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

3 views (last 30 days)
qilin on 13 Feb 2024
Commented: Matt J on 13 Feb 2024
if x=0, expected output of 1/x is Inf, but matlab output -Inf
x==0
Unrecognized function or variable 'x'.
1/0
1/x
Matt J on 13 Feb 2024
Your attached file doesn't contain a variable called x.
Image Analyst on 13 Feb 2024
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.

Stephen23 on 13 Feb 2024
Edited: Stephen23 on 13 Feb 2024
"A bug? input: 1/0 output: -Inf"
Not a bug. Because your value x is actually negative zero not positive 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
Dyuman Joshi on 13 Feb 2024
How did you obtain that value?
Stephen23 on 13 Feb 2024
"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.

Matt J on 13 Feb 2024
Edited: Matt J on 13 Feb 2024
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 on 13 Feb 2024
In other words, when we say , actually it's , here , so the second
''='' of ''1/0 = (-1)/(-0) =...'' should be modified.
Matt J on 13 Feb 2024
You took it way too seriously...

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