5 ms·

NaN literally means "not a number." Lots of things aren't numbers. The letter a is not a number. The square root of negative 1 is not a number (at least, not on

by cwyers 12y ago

NaN literally means "not a number." Lots of things aren't numbers. The letter a is not a number. The square root of negative 1 is not a number (at least, not one representable in floating point math). a is not equal to the square root of negative 1.

kazinator 12y ago
I don't understand your point. This is not a number: char *p = "abc"; Yet, this yields true: p == p The square root of -1 is easily representable in floating math. You just need two floats to make a complex.
- hueving 12y ago
  The point is that you can have very different things that are not a number. It's not safe to say that multiple things that are not a number are equal.
  - kazinator 12y ago
    The values of two expressions that evaluate the same variable are not "multiple things".
- cwyers 12y ago
  Okay, but while "abc" is not a number, it is also the string "abc". NaN is special in that all it tells you is that the value is not a number -- it doesn't tell you what it is. In order to return a true value from an equality test, it isn't enough to know that both values are not a number, you have to know what they actually are.
  - kazinator 12y ago
    If it is not known what is in the p variable, then the variable is indeterminate; it has exactly the same status as a variable that has not been initialized. In this case, the behavior upon accessing the variable should be undefined. I agree with making accesses of NaN-valued variables undefined behavior, so that not comparing equal is then a possible consequence of undefined behavior. I don't agree with defining the unequal comparison as the required behavior. To define the behavior is tantamount to the recognition that a NaN is something: an object. A variable can have a defined value which is that something, and that value must obey the Law of Identity.
pcl 12y ago
In practice, NaN as a literal means that the outcome of a mathematical statement is not expressible. So, the letter 'a' is not equal to NaN (with either two or three = signs). NaN, in other words, has a special meaning. Dismayingly, JavaScript's isNaN() function diverges from this special meaning, and does something close to what you said -- it tests to see if something is at least almost a number. So isNaN(13/0) and isNaN('foo') both evaluate to 'true', whereas isNaN('1') and isNaN(42) both evaluate to 'false'.
- flomo 12y ago
  Javascript type coercions might make sense if you realize it was originally designed for a close integration with HTML forms using a simplified proto-DOM ("Level 0"). The idea was more like "isNaN(myForm.myField)"? For all I know, the above might actually still work. But either way, that explains Javascript type conversion logic.

by cwyers 12y ago

kazinator 12y ago

I don't understand your point. This is not a number: char *p = "abc"; Yet, this yields true: p == p The square root of -1 is easily representable in floating math. You just need two floats to make a complex.

hueving 12y ago
The point is that you can have very different things that are not a number. It's not safe to say that multiple things that are not a number are equal.
- kazinator 12y ago
  The values of two expressions that evaluate the same variable are not "multiple things".
cwyers 12y ago
Okay, but while "abc" is not a number, it is also the string "abc". NaN is special in that all it tells you is that the value is not a number -- it doesn't tell you what it is. In order to return a true value from an equality test, it isn't enough to know that both values are not a number, you have to know what they actually are.
- kazinator 12y ago
  If it is not known what is in the p variable, then the variable is indeterminate; it has exactly the same status as a variable that has not been initialized. In this case, the behavior upon accessing the variable should be undefined. I agree with making accesses of NaN-valued variables undefined behavior, so that not comparing equal is then a possible consequence of undefined behavior. I don't agree with defining the unequal comparison as the required behavior. To define the behavior is tantamount to the recognition that a NaN is something: an object. A variable can have a defined value which is that something, and that value must obey the Law of Identity.

pcl 12y ago

In practice, NaN as a literal means that the outcome of a mathematical statement is not expressible. So, the letter 'a' is not equal to NaN (with either two or three = signs). NaN, in other words, has a special meaning. Dismayingly, JavaScript's isNaN() function diverges from this special meaning, and does something close to what you said -- it tests to see if something is at least almost a number. So isNaN(13/0) and isNaN('foo') both evaluate to 'true', whereas isNaN('1') and isNaN(42) both evaluate to 'false'.

flomo 12y ago
Javascript type coercions might make sense if you realize it was originally designed for a close integration with HTML forms using a simplified proto-DOM ("Level 0"). The idea was more like "isNaN(myForm.myField)"? For all I know, the above might actually still work. But either way, that explains Javascript type conversion logic.