• Tsaukpaetra (disco)

    E_TOPIC_FOUND: Success

  • Onyx (disco) in reply to Tsaukpaetra


  • cheong (disco)
    Comment held for moderation.
  • Kenneth_Mitchell (disco)

    At the University of Kansas in 1976, basic Boolean logic in the CS/Engineering department was "Yes", "No", and "Don't Care". You'd get dinged in your projects if you didn't properly allow for the "Don't Care" conditions.

  • Fox (disco) in reply to Kenneth_Mitchell

    Why have a boolean if you didn't care what it was?

  • FrostCat (disco) in reply to Fox
    Why have a boolean if you didn't care what it was?

    Because sometimes a variable's value isn't relevant to the solution.

  • Kenneth_Mitchell (disco) in reply to Fox

    You have multiple conditions; for some situations, one set of the values are more important than some other condition. In that case, the condition could be yes, or no, but you really DON'T CARE.

  • HardwareGeek (disco) in reply to Fox

    One use case is comparing values. Suppose you have an 8-bit value, and you want to do something if bits 7, 6 and 3 are true, and bit 5 is false. Which is clearer and easier to understand: x[7] & x[6] & !x[5] & x[3] or (x & 11101000) == 11001000 or x == 110?1??? ?

    In modern logic design, boolean logic can have 4 values, false, true, and two kinds of "I don't know," typically denoted by the enumerated values 0, 1, X and Z. X means "I can't figure out what the value is," either because variable was never initialized, or the state is (possibly) invalid, e.g., the logic is powered-down or a timing error occurred that could put a device into a metastable state, or because one of the devices inputs is unknown. Z is essentially a special kind of unknown that means "no device is driving this signal," which turns into an X when it goes through any kind of logic gate. Zs (and the resulting Xs) are expected and often necessary on any kind of half-duplex bi-directional bus (where one device asks another for data, and the other device sends the data back on the same wires) or removable device. (It is possible to avoid the Z case by pulling the wires to a valid logic level, but at the expense of higher power, which is not acceptable in a lot of applications.)

  • Yazeran (disco) in reply to FrostCat

    Yes for instance, if the reset pin on a J-K-flip-flop is pulled low and the set pin is kept high, then the state of the j,k and CP inputs really does not matter, as the state of the output (Q and not-Q) are solely determined by the reset and set pins

  • Cbuttius (disco)

    That article was 10 years old last week and is still one of the clbuttics.. I just brought it up in a code review today.

  • Quite (disco)


  • ixvedeusi (disco) in reply to Quite

    FILE_NOT_FOUND is a weird name for the hi-z state, though...

  • Thad (disco)

    Yeah obvious nonsense. No-one would ever put a third value in a boolean field, like NULL.

  • David_C (disco) in reply to Fox
    Comment held for moderation.
  • ScholRLEA (disco)

    Tri-state (and multi-state) logics - of which there are several - can be quite useful, but they are quite different from Boolean logic. Boolean logic in multiple variables can always be substituted for multi-state logics (or any other finite discrete value domain, for that matter), but not fuzzy logics, which are continuous, or quantum logics, which can have superposition. There are definitely cases where having a single inferential value be multi-state is useful, such as in relational calculus, however, and multi-state values are often valuable for encoding information being transmitted across a medium with multiple phases (such as the use of simultaneous amplitude, frequency and phase encodings to increase the effective bandwidth of an optical fiber).

  • blakeyrat (disco) in reply to Kenneth_Mitchell


    Modern day SQL has True/False/NULL as possible boolean values.

  • David_C (disco) in reply to Thad
    Yeah obvious nonsense. No-one would ever put a third value in a boolean field, like NULL.

    Well, if you're dealing with some kind of cascading properties, then any property may be in an "unconfigured" state in addition to a specific value (which could be Boolean). The precise meaning of "unconfigured" would depend on the application - it might mean that a default value (constant or computed) should be used, or that a value should be inherited from a parent object, or it could have some other application-specific meaning.

    Whether you would call "unconfigured" a separate value for the property (e.g. a so-called 3-state "boolean") or an independent property is really a meaningless discussion. There are many possible ways to represent the concept in code, all of which (should) have the same effect at the user interface level.

  • code.goddess (disco)

    Are they false dichotomies or are they dichotomies that are neither true nor false?

  • dkf (disco) in reply to HardwareGeek
    Z is essentially a special kind of unknown that means "no device is driving this signal," which turns into an X when it goes through any kind of logic gate.

    It's useful because when you're describing the behaviour of a device that's driving a bus line, you can either force the bus low or force it high, but you can also leave it alone (by switching the transistors that drive it either way off), and when you've got several places that can drive the same place, you only want one of them to drive the bus at a time. (Two things driving it in the same direction wouldn't be too bad I suppose, but two in the opposite directions Would Be Bad, as it would let the magic smoke out, so it's easier to just think in terms of explicitly producing Z.)

    Boolean logic is useful in other ways, however, as it was one of the key components of finally working out which were the sound syllogisms, and which were ill-founded. This went a long way towards setting much of what had classically been considered to be philosophy on a surer footing, and in the process it became a field of mathematics. The philosophers went off in a huff to ponder the meanings of intelligence and reality, and promptly achieved nothing much of consequence for a century or more. :smiling_imp:

  • Axel (unregistered) in reply to HardwareGeek

    Quote from HardwareGeek:

    "X means 'I can't figure out what the value is,'..."

    No, it doesn't. X means "I don't CARE what the value is." There's a difference. When you're filling in a Karnaugh map to reduce logic, an "X" in a square means the logic can produce either a 1 or 0; doesn't matter. You will KNOW what the value is; it's deterministic. But you don't CARE what it is because it doesn't affect the downstream logic.

    Another random observation: we (all of us) have done call-backs to this SOD as "FILE_NOT_FLUND." I only just now noticed that the correct citation is "FileNotFound." /pedant

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