Comment on Is "If A then B" equal to "B if and only if A"?

Boinkage@lemmy.world ⁨1⁩ ⁨year⁩ ago

• pineapplelover@lemm.ee ⁨1⁩ ⁨year⁩ ago

  First thing I thought lmao. Somebody is taking logic

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• XeroxCool@lemmy.world ⁨1⁩ ⁨year⁩ ago

  That’s not equivalent either. “if not b, then not a” works if it’s a sequence but doesn’t work for options in which multiple inputs can lead to the same output. If you get pizza every Tuesday and Friday, then answering “what’s for lunch” with “if Tuesday, then pizza” and “if Friday, then pizza” doesn’t let it work in reverse. “what day is it” can’t be answered with “if pizza lunch, then Tuesday”

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  • Boinkage@lemmy.world ⁨1⁩ ⁨year⁩ ago

    Ya wrong.

    If Tuesday, then pizza. And, if Friday, then pizza.

    The contrapositive : if not pizza, then not Tuesday and not Friday.

    What day is it? We’re not having pizza. So it’s not Tuesday or Friday.

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    • XeroxCool@lemmy.world ⁨1⁩ ⁨year⁩ ago

      That’s a key detail I now see

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  • pruwybn@discuss.tchncs.de ⁨1⁩ ⁨year⁩ ago

    You left out the “not” part - “If not pizza lunch, then not Tuesday” does indeed work.

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  • kogasa@programming.dev ⁨1⁩ ⁨year⁩ ago

    Using standard definitions from propositional logic they are equivalent.

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• monotremata@kbin.social ⁨1⁩ ⁨year⁩ ago

  Honestly what the homework is probably looking for is that it's equivalent to "B or not A." But yeah.

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