A bug? input: 1/0 output: -Inf

if x=0, expected output of 1/x is Inf, but matlab output -Inf

load('x.mat')
x==0

Unrecognized function or variable 'x'.

1/0
1/x

[Matt J]

Your attached file doesn't contain a variable called x.

[Image Analyst]

x==0 is a comparison, not an assignment. Since you didn't assign x yet, you can't see if it's equal to zero, hence the error. I think you meant x=0, not x==0.

Answer 1

[Stephen23]

"A bug? input: 1/0 output: -Inf"

Not a bug. Because your value x is actually negative zero not positive zero:

https://en.wikipedia.org/wiki/Signed_zero

Lets try it now:

x = -0

x = 0

1/x

ans = -Inf

num2hex(x) % yep, definitely negative zero

ans = '8000000000000000'

x = +0

x = 0

1/x

ans = Inf

num2hex(x) % yep, that is positive zero

ans = '0000000000000000'

So far everything is working exactly as expected. Note that a few simple arithmetic operations will convert negative zero to positive zero without affecting any other values, e.g. adding zero:

x = -0;
1/x

ans = -Inf

x = 0+x;
1/x

ans = Inf

[Stephen23]

Lets take a look at your uploaded data:

S = load('x.mat');
x = S.deltaX

x = 0

1/x

ans = -Inf

num2hex(x) % yep, definitely negative zero

ans = '8000000000000000'

[qilin]

It is helpful, maybe the next step is to figure out why my ''deltaX'' is negative zero, thank you.

[Dyuman Joshi]

How did you obtain that value?

[Stephen23]

"maybe the next step is to figure out why my ''deltaX'' is negative zero"

Most likely it makes zero(!) difference: note that by definition negative and positive zeros have the same value, so if you are happy with your array being zero-value then simply multiply your array by one (or add zero) and move on to more important tasks.

Answer 2

[Matt J]

Theorem: 1/0 = Inf if and only if 1/0 is also -Inf.

Proof: Assume first that 1/0=Inf and let us deduce the implication that 1/0=-Inf. By multiplying the numerator and denominator by -1, we obtain,

1/0 = (-1)/(-0) = (-1)/0 = -(1/0) = -Inf

proving the implication Now assume that 1/0=-Inf. Proceeding similarly,

1/0 = (-1)/(-0) = (-1)/0 = -(1/0) = -(-Inf) = +Inf

and the reverse implication is also proved. Q.E.D.

[qilin]

Thank you for your answer, it's interesting, but: .

In other words, when we say , actually it's , here , so the second

''='' of ''1/0 = (-1)/(-0) =...'' should be modified.

[Matt J]

You took it way too seriously...