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https://issues.apache.org/jira/browse/MATH-306?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12769670#action_12769670
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Phil Steitz commented on MATH-306:
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Thanks for reviewing the code.
The code is misleading, but should not lead to incorrect results. The line you
refer to above is actually in a block that will never get executed - so should
be deleted.
{code}
if (Math.abs(c) < Math.abs(d)) {
if (d == 0.0) { <---- impossible to have abs(c) < 0
return createComplex(real/c, imaginary/c);
}
double q = c / d;
double denominator = c * q + d;
return createComplex((real * q + imaginary) / denominator,
(imaginary * q - real) / denominator);
} else {
if (c == 0.0) { <-- to get here, we need c = d = 0, but this is
handled above.
return createComplex(imaginary/d, -real/c); <-- incorrect fmla
is harmless because never executed.
}
{code}
Interesting that static analyzers did not catch this as dead code. Unless I am
missing something, both of the if(* == 0) tests in the block above can be
removed (with no effect on results).
Appreciate comments on this, but leaning toward code cleanup and closing as
invalid.
> Method 'divide' in class 'Complex' uses a false formula for a special case
> resulting in erroneous division by zero.
> -------------------------------------------------------------------------------------------------------------------
>
> Key: MATH-306
> URL: https://issues.apache.org/jira/browse/MATH-306
> Project: Commons Math
> Issue Type: Bug
> Environment: all
> Reporter: Joerg Huber
>
> The formula that 'divide' wants to implement is
> ( a + bi ) / ( c + di ) = ( ac + bd + ( bc - ad ) i ) / ( c^2 + d^2 )
> as correctly written in the description.
> When c == 0.0 this leads to the special case
> ( a + bi ) / di = ( b / d ) - ( a / d ) i
> But the corresponding code is:
> if (c == 0.0) {
> return createComplex(imaginary/d, -real/c);
> }
> The bug is the last division -real/c, which should obviously be -real/d.
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