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POExtracting numbers from a 32-bit integer
 

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  1. POExtracting numbers from a 32-bit integer
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    <p>I'm trying to solve a riddle in a programming test. </p> <p>Disclaimer: It's a test for a job, but I'm not looking for an answer. I'm just looking for an understanding of how to do this. The test requires that I come up with a set of solutions to a set of problems within 2 weeks, and it doesn't state a requirement that I arrive at the solutions in isolation.</p> <p>So, the problem:</p> <p>I have a 32-bit number with the bits arranged like this:</p> <pre><code>siiiiiii iiiiiiii ifffffff ffffffff </code></pre> <p>Where:</p> <ul> <li>s is the sign bit (1 == negative) </li> <li>i is 16 integer bits </li> <li>f is 15 fraction bits</li> </ul> <p>The assignment is to write something that decodes a 32-bit integer into a floating-point number. Given the following inputs, it should produce the following outputs:</p> <pre><code>input output 0x00008000 1.0 0x80008000 -1.0 0x00010000 2.0 0x80014000 -2.5 0x000191eb 3.14 0x00327eb8 100.99 </code></pre> <p>I'm having no trouble getting the sign bit or the integer part of the number. I get the sign bit like this:</p> <pre><code>boolean signed = ((value &amp; (1 &lt;&lt; 31)) != 0); </code></pre> <p>I get the integer and fraction parts like this:</p> <pre><code>int wholePart = ((value &amp; 0x0FFFFFFF) &gt;&gt; 15); int fractionPart = ((value &amp; 0x0000FFFF &gt;&gt; 1)); </code></pre> <p>The part I'm having an issue with is getting the number in the last 15 bits to match the expected values. Instead of 3.14, I get 3.4587, etc.</p> <p>If someone could give me a hint about what I'm doing wrong, I'd appreciate it. More than anything else, the fact that I haven't figured this out after hours of messing with it is kind of driving me nuts. :-)</p>
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    1. COYou have a problem with the wholePart mask that the test data won't reveal. The doubled parentheses in the fractionPart expression don't match the parentheses in the wholePart expression; but I don't see a reason to shift the fractionPart anyway. The fractionPart mask is wrong, too.
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      1. USJonathan Leffler
 

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