Hello. I have a simple question - how to detect if a variable contains a Float of value -0.0? Is there any special method for this, or any comparison that distinguishes negative zero from a the regular zero? Thanks in advance. TPR.

on 2009-04-29 00:07

on 2009-04-29 00:13

Thomas B. wrote: > Hello. > > I have a simple question - how to detect if a variable contains a Float > of value -0.0? Is there any special method for this, or any comparison > that distinguishes negative zero from a the regular zero? > > Thanks in advance. > TPR. irb(main):001:0> x = -0.0 => -0.0 irb(main):002:0> x == 0.0 => true irb(main):003:0> x.eql?(0.0) => true irb(main):004:0> x.equal?(0.0) => false

on 2009-04-29 00:14

Thomas B. wrote: > I have a simple question - how to detect if a variable contains a Float > of value -0.0? Is there any special method for this, or any comparison > that distinguishes negative zero from a the regular zero? irb(main):001:0> 1/-0.0 < 0 => true irb(main):002:0> 1/0.0 < 0 => false -Matthias

on 2009-04-29 00:17

Joel VanderWerf wrote: > Thomas B. wrote: >> Hello. >> >> I have a simple question - how to detect if a variable contains a Float >> of value -0.0? Is there any special method for this, or any comparison >> that distinguishes negative zero from a the regular zero? >> >> Thanks in advance. >> TPR. > > irb(main):001:0> x = -0.0 > => -0.0 > irb(main):002:0> x == 0.0 > => true > irb(main):003:0> x.eql?(0.0) > => true > irb(main):004:0> x.equal?(0.0) > => false irb(main):001:0> x = 0.0 => 0.0 irb(main):002:0> x == 0.0 => true irb(main):003:0> x.eql?(0.0) => true irb(main):004:0> x.equal?(0.0) => false In short: you cannot distinguish between 0.0 and -0.0 this way. -Matthias

on 2009-04-29 00:25

Thank you :) Joel, your answer is not very helpful as 0.0.equals?(0.0) also returns false. Matthias, > irb(main):001:0> 1/-0.0 < 0 > => true > irb(main):002:0> 1/0.0 < 0 > => false thanks, that's just the obvious thing that I have overlooked! Changing the difference between zeros to the difference between infinities. TPReal.

on 2009-04-29 00:26

Joel VanderWerf wrote: > irb(main):001:0> x = -0.0 > => -0.0 > irb(main):002:0> x == 0.0 > => true > irb(main):003:0> x.eql?(0.0) > => true > irb(main):004:0> x.equal?(0.0) > => false What does that accomplish? irb(main):001:0> (-0.0).equal? -0.0 => false

on 2009-04-29 00:30

Thomas B. wrote: > Hello. > > I have a simple question - how to detect if a variable contains a Float > of value -0.0? Is there any special method for this, or any comparison > that distinguishes negative zero from a the regular zero? NB: I'm not sure how portable this is. def negative_zero?(number) inverse = 1 / number.to_f inverse < 0 && inverse.infinite? ? true : false end [ 0.0, # false -0.0, # true -1.0, # false 1.0 # false ].each do |f| puts negative_zero?(f) end

on 2009-04-29 00:36

Jeff S. wrote: >> > > irb(main):001:0> (-0.0).equal? -0.0 > => false Oops. That never seemed right to me. Float#equal? should hide the fact that floats are allocated, and not immediate. It has always seemed like a leaky abstraction this way.

on 2009-04-29 00:43

Joel VanderWerf wrote: > Oops. That never seemed right to me. Float#equal? should hide the fact > that floats are allocated, and not immediate. Why would it? It doesn't do it for any other type of object. And if it would, what would be the difference to == ?

on 2009-04-29 01:00

Sebastian H. wrote: > Joel VanderWerf wrote: >> Oops. That never seemed right to me. Float#equal? should hide the fact >> that floats are allocated, and not immediate. > > Why would it? It doesn't do it for any other type of object. And if it would, > what would be the difference to == ? Because it is only a quirk of cpu architecture that forces them to be allocated rather than immediate. Maybe some future ruby implementation (on >32 bit systems) will use immediate double-precision floats. Other types will never be immediate. The difference to #== would be the same as today: #== performs numeric type conversions (and consequently erases the difference between 0.0 and -0.0). But #equal? never would.

on 2009-04-29 01:55

On 28.04.2009 22:27, Jeff S. wrote: > inverse = 1 / number.to_f > inverse < 0 && inverse.infinite? ? true : false Why do you use the ternary operator to convert a boolean into a boolean? > end > > [ 0.0, # false > -0.0, # true > -1.0, # false > 1.0 # false > ].each do |f| > puts negative_zero?(f) > end My 0.02 EUR: normally there should not be any distinction between 0.0 and -0.0. Even though computer math is not the same as real math, the distinction does not seem to make sense to me. Kind regards robert

on 2009-04-29 02:40

On Apr 28, 2009, at 4:59 PM, Joel VanderWerf wrote: > implementation (on >32 bit systems) will use immediate double- > precision floats. Other types will never be immediate. A quick look at the IEEE format suggests that there are (2^51 - 1) bit patterns that are all considered NaN (not a number). Seems like it might be possible to encode Ruby references in there. Does anyone know of a language implementation that tags references in that way? Gary W.

on 2009-04-29 03:20

Gary W. <removed_email_address@domain.invalid> writes: > [...] > A quick look at the IEEE format suggests that there are (2^51 - 1) bit > patterns that are all considered NaN (not a number). > > Seems like it might be possible to encode Ruby references in there. > > Does anyone know of a language implementation that tags references in > that way? Some specific NaN are produced by arithmetic operations. Does IEEE forbid a FP unit to generate any NaN for some operations? It would seem to me to be dangerous to be able to produce random references from arithmetic operations...

on 2009-04-29 05:11

Pascal J. Bourguignon wrote: > Some specific NaN are produced by arithmetic operations. > Does IEEE forbid a FP unit to generate any NaN for some operations? > > It would seem to me to be dangerous to be able to produce random > references from arithmetic operations... The interpreter would have to add code to check for that and replace with some canonical NaN that doesn't conflict. The ruby runtime already has to call rb_float_new() on the result of every floating point function. Doing this check instead would be less overhead.

on 2009-04-29 19:48

Robert K. wrote: >> def negative_zero?(number) >> inverse = 1 / number.to_f >> inverse < 0 && inverse.infinite? ? true : false > > Why do you use the ternary operator to convert a boolean into a boolean? I don't. Float#infinite? can return nil, -1, or +1, but never true or false. Without the ternary, the original client code produces the following, far less meaningful output, including the blank line: false -1 false >> end >> >> [ 0.0, # false >> -0.0, # true >> -1.0, # false >> 1.0 # false >> ].each do |f| >> puts negative_zero?(f) >> end > My 0.02 EUR: normally there should not be any distinction between 0.0 > and -0.0. Even though computer math is not the same as real math, the > distinction does not seem to make sense to me. "Real" math? Whatever you say. In EE college courses, professors often use -0 to represent the limit of an asymptotic function that approaches zero from the negative side, e.g. the voltage decay of a negatively charged capacitor. The use has nothing to do with computers or IEEE floats. Anyway, take it up with the OP; AFAIK, his question was academic, but maybe he has an interesting use case.

on 2009-04-29 21:05

Jeff S. wrote: > "Real" math? Whatever you say. In EE college courses, professors often > use -0 to represent the limit of an asymptotic function that approaches > zero from the negative side, e.g. the voltage decay of a negatively > charged capacitor. The use has nothing to do with computers or IEEE > floats. > > Anyway, take it up with the OP; AFAIK, his question was academic, but > maybe he has an interesting use case. In fact, the -0.0 in programming is not very similar to the real math lim_{x->0-}(x). The simplest proof of this is the fact that -(1.0-1.0) gives -0.0, while after pushing the minus into the parenthesis we get -1.0+1.0 which gives 0.0. So I wouldn't say that -0.0 resembles the limes of capacitor charge, maybe only a bit. But if we wanted some more real math logic, we would need also +0.0 (different from 0.0), begin the result of 1.0/infinity. Then we would have -(+0.0) = -0.0, but -(0.0) = 0.0. But still it's only some approximation of "real math". Because of these inconsistency in IEEE (inconsistency with the real math or physics, I mean, not in IEEE itself), I'm not trying to use -0.0 as a real limes of something. The real use case is as follows (if anybody should be interested): A car can drive forward or reverse, but after it brakes to stop after, say, going forward, it needs to spend a short time staying still before it can start going backwards. This is a way of modelling the time needed to switch the gear from 1 and R (and the same applies to switching from R to 1). So now if I'm controlling the car, then I should be able to give it the desired velocity (the set point to a controller). So I decided that 0.0 means "don't move and be ready to go forward immediately (while I know there will be a moment's pause if I want to go backwards now)", while -0.0 means "don't move but stay switched to reverse, so that there's no time needed to start driving reverse (while a moment will be needed should I decide to go forward)". That's it, just one more bit of information pushed into the value of zero, which is exactly where I need it. TPR.

on 2009-04-29 23:36

On 29.04.2009 17:39, Jeff S. wrote: >>> > false > -1 > > false Well, but all these are perfectly boolean values in Ruby. There is no need to convert the expression other than for display maybe. But in that case I'd do the conversion outside the method as it does not add any semantics to the method implementation but costs time. Kind regards robert

on 2009-04-30 01:11

Robert K. wrote: >>>> >> following, far less meaningful output, including the blank line: >> >> false >> -1 >> >> false > > Well, but all these are perfectly boolean values in Ruby. They can be used seamlessly in boolean contexts, but that does not make them boolean values in the sense that true and false are. > There is no > need to convert the expression other than for display maybe. You mean, like, maybe in a Usenet post? > But in > that case I'd do the conversion outside the method as it does not add > any semantics to the method implementation but costs time. Have you done enough profiling to demonstrate that there is in fact a performance penalty, and that it justifies adding complexity to the client code? Silliness. Let the function return consistent, predictable values, and quit your whining.

on 2009-04-30 04:10

On Apr 28, 3:07 pm, "Thomas B." <removed_email_address@domain.invalid> wrote: > Hello. > > I have a simple question - how to detect if a variable contains a Float > of value -0.0? Is there any special method for this, or any comparison > that distinguishes negative zero from a the regular zero? > > Thanks in advance. > TPR. > -- > Posted viahttp://www.ruby-forum.com/. shouldnt converting it to a String and then testing for the presence of '-' work?