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POfloat/double equality with exact zero
 

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    <p>I have an algorithm which uses <code>floats</code> or <code>doubles</code> to perform some calculations.</p> <p>Example:</p> <pre><code>double a; double b; double c; ... double result = c / (b - a); if ((result &gt; 0) &amp;&amp; (result &lt; small_number)) { // result is relevant... } else { // result not required... } </code></pre> <p>Now, I am worried about <code>(b - a)</code> might be zero. If it is close to zero but not zero, it does not matter because the <code>result</code> will be out of range to be useful, and I already detect that (as <code>(b - a)</code> approaches zero, <code>result</code> will approach +/- inf, which is not in the range <code>0</code>-<code>small_number</code>...)</p> <p>But if the result of <code>(b - a)</code> is exactly zero, I expect that something platform dependant will happen due to divide by zero. I could change the <code>if</code> statement to:</p> <pre><code>if ((!((b-a) == 0.0)) &amp;&amp; ((result = c/(b-a)) &gt; 0) &amp;&amp; (result &lt; small_number)) { </code></pre> <p>but I don't know if <code>(b-a) == 0.0</code> will always detect equality with zero. I have seen there are multiple representations for exact zero in floating point? How can you test for them all without doing some epsilon check, which I don't need (a small epsilon will be ignored in my algorithm)?</p> <p>What is the platform independant way to check?</p> <p><strong>EDIT:</strong></p> <p>Not sure if it was clear enough to people. Basically I want to know how to find if an expression like:</p> <pre><code>double result = numerator / denominator; </code></pre> <p>will result in a floating point exception, a cpu exception, a signal from the operating system or something else.... without actually performing the operating and seeing if it will "throw"... because detecting a "throw" of this nature seems to be complicated and platform specific.</p> <p>Is <code>( (denominator==0.0) || (denominator==-0.0) ) ? "Will 'throw'" : "Won't 'throw'";</code> enough?</p>
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    1. COEven if `b - a` isn't exactly zero, the operation `c / (b - a)` could still overflow and send the value to `+/-INF`.
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    2. CO@Mystical That's fine, it would put the result outside of the range of interest for my problem.
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    3. COIt's hard to find an implementation that doesn't use IEEE floating-point. So I think it's safe to assume that division by `0` will just yield `+INF` or `-INF`? Someone care to correct me if I'm wrong?
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