | 1 = 1
        • 1 | 0 = 1
        • 0 | 0 = 0 for all the rest

        So the result is 0b00011011 which is 27.

        So now you’re asking “when would I ever need to do such a thing?” and the flippant answer is “you’ll know when you need it”.

        You’re looking for more though, I know. Basically computers often put multiple bits of data into bitstreams (long sequences of bits). Think networking and file storage. Constructing these bitstreams is done with bitwise operators like |, &, ^, << and >>. Together they form a different type of maths to what you’re used to.

        These operators work in a very similar way to how +, -, * and / work. They take two numbers and return a third. If we rewrite your code using operators you’re more familiar with…

            if coin == 25 | 10 | 5:  # if coin == 31
        ...
    if coin == 25 + 10 + 5:  # if coin == 40
        ...

        …you can see it’s obviously wrong because you’re doing one comparison with the result of the operation (addition or bitwise-or), not three comparisons.

      • driving_crooner@lemmy.eco.br
        1·
        2 months ago

        I use that in match case operations, but usually when is just two possibilities, try something like this and see if works

        match coin:
        case 5 | 10 | 20:
        …

        Edit: just tested and it’s works.

      • ExtremeDullard@lemmy.sdf.org
        1·
        2 months ago

        But what is an example of where I can use it?

        Aside from operations on bitfields, a bitwise operator can be useful in several “non bits” cases. For instance:

        value & 1 evaluates to 1 if value is odd (and will evaluate to True in an if statement)
        value >> 1 divides value by 2 (integer division)

        But usually bitwise operators are for when you want to manipulate bits in values. For instance:

        value | 5 returns value with bits 1 and 3 set to True
        value & 0xffff returns the 16 least-significant bits in value (usually you do this to make sure it will fit in 2 bytes in memory for example)
        value & (0xffff ^ 5) returns the lower 16 bits of value with bits 1 and 3 set to False

        Etc.

  • Eggscellent@sh.itjust.worksEnglish
    1·
    2 months ago

    Part of the problem is operator precedence - it’s ORing together the three numbers, then comparing that to “coin”.

    5 = 00101
10= 01010
25= 11001
    11111 = 31

    It’s testing if coin equals 31.

      • MinekPo1 [it/she]@lemmygrad.ml
        1·
        2 months ago

        one small gripe i have is that the repr¹ doesn’t handle aliases (ie items with more than one bit set) that well , but tbh its not an easy problem

        example to illustrate the problem :

        class Foo(enum.Flag):
    A = auto()
    B = auto()
    C = auto()
    # no D
    AB = 0b0011
    CD = 0b1100

print(Foo.AB | Foo.C)  # <Foo.A|B|C: 7> NOT <Foo.AB|C: 7>
print(Foo.CD | Foo.A)  # <Foo.A|C|CD: 13> NOT <Foo.A|CD: 7>

        its a minor thing but it annoys me a bit

        [1]: the _name_ member , which is used by __repr__ , of an enum member is actually generated either when the individual enum class is created or when the value is first needed by _missing_ . also the docs call these names surrounded by single underscores “sunder” names