## digitalmars.D - nan question

- bobef (17/17) Sep 25 2007 Hello,
- Regan Heath (17/39) Sep 25 2007 See:
- BCS (2/23) Sep 25 2007 if you want to check for nan without a function call uses: fpVar !<>= 0
- bobef (2/6) Sep 25 2007 HAHAHAHAHAHAHAHA. I am not laughing at you, thanks for the help, but som...
- Nathan Reed (5/14) Sep 25 2007 Read it as "fpVar is not less than, greater than, or equal to zero."
- Janice Caron (4/5) Sep 25 2007 Well consider, the imaginary number i is not less than one. It is also
- bobef (2/8) Sep 25 2007 I can't imagine such number, sorry :) But let aside my imagination. I do...
- Nathan Reed (8/17) Sep 25 2007 I'm assuming you meant to say that it makes no sense if double.nan ==
- Don Clugston (7/28) Sep 25 2007 Yup, that's the reasoning.
- Janice Caron (6/6) Sep 26 2007 Think of NaN as meaning "I don't know the answer".
- Don Clugston (7/14) Sep 26 2007 Yes, we're forcing 3 states (yes, no, maybe) into 2 states.
- Steven Schveighoffer (7/18) Sep 25 2007 From what I can recall reading on this message board, the behavior is
- BCS (6/9) Sep 25 2007 NaN is "Not A Number" and considering that we are talking about numeric ...
- Jarrett Billingsley (4/6) Sep 25 2007 An imaginary number is an ifloat, idouble, or ireal. ;)

Hello, I have a little question. This program outputs gggg on dmd 1.21. Yes, I suck at math, but is this right (i.e. buggy) or nans must be compared another way? =========================== import tango.io.Stdout; void main() { double a=double.nan; if(a==double.nan) Stdout("1\n"); else Stdout("g\n"); if(a is double.nan) Stdout("2\n"); else Stdout("g\n"); if(double.nan==double.nan) Stdout("3\n"); else Stdout("g\n"); if(double.nan is double.nan) Stdout("4\n"); else Stdout("g\n"); }

Sep 25 2007

bobef wrote:Hello, I have a little question. This program outputs gggg on dmd 1.21. Yes, I suck at math, but is this right (i.e. buggy) or nans must be compared another way? =========================== import tango.io.Stdout; void main() { double a=double.nan; if(a==double.nan) Stdout("1\n"); else Stdout("g\n"); if(a is double.nan) Stdout("2\n"); else Stdout("g\n"); if(double.nan==double.nan) Stdout("3\n"); else Stdout("g\n"); if(double.nan is double.nan) Stdout("4\n"); else Stdout("g\n"); }See: http://www.digitalmars.com/d/expression.html#floating_point_comparisons In particular notice that the == row contains F in the unordered column. This means (if I'm reading it correctly) that if either operand is nan == will always give false. I think you want to have a look at isnan from std.math (not sure what Tango has), eg //example phobos-ified import std.math; void main() { double a = double.nan; if (isnan(a)) writefln("1"); else writefln("g"); } Regan

Sep 25 2007

Reply to bobef,Hello, I have a little question. This program outputs gggg on dmd 1.21. Yes, I suck at math, but is this right (i.e. buggy) or nans must be compared another way? =========================== import tango.io.Stdout; void main() { double a=double.nan; if(a==double.nan) Stdout("1\n"); else Stdout("g\n"); if(a is double.nan) Stdout("2\n"); else Stdout("g\n"); if(double.nan==double.nan) Stdout("3\n"); else Stdout("g\n"); if(double.nan is double.nan) Stdout("4\n"); else Stdout("g\n"); }if you want to check for nan without a function call uses: fpVar !<>= 0

Sep 25 2007

BCS Wrote:if you want to check for nan without a function call uses: fpVar !<>= 0HAHAHAHAHAHAHAHA. I am not laughing at you, thanks for the help, but something is totally wrong. Just look at this. "!<>=" it looks almost like bytecode or 1337 or something :)

Sep 25 2007

bobef wrote:BCS Wrote:Read it as "fpVar is not less than, greater than, or equal to zero." The only 'number' that satisfies all these conditions is NaN. Thanks, Nathan Reedif you want to check for nan without a function call uses: fpVar !<>= 0HAHAHAHAHAHAHAHA. I am not laughing at you, thanks for the help, but something is totally wrong. Just look at this. "!<>=" it looks almost like bytecode or 1337 or something :)

Sep 25 2007

On 9/25/07, bobef <bobef abv_nospam.bg> wrote:something is totally wrong. Just look at this. "!<>="Well consider, the imaginary number i is not less than one. It is also not greater than one. It is also not equal to one. It makes perfect sense.

Sep 25 2007

Janice Caron Wrote:On 9/25/07, bobef <bobef abv_nospam.bg> wrote:I can't imagine such number, sorry :) But let aside my imagination. I don't know what imaginary number is, so I am not commenting if it makes sense. All I'm saying it that it is nonsense that if(double.nan!=double.nan) returns false (IMHO).something is totally wrong. Just look at this. "!<>="Well consider, the imaginary number i is not less than one. It is also not greater than one. It is also not equal to one. It makes perfect sense.

Sep 25 2007

bobef wrote:Janice Caron Wrote:I'm assuming you meant to say that it makes no sense if double.nan == double.nan returns false? But if NaNs compared equal to each other, then this: sqrt(-1) == acos(2) would be true, since both return a NaN. I bet you don't really want that. Thanks, Nathan ReedOn 9/25/07, bobef <bobef abv_nospam.bg> wrote:I can't imagine such number, sorry :) But let aside my imagination. I don't know what imaginary number is, so I am not commenting if it makes sense. All I'm saying it that it is nonsense that if(double.nan!=double.nan) returns false (IMHO).something is totally wrong. Just look at this. "!<>="Well consider, the imaginary number i is not less than one. It is also not greater than one. It is also not equal to one. It makes perfect sense.

Sep 25 2007

Nathan Reed wrote:bobef wrote:Yup, that's the reasoning. Even so, I think it probably was a mistake by the IEEE standard to violate x == x; I think the problems created by this decision are worse than the ones that were solved. (Note that they could even have used NaN payloads to detect the worst kinds of errors). But it's built into the hardware everywhere, so it's Too Late Now.Janice Caron Wrote:I'm assuming you meant to say that it makes no sense if double.nan == double.nan returns false? But if NaNs compared equal to each other, then this: sqrt(-1) == acos(2) would be true, since both return a NaN. I bet you don't really want that.something is totally wrong. Just look at this. "!<>="Well consider, the imaginary number i is not less than one. It is also not greater than one. It is also not equal to one. It makes perfect sense.

Sep 25 2007

Think of NaN as meaning "I don't know the answer". With that understanding, when you compare two NaNs with ==, you are asking "Is one thing I don't know the answer to equal to another thing I don't know the answer to". The absolutely correct answer should really be "I don't know, but probably not", but since that can't be expressed in a bool, we go with the second best answer: no.

Sep 26 2007

Janice Caron wrote:Think of NaN as meaning "I don't know the answer". With that understanding, when you compare two NaNs with ==, you are asking "Is one thing I don't know the answer to equal to another thing I don't know the answer to". The absolutely correct answer should really be "I don't know, but probably not", but since that can't be expressed in a bool, we go with the second best answer: no.Yes, we're forcing 3 states (yes, no, maybe) into 2 states. IMHO, it would have been better to retain the crucial identity x==x, rather than try to detect obscure floating-point bugs. (and introduce a different operator to mean, equal-and-not-both-NaN). I use NaNs a lot (probably more than anyone else on this ng), and have not found many cases where the IEEE behaviour is helpful.

Sep 26 2007

"bobef" <bobef abv_nospam.bg> wrote in message news:fdbedv$2pu2$1 digitalmars.com...something is totally wrong. Just look at this. "!<>="Well consider, the imaginary number i is not less than one. It is also not greater than one. It is also not equal to one. It makes perfect sense.

Sep 25 2007

Reply to bobef,All I'm saying it that it is nonsense that if(double.nan!=double.nan) returns false (IMHO).NaN is "Not A Number" and considering that we are talking about numeric equality, it is reasonable for something that isn't a NUMber to not be NUMerically equal to anything. It is sort of like comparing to nothing (not the state of nothing or a set of nothing, but the absolute omission or the entity) this assertion "x is equal to" is syntactically broken so how can it be true?

Sep 25 2007

"bobef" <bobef abv_nospam.bg> wrote in message news:fdbedv$2pu2$1 digitalmars.com...I can't imagine such number, sorry :) But let aside my imagination. I don't know what imaginary number isAn imaginary number is an ifloat, idouble, or ireal. ;) (Everything I know I learned from D.)

Sep 25 2007