if and only if

[danzibr]

I did a lot of math back in the day, and encountered many iff's. Recently I've been trying to think of a good non-mathy English sentence where iff would make sense. I did things like... (speaking to my son) we'll leave the playground if you misbehave. Well, that's not an only if. Or I'll grab an umbrella if it rains. That one doesn't work too well.

Just the other day I thought of a good one. Tax collectors are coming after you if/only if you didn't pay your taxes. It works both directions! Not the most cheery sentence, but I think it's good.

Any more examples come to mind?

[Fragenstein]

I have a chance, albeit a very small one, of winning the lottery if/only if I buy a ticket.

Bonus points for working out the math on the odds without googling.

[Kato]

I have a chance, albeit a very small one, of winning the lottery if/only if I buy a ticket.

Bonus points for working out the math on the odds without googling.

Odds of what? Winning the lottery? Well, depends on the lottery... standard German is think is 7 out of 49, so about 1 in 86 mio.


Well, there's a bunch of "only if"s depending on the situation, though I guess you could get semantic with a lot of them. (e.g. your tax collectors might also come after you if they or you made a mistake when doing your taxes :smalltongue:)
If/only if you don't die, you become older. (Okay, that's a somewhat weird one)
if/ only if you are a good boy Santa will bring you presents for Christmas. (Sufficiently strict parents required)
You can move at the speed of light if/only if you are a photon. (Okay, you can dissect even this...)

[Lethologica]

I'm planning to go to the beach today. I will go iff nothing urgent comes up.
I'm at the beach. This kite will fly iff there's enough wind.
The kite fell into the water. It will float iff it's less dense than the water.


Bonus points for working out the math on the odds without googling.
Cannot solve with the information given.

[Mando Knight]

Odds of what? Winning the lottery? Well, depends on the lottery... standard German is think is 7 out of 49, so about 1 in 86 mio.

It also depends on what you define as "winning the lottery", as many lotteries have smaller prizes to keep the players engaged while they seek the jackpot.

[Lethologica]

We can determine the odds of winning the lottery iff the lottery's parameters are specified. :smalltongue:

[crayzz]

"You'll pass this class iff you get at least 50% in both the lecture and lab section," is one I've encountered often.

[Cespenar]

If you define the problem to the point where if and iff makes no difference, then it's easy to use, as demonstrated by many of the examples in this thread.


I'm planning to go to the beach today. I will go iff nothing urgent comes up.
I'm at the beach. This kite will fly iff there's enough wind.
The kite fell into the water. It will float iff it's less dense than the water.

To the second: The kite may also fly if you tie it to, say, a drone.
To the third: The kite may also float if you tie it to a life buoy.

[NichG]

Rather than finding examples of iff, maybe it makes more sense to find examples where if vs iff makes a difference? That's kind of nontrivial conversationally, because a lot of the difference between 'if' and 'only if' comes up in situations where it'd be nonsensical for a person to express the thought that way.

For example:

If it rains, I will go to work. (But if it doesn't rain, I will also go to work).

That's a case where 'if' is not 'only if', but why is the speaker telling you this? In terms of communication, the lone 'if' tends to take on a role of 'even if', which isn't mathematically distinct, but certain is conversationally significant.

It can also show up in cases where there's a high multiplicity of possibilities, where the speaker is calling attention to one possibility for some reason:

If I get a full house and you get a pair, I win. (But that is not the only way I can win)

[Lethologica]

It's relatively easy to come up with examples where if and iff differ--less so to come up with ones where the iff version is something someone would actually say.

[danzibr]

It's relatively easy to come up with examples where if and iff differ--less so to come up with ones where the iff version is something someone would actually say.
Yeah, that's what I was after.

And thanks for the examples all! The one I ended up using was the lottery one.

[Jay R]

It's relatively easy to come up with examples where if and iff differ--less so to come up with ones where the iff version is something someone would actually say.

True, but only because it is so common that there's usually no need to say it.

I will wash these dishes if and only if they are dirty.

I will put the DVD in the player if and only if I want to watch the movie.

I will scratch if and only if it itches.

etc.

[Lethologica]

True, but only because it is so common that there's usually no need to say it.

I will wash these dishes if and only if they are dirty.

I will put the DVD in the player if and only if I want to watch the movie.

I will scratch if and only if it itches.

etc.
Sorry, I meant cases where both conditions were true, not cases where only the "meaningful iff" condition was true.

[Lorsa]

We can determine the odds of winning the lottery iff the lottery's parameters are specified. :smalltongue:

The odds of "winning" the lottery is always 1 if you're the one running it.