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It's not only NASA. Zero is just not a reasonable default value for an average. Imagine average credit card balance for FICO score, average blood pressure, aver

by pps43 8y ago

It's not only NASA. Zero is just not a reasonable default value for an average. Imagine average credit card balance for FICO score, average blood pressure, average temperature in a freezer, etc. UB is not the only way. You can have NULLs (they come with their own sets of problems, but still). In many cases no value is better than incorrect value.

jeremyjh 8y ago
You can check for 0 and throw in those cases, and then propagate the error path appropriately. And as they've already said in this thread, they will be providing a checked arithmetic in the standard library to do that for you.
- Giroflex 8y ago
  Well, sometimes the average is 0, e.g. (-2 + 2)/2. And the whole point is that this is meant to catch unforeseen interactions as soon as possible. If you add a check it's no longer unforeseen, and it may easily slip the programmer's mind.
  - seandougall 8y ago
    If I'm not mistaken, jeremyjh was referring to checking for 0 in the denominator, not the numerator.
Veedrac 8y ago
It's a value type, so you can't really have NULL. I don't see the issue with those examples; if you have no samples, there's nothing to mislabel. An average blood pressure of 0 across 0 patients is only going to harm 0 patients.
- pps43 8y ago
  > I don't see the issue with those examples Let me elaborate then. Average credit card balance predicts probability of default. Joe who maxed out his credit card is higher risk than Jane who pays back her entire credit card balance every month. Now we have Jack with no credit card. We predict that Jack is low risk because his average credit card balance is zero. Second example, defibrillator that monitors blood pressure and shocks the patient when his heart stops (blood pressure drops to zero). We attach the defibrillator to a patient, turn it on, and it shocks the patient immediately because there are no blood pressure measurements yet and therefore it thinks the average is zero. Third example, a thermostat that turns on the freezer if average temperature is above a set threshold. It never turns on because average (of zero observations) is already zero. Of course you can hard-code handling of those special cases, but if you did not, failing would be preferable to continuing to work incorrectly.
  - labster 8y ago
    https://en.m.wikipedia.org/wiki/Credit_score_in_the_United_States#FICO_score https://en.m.wikipedia.org/wiki/Credit_score_in_the_United_S... Credit scores don't work this way. Zero is not a valid credit score, and the things you mentioned describe different components of the score. I work a lot with survey data, to be interpreted by humans. In this problem domain, zero is a sane value for an empty average, though "N/A" is usually better. I wish our language would let us define it as zero so long reports don't die in the middle. But it's about your problem domain, as usual.
    - pps43 8y ago
      There are many different credit scores. The range of valid FICO scores does not include zero, but internal acquisition and behavior scores that banks use might. But in this case it's irrelevant because average balance is independent variable (input of the scoring model).
  - seandougall 8y ago
    It seems to me taking averages would be inappropriate in all three of those scenarios. Credit risk is based on total debt, and since credit cards are a revolving line of credit, the entire credit limit is considered debt, even if the balance is zero. That's why you can improve your credit score by closing out a credit card that you never use and that carries a zero balance (which we did in order to qualify for a mortgage several years ago). In other words, it's a sum, not an average; there's no division involved. Setting aside the fact that there are much more reliable ways to detect a stopped heart than blood pressure: If you're taking a running average of BP over time, a low reading after a heart attack would just be a single data point, and you'd probably have to accumulate a bunch of them in order for it to register (particularly if you got a high reading just before the event). Even if you take a reading every five minutes, which is ridiculously frequent for BP, the patient will be long dead by the time you notice. You should be acting based on the last reading; again, no division involved. I don't see why a freezer would be based on running averages rather than the most recent reading either. Thermostats generally work off of two thresholds: a higher one above which the compressor turns on, and a lower one below which it turns off. That smooths out any measurement noise without taking averages. Even if you do use averages for some reason, though, presumably it will start measuring at _some_ point and the compressor will turn on; I don't see why the average would stay at zero. I'm not convinced that `1/0 = 0` is correct in any meaningful way, but I feel like any situation where it would cause bugs more critical than a UI issue probably points to a deeper design flaw. After all, if the alternative is to crash on a failed assertion, that's not necessarily preferable in a life-or-death situation.

by pps43 8y ago

jeremyjh 8y ago

You can check for 0 and throw in those cases, and then propagate the error path appropriately. And as they've already said in this thread, they will be providing a checked arithmetic in the standard library to do that for you.

Giroflex 8y ago
Well, sometimes the average is 0, e.g. (-2 + 2)/2. And the whole point is that this is meant to catch unforeseen interactions as soon as possible. If you add a check it's no longer unforeseen, and it may easily slip the programmer's mind.
- seandougall 8y ago
  If I'm not mistaken, jeremyjh was referring to checking for 0 in the denominator, not the numerator.

Veedrac 8y ago

It's a value type, so you can't really have NULL. I don't see the issue with those examples; if you have no samples, there's nothing to mislabel. An average blood pressure of 0 across 0 patients is only going to harm 0 patients.

pps43 8y ago
> I don't see the issue with those examples Let me elaborate then. Average credit card balance predicts probability of default. Joe who maxed out his credit card is higher risk than Jane who pays back her entire credit card balance every month. Now we have Jack with no credit card. We predict that Jack is low risk because his average credit card balance is zero. Second example, defibrillator that monitors blood pressure and shocks the patient when his heart stops (blood pressure drops to zero). We attach the defibrillator to a patient, turn it on, and it shocks the patient immediately because there are no blood pressure measurements yet and therefore it thinks the average is zero. Third example, a thermostat that turns on the freezer if average temperature is above a set threshold. It never turns on because average (of zero observations) is already zero. Of course you can hard-code handling of those special cases, but if you did not, failing would be preferable to continuing to work incorrectly.
- labster 8y ago
  https://en.m.wikipedia.org/wiki/Credit_score_in_the_United_States#FICO_score https://en.m.wikipedia.org/wiki/Credit_score_in_the_United_S... Credit scores don't work this way. Zero is not a valid credit score, and the things you mentioned describe different components of the score. I work a lot with survey data, to be interpreted by humans. In this problem domain, zero is a sane value for an empty average, though "N/A" is usually better. I wish our language would let us define it as zero so long reports don't die in the middle. But it's about your problem domain, as usual.
  - pps43 8y ago
    There are many different credit scores. The range of valid FICO scores does not include zero, but internal acquisition and behavior scores that banks use might. But in this case it's irrelevant because average balance is independent variable (input of the scoring model).
- seandougall 8y ago
  It seems to me taking averages would be inappropriate in all three of those scenarios. Credit risk is based on total debt, and since credit cards are a revolving line of credit, the entire credit limit is considered debt, even if the balance is zero. That's why you can improve your credit score by closing out a credit card that you never use and that carries a zero balance (which we did in order to qualify for a mortgage several years ago). In other words, it's a sum, not an average; there's no division involved. Setting aside the fact that there are much more reliable ways to detect a stopped heart than blood pressure: If you're taking a running average of BP over time, a low reading after a heart attack would just be a single data point, and you'd probably have to accumulate a bunch of them in order for it to register (particularly if you got a high reading just before the event). Even if you take a reading every five minutes, which is ridiculously frequent for BP, the patient will be long dead by the time you notice. You should be acting based on the last reading; again, no division involved. I don't see why a freezer would be based on running averages rather than the most recent reading either. Thermostats generally work off of two thresholds: a higher one above which the compressor turns on, and a lower one below which it turns off. That smooths out any measurement noise without taking averages. Even if you do use averages for some reason, though, presumably it will start measuring at _some_ point and the compressor will turn on; I don't see why the average would stay at zero. I'm not convinced that `1/0 = 0` is correct in any meaningful way, but I feel like any situation where it would cause bugs more critical than a UI issue probably points to a deeper design flaw. After all, if the alternative is to crash on a failed assertion, that's not necessarily preferable in a life-or-death situation.