5 ms·

I once encountered an old database using floats as its primary keys. This was in a fairly popular piece of manufacturing software from the 90s.

shabbyrobe 8y ago
Wow, I'd love to hear more about this.
mcv 8y ago
Useful in case you want multiple entries with an id of approximately 1?
- Const-me 8y ago
  Potentially, might be useful if you want small fixed-length keys that are fast to compare, and also you want to be able to insert new items between existing ones.
  - jmiserez 8y ago
    There was this article posted a while ago that used a Stern-Brocot tree and/or floats for ordering: https://news.ycombinator.com/item?id=16635440 https://news.ycombinator.com/item?id=16635440 It doesn't work in the pathological case though as any float has a fixed number of bits, and I see the article has been amended with a note.
maxxxxx 8y ago
What type of data would that be? I once did this when I did my own primitive version of a geospatial database in the 90s.
sevensor 8y ago
I sometimes, in Python, cautiously use floating point numbers as dictionary keys. I do this when what I mean is "this exact floating-point number" and not "the result of an equivalent computation." It's almost never a good idea, because it's really easy to trip yourself up by assuming the latter when only the former actually holds.
- deleted 8y ago
  [deleted]
- jmiserez 8y ago
  And even the former apparently isn't guaranteed on all hardware. I found this gem in the Java documentation, I don't know which processors actually do this: >"Arithmetic operations on signaling NaNs turn them into quiet NaNs with a different, but often similar, bit pattern. However, on some processors merely copying a signaling NaN also performs that conversion. In particular, copying a signaling NaN to return it to the calling method may perform this conversion." Source: https://docs.oracle.com/javase/7/docs/api/java/lang/Double.html https://docs.oracle.com/javase/7/docs/api/java/lang/Double.h...
  - sevensor 8y ago
    This is the stuff of nightmares.
  - Veedrac 8y ago
    If you're putting floats in a dict, you can't use NaNs anyway, since they aren't equal to each other.
- kazinator 8y ago
  You could hardly avoid this if you, say, memoized some function that has floating-point inputs.
  - sevensor 8y ago
    Indeed, and for that reason I'd be very cautious when attempting to memoize a function with floating-point inputs.

shabbyrobe 8y ago

Wow, I'd love to hear more about this.

mcv 8y ago

Useful in case you want multiple entries with an id of approximately 1?

Const-me 8y ago
Potentially, might be useful if you want small fixed-length keys that are fast to compare, and also you want to be able to insert new items between existing ones.
- jmiserez 8y ago
  There was this article posted a while ago that used a Stern-Brocot tree and/or floats for ordering: https://news.ycombinator.com/item?id=16635440 https://news.ycombinator.com/item?id=16635440 It doesn't work in the pathological case though as any float has a fixed number of bits, and I see the article has been amended with a note.

maxxxxx 8y ago

What type of data would that be? I once did this when I did my own primitive version of a geospatial database in the 90s.

sevensor 8y ago

I sometimes, in Python, cautiously use floating point numbers as dictionary keys. I do this when what I mean is "this exact floating-point number" and not "the result of an equivalent computation." It's almost never a good idea, because it's really easy to trip yourself up by assuming the latter when only the former actually holds.

deleted 8y ago
[deleted]
jmiserez 8y ago
And even the former apparently isn't guaranteed on all hardware. I found this gem in the Java documentation, I don't know which processors actually do this: >"Arithmetic operations on signaling NaNs turn them into quiet NaNs with a different, but often similar, bit pattern. However, on some processors merely copying a signaling NaN also performs that conversion. In particular, copying a signaling NaN to return it to the calling method may perform this conversion." Source: https://docs.oracle.com/javase/7/docs/api/java/lang/Double.html https://docs.oracle.com/javase/7/docs/api/java/lang/Double.h...
- sevensor 8y ago
  This is the stuff of nightmares.
- Veedrac 8y ago
  If you're putting floats in a dict, you can't use NaNs anyway, since they aren't equal to each other.
kazinator 8y ago
You could hardly avoid this if you, say, memoized some function that has floating-point inputs.
- sevensor 8y ago
  Indeed, and for that reason I'd be very cautious when attempting to memoize a function with floating-point inputs.