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bc freezes evaluating cbrt(0.01) #64

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ikolesnikes opened this issue Mar 13, 2023 · 4 comments
Closed

bc freezes evaluating cbrt(0.01) #64

ikolesnikes opened this issue Mar 13, 2023 · 4 comments

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@ikolesnikes
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bin/bc enters an infinite loop while evaluating the cube-root of 0.01

bin/bc -l -e 'cbrt(0.1)'
returns .46415888336127788924 (which is expected)

bin/bc -l -e 'cbrt(0.01)'
freezes

Looking at the definition of root(x,n) in lib2.bc the following loop never exits

while(r!=q){
	r=q
	q=(p*r+x/r^p)/n
}

A simple hack makes the cbrt function working.

while(abs(r-q)>0.000001){
	r=q
	q=(p*r+x/r^p)/n
}
@gavinhoward
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Yes, this is a bug. I have reproduced it.

Your fix pointed right to the problem, which I suspected: it ran into a pathological case where the two values, r and q, never equaled each other because they would keep switching, i.e., r would equal x, and q would equal y, then on the next iteration, r would equal y, and q would equal x.

This is the reason I hate algorithms that are calculated by series. They can easily have problems like this.

I tried to fix it with 01230fc. It increases the precision, but then does the comparison at a smaller precision.

Can you pull and test it for me? I'll start my release process in the meantime.

@ikolesnikes
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ikolesnikes commented Mar 13, 2023 •
edited
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I tested your solution and it worked!
Maybe it's worth to add a test for this case.

Never mind, you already did it.

@gavinhoward
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Yes, but you are right that if I did not, I should have!

I will put out a release with the fix as soon as I can. I'll close this issue at that time.

@gavinhoward
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Release 6.5.0 is out! Thank you for the report!

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@gavinhoward @ikolesnikes